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Efficiency of MFJ remotely-tuned loop antennas

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Alan Bloom

Efficiency of MFJ remotely-tuned loop antennas

MFJ makes a pair of small, remotely-tuned loop antennas, the MFJ-1786
that covers 10-30 MHz and the MFJ-1788 that covers 7 to 21+ MHz. As far
as I can tell, the two antennas are identical except for the size of the
tuning capacitor. Each consists of a 3 foot (91 cm) diameter loop made
of aluminum tubing and a plastic housing that contains the tuning
capacitor, motor, and coupling loop. No control cable is required since
the control voltage is sent from the control box in the shack to the
motor in the antenna via the coaxial cable.

Before I purchase one of these I wanted to get an idea of the efficiency
of such a small loop. MFJ is silent on the subject so I did my own
calculations. The calculations and results are on a 1-page document
that I uploaded to Dropbox and can be downloaded here:

https://www.dropbox.com/s/l8mv67cjrck2ssn/MFJ-1786-1788.pdf?dl=0

My calculations are based on the assumption that the efficiency of the
MFJ antennas is similar to the (no longer manufactured) AEA Isoloop (my
reasoning for that is in the document) and that AEA's specification of
72% efficiency at 14 MHz is correct. From that number I can calculate
the efficiency and gain on all the other bands.

If you don't want to download the document, here is a summary of the
results:

Freq Eff Gain with respect to a half-wave dipole
MHz dB dBd
7.0 -7.3 -7.7
10.1 -3.5 -3.9
14.0 -1.4 -1.8
18.068 -0.6 -1.0
21.0 -0.4 -0.8
24.89 -0.2 -0.6
28.0 -0.15 -0.5

I'd be interested in any comments people may have on the accuracy of
my assumptions and calculations in the document.

Alan N1AL
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wayne burdick

Re: Efficiency of MFJ remotely-tuned loop antennas

Administrator

Hi Alan,

72% sounds a bit high. Is this number based on loop size alone ("in theory")? Or are they taking conductor geometry and other losses into account?

Wayne
N6KR

> On Jan 18, 2021, at 2:05 PM, Alan Bloom <[hidden email]> wrote:
>
> MFJ makes a pair of small, remotely-tuned loop antennas, the MFJ-1786 that covers 10-30 MHz and the MFJ-1788 that covers 7 to 21+ MHz. As far as I can tell, the two antennas are identical except for the size of the tuning capacitor. Each consists of a 3 foot (91 cm) diameter loop made of aluminum tubing and a plastic housing that contains the tuning capacitor, motor, and coupling loop. No control cable is required since the control voltage is sent from the control box in the shack to the motor in the antenna via the coaxial cable.
>
> Before I purchase one of these I wanted to get an idea of the efficiency of such a small loop. MFJ is silent on the subject so I did my own calculations. The calculations and results are on a 1-page document that I uploaded to Dropbox and can be downloaded here:
>
> https://www.dropbox.com/s/l8mv67cjrck2ssn/MFJ-1786-1788.pdf?dl=0
>
> My calculations are based on the assumption that the efficiency of the MFJ antennas is similar to the (no longer manufactured) AEA Isoloop (my reasoning for that is in the document) and that AEA's specification of 72% efficiency at 14 MHz is correct. From that number I can calculate the efficiency and gain on all the other bands.
>
> If you don't want to download the document, here is a summary of the results:
>
> Freq Eff Gain with respect to a half-wave dipole
> MHz dB dBd
> 7.0 -7.3 -7.7
> 10.1 -3.5 -3.9
> 14.0 -1.4 -1.8
> 18.068 -0.6 -1.0
> 21.0 -0.4 -0.8
> 24.89 -0.2 -0.6
> 28.0 -0.15 -0.5
>
> I'd be interested in any comments people may have on the accuracy of
> my assumptions and calculations in the document.
>
> Alan N1AL
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
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k6dgw

Re: Efficiency of MFJ remotely-tuned loop antennas

In reply to this post by Alan Bloom

Alan,

I think 72% is really optimistic for both the AEA Isoloop and the MFJ.
For a 3/4" copper tubing loop 3 ft in diameter, at 14 MHz, I come up with:

Antenna efficiency: 38% (-4.2 dB below 100%)
Antenna bandwidth: 31.5 kHz
Tuning Capacitance: 85 pF

Capacitor voltage: 2,445 volts RMS
Resonant circulating current: 18.2 A
Radiation resistance: 0.058 ohms
Loss Resistance: 0.093 ohms
Inductance: 1.53 microhenrys
Inductive Reactance: 134 ohms
Quality Factor (Q): 445
Distributed capacity: 8 pF

TX power was arbitrarily set at 100 W. The conductivity of Cu is about
59E10 Se vs 38E10 Se for Al and I once saw an analysis that showed
silver soldering lengths of copper pipe and 45 deg fittings into an
octagon raised the loss by about 20 % @ 14 MHz. The best configuration
seems to be a continuous copper strip [wider is better within reason].
There will be some additional loss in the connection to the capacitor.
Since the radiation resistance is in the tens of milliohms range, this
is a case where antenna DX wax might actually work.😉

I have an Alexloop I use with my K2. It works, sort of, best on 20 and
up. One caveat to any small mag loop: They are essentially resonant
transformers and the operative word is "resonant." Any ATU must be
bypassed and the loop tuned exactly to resonance [R+j0X]. The swr is a
function of the area ratio between the large loop and the small driven
loop. Letting the ATU "find the match" results in a very good
approximation to a dummy load.

The above calcs came from

www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx

which I like, there are a number of others out there. I distrust the BW
number, my Alexloop has about 60% of the calculated value before it
assumes the shape of a dummy load.

73,

Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County

On 1/18/2021 2:05 PM, Alan Bloom wrote:

> MFJ makes a pair of small, remotely-tuned loop antennas, the MFJ-1786
> that covers 10-30 MHz and the MFJ-1788 that covers 7 to 21+ MHz. As
> far as I can tell, the two antennas are identical except for the size
> of the tuning capacitor. Each consists of a 3 foot (91 cm) diameter
> loop made of aluminum tubing and a plastic housing that contains the
> tuning capacitor, motor, and coupling loop. No control cable is
> required since the control voltage is sent from the control box in the
> shack to the motor in the antenna via the coaxial cable.
>
> Before I purchase one of these I wanted to get an idea of the
> efficiency of such a small loop. MFJ is silent on the subject so I
> did my own calculations. The calculations and results are on a 1-page
> document that I uploaded to Dropbox and can be downloaded here:
>
> https://www.dropbox.com/s/l8mv67cjrck2ssn/MFJ-1786-1788.pdf?dl=0
>
> My calculations are based on the assumption that the efficiency of the
> MFJ antennas is similar to the (no longer manufactured) AEA Isoloop
> (my reasoning for that is in the document) and that AEA's
> specification of 72% efficiency at 14 MHz is correct. From that
> number I can calculate the efficiency and gain on all the other bands.
>
> If you don't want to download the document, here is a summary of the
> results:
>
> Freq    Eff    Gain with respect to a half-wave dipole
> MHz    dB    dBd
> 7.0    -7.3    -7.7
> 10.1    -3.5    -3.9
> 14.0    -1.4    -1.8
> 18.068    -0.6    -1.0
> 21.0    -0.4    -0.8
> 24.89    -0.2    -0.6
> 28.0    -0.15    -0.5
>
> I'd be interested in any comments people may have on the accuracy of
> my assumptions and calculations in the document.
>
> Alan N1AL

David Gilbert-2

Re: Efficiency of MFJ remotely-tuned loop antennas

In reply to this post by wayne burdick

72% sounds way high to me, and I also think it's a big leap in logic to
assume that MFJ quality (materials, connections, etc) is similar to any
other reputable company anyway.

Dave AB7E

On 1/18/2021 3:39 PM, Wayne Burdick wrote:

> Hi Alan,
>
> 72% sounds a bit high. Is this number based on loop size alone ("in theory")? Or are they taking conductor geometry and other losses into account?
>
> Wayne
> N6KR
>
>
>> On Jan 18, 2021, at 2:05 PM, Alan Bloom <[hidden email]> wrote:
>>
>> MFJ makes a pair of small, remotely-tuned loop antennas, the MFJ-1786 that covers 10-30 MHz and the MFJ-1788 that covers 7 to 21+ MHz. As far as I can tell, the two antennas are identical except for the size of the tuning capacitor. Each consists of a 3 foot (91 cm) diameter loop made of aluminum tubing and a plastic housing that contains the tuning capacitor, motor, and coupling loop. No control cable is required since the control voltage is sent from the control box in the shack to the motor in the antenna via the coaxial cable.
>>
>> Before I purchase one of these I wanted to get an idea of the efficiency of such a small loop. MFJ is silent on the subject so I did my own calculations. The calculations and results are on a 1-page document that I uploaded to Dropbox and can be downloaded here:
>>
>> https://www.dropbox.com/s/l8mv67cjrck2ssn/MFJ-1786-1788.pdf?dl=0
>>
>> My calculations are based on the assumption that the efficiency of the MFJ antennas is similar to the (no longer manufactured) AEA Isoloop (my reasoning for that is in the document) and that AEA's specification of 72% efficiency at 14 MHz is correct. From that number I can calculate the efficiency and gain on all the other bands.
>>
>> If you don't want to download the document, here is a summary of the results:
>>
>> Freq Eff Gain with respect to a half-wave dipole
>> MHz dB dBd
>> 7.0 -7.3 -7.7
>> 10.1 -3.5 -3.9
>> 14.0 -1.4 -1.8
>> 18.068 -0.6 -1.0
>> 21.0 -0.4 -0.8
>> 24.89 -0.2 -0.6
>> 28.0 -0.15 -0.5
>>
>> I'd be interested in any comments people may have on the accuracy of
>> my assumptions and calculations in the document.
>>
>> Alan N1AL
>> ______________________________________________________________
>> Elecraft mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:[hidden email]
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>> Message delivered to [hidden email]
> ______________________________________________________________
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>
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Alan Bloom

Re: Efficiency of MFJ remotely-tuned loop antennas

In reply to this post by wayne burdick

Well let's see...

Radiation resistance of a small loop is 31,171 * (Area / wavelength^2)^2

For a loop with a 91cm diameter at 14 MHz, I believe that comes out to
0.064 ohms.

Assuming the loss is due to the RF resistance of the loop:

From the internet I get the volume resistivity and skin depth for 6063
aluminum is 0.03 microohms-meter and 23.3 micrometers respectively, so
the surface resistivity is 0.03/23.3 = 0.0013 ohms per square. The
outside circumference of the tubing is PI * 1.05" = 3.3" and the loop
length is PI * 36" = 113" so the loss resistance is .0013 * 113/3.3 =
0.045 ohms.

So I calculate an efficiency of 0.064 / (0.064 + 0.045) = 59%

So worse than AEA claimed, but in the ballpark.

Alan N1AL

On 1/18/2021 3:39 PM, Wayne Burdick wrote:

NK7Z

Re: Efficiency of MFJ remotely-tuned loop antennas

In reply to this post by Alan Bloom

Thank you for sharing that... I have saved it to my system.

73, and thanks,
Dave (NK7Z)
https://www.nk7z.net
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources

On 1/18/21 2:05 PM, Alan Bloom wrote:

> MFJ makes a pair of small, remotely-tuned loop antennas, the MFJ-1786
> that covers 10-30 MHz and the MFJ-1788 that covers 7 to 21+ MHz. As far
> as I can tell, the two antennas are identical except for the size of the
> tuning capacitor. Each consists of a 3 foot (91 cm) diameter loop made
> of aluminum tubing and a plastic housing that contains the tuning
> capacitor, motor, and coupling loop. No control cable is required since
> the control voltage is sent from the control box in the shack to the
> motor in the antenna via the coaxial cable.
>
> Before I purchase one of these I wanted to get an idea of the efficiency
> of such a small loop. MFJ is silent on the subject so I did my own
> calculations. The calculations and results are on a 1-page document
> that I uploaded to Dropbox and can be downloaded here:
>
> https://www.dropbox.com/s/l8mv67cjrck2ssn/MFJ-1786-1788.pdf?dl=0
>
> My calculations are based on the assumption that the efficiency of the
> MFJ antennas is similar to the (no longer manufactured) AEA Isoloop (my
> reasoning for that is in the document) and that AEA's specification of
> 72% efficiency at 14 MHz is correct. From that number I can calculate
> the efficiency and gain on all the other bands.
>
> If you don't want to download the document, here is a summary of the
> results:
>
> Freq    Eff    Gain with respect to a half-wave dipole
> MHz    dB    dBd
> 7.0    -7.3    -7.7
> 10.1    -3.5    -3.9
> 14.0    -1.4    -1.8
> 18.068    -0.6    -1.0
> 21.0    -0.4    -0.8
> 24.89    -0.2    -0.6
> 28.0    -0.15    -0.5
>
> I'd be interested in any comments people may have on the accuracy of
> my assumptions and calculations in the document.
>
> Alan N1AL
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

David Gilbert-2

Re: Efficiency of MFJ remotely-tuned loop antennas

In reply to this post by Alan Bloom

You are neglecting the losses in various connections in the system ...
including possibly the construction of the capacitor itself. I don't
believe that they are insignificant. There is a reason why top quality
variable capacitors often use welded plates.

I would also guess that contact resistance is worse for dissimilar
materials, such as a copper wire to an aluminum tube.

Yours is a limited theoretical analysis ... not a practical one.

Dave AB7E

On 1/18/2021 5:38 PM, Alan Bloom wrote:

> Well let's see...
>
> Radiation resistance of a small loop is 31,171 * (Area / wavelength^2)^2
>
> For a loop with a 91cm diameter at 14 MHz, I believe that comes out to
> 0.064 ohms.
>
> Assuming the loss is due to the RF resistance of the loop:
>
> From the internet I get the volume resistivity and skin depth for 6063
> aluminum is 0.03 microohms-meter and 23.3 micrometers respectively, so
> the surface resistivity is 0.03/23.3 = 0.0013 ohms per square. The
> outside circumference of the tubing is PI * 1.05" = 3.3" and the loop
> length is PI * 36" = 113" so the loss resistance is .0013 * 113/3.3 =
> 0.045 ohms.
>
> So I calculate an efficiency of 0.064 / (0.064 + 0.045) = 59%
>
> So worse than AEA claimed, but in the ballpark.
>
> Alan N1AL
>
>
>
>
> On 1/18/2021 3:39 PM, Wayne Burdick wrote:
>> Hi Alan,
>>
>> 72% sounds a bit high. Is this number based on loop size alone ("in
>> theory")? Or are they taking conductor geometry and other losses into
>> account?
>>
>> Wayne
>> N6KR
>>
>>
>>> On Jan 18, 2021, at 2:05 PM, Alan Bloom <[hidden email]> wrote:
>>>
>>> MFJ makes a pair of small, remotely-tuned loop antennas, the
>>> MFJ-1786 that covers 10-30 MHz and the MFJ-1788 that covers 7 to 21+
>>> MHz. As far as I can tell, the two antennas are identical except
>>> for the size of the tuning capacitor. Each consists of a 3 foot (91
>>> cm) diameter loop made of aluminum tubing and a plastic housing that
>>> contains the tuning capacitor, motor, and coupling loop. No control
>>> cable is required since the control voltage is sent from the control
>>> box in the shack to the motor in the antenna via the coaxial cable.
>>>
>>> Before I purchase one of these I wanted to get an idea of the
>>> efficiency of such a small loop. MFJ is silent on the subject so I
>>> did my own calculations. The calculations and results are on a
>>> 1-page document that I uploaded to Dropbox and can be downloaded here:
>>>
>>> https://www.dropbox.com/s/l8mv67cjrck2ssn/MFJ-1786-1788.pdf?dl=0
>>>
>>> My calculations are based on the assumption that the efficiency of
>>> the MFJ antennas is similar to the (no longer manufactured) AEA
>>> Isoloop (my reasoning for that is in the document) and that AEA's
>>> specification of 72% efficiency at 14 MHz is correct. From that
>>> number I can calculate the efficiency and gain on all the other bands.
>>>
>>> If you don't want to download the document, here is a summary of the
>>> results:
>>>
>>> Freq    Eff    Gain with respect to a half-wave dipole
>>> MHz    dB    dBd
>>> 7.0    -7.3    -7.7
>>> 10.1    -3.5    -3.9
>>> 14.0    -1.4    -1.8
>>> 18.068    -0.6    -1.0
>>> 21.0    -0.4    -0.8
>>> 24.89    -0.2    -0.6
>>> 28.0    -0.15    -0.5
>>>
>>> I'd be interested in any comments people may have on the accuracy of
>>> my assumptions and calculations in the document.
>>>
>>> Alan N1AL
>>> ______________________________________________________________
>>> Elecraft mailing list
>>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>>> Help: http://mailman.qth.net/mmfaq.htm
>>> Post: mailto:[hidden email]
>>>
>>> This list hosted by: http://www.qsl.net
>>> Please help support this email list: http://www.qsl.net/donate.html
>>> Message delivered to [hidden email]
>>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

Alan Bloom

Re: Efficiency of MFJ remotely-tuned loop antennas

> There is a reason why top quality variable capacitors often use
welded plates.

I believe they do weld the capacitor plates and also weld the loop to
the capacitor. (I don't have one, but that's what I've read.)

> Yours is a limited theoretical analysis ... not a practical one.

A number of reviews I have read (including the QST review of August
1994) have reported comparable performance to full-sized wire antennas
located on the same site. If the loop is down by, say, 3 dB, that's
only half an S unit, which would hardly be noticeable in the QSB of a
typical amateur band.

As I see it, the advantages of the MFJ-1786 10-30 MHz loop are:

- Continuous coverage on 6 amateur bands. A convenient way to cover all
the WARC bands.
- Small and light.
- Omni-directional (when mounted horizontally) so does not need a rotor.
- No control cable required - control voltage is fed through the coax.
- Narrow bandwidth provides excellent RF selectivity. Might be good on
Field Day to reduce inter-station QRM.
- Users have reported lower receiver noise compared to wire antennas.
No doubt that is because the isolated pickup loop prevents feedline
radiation/pickup.

And the disadvantages:

- Expensive ($500 list price)
- Less gain than a simple dipole (although you would theoretically need
6 of them).
- Fiddly to tune. If you QSY too far you have to re-tune.
- MFJ quality control leaves something to be desired. (You may have to
open it up when you get it and make minor repairs.)
- You have to pay attention to the problem of entry of water and/or bugs
into the housing.
- The controller can be damaged by a DC short in the coax e.g. from an
shorting-type antenna switch. (I don't understand why MFJ didn't
include a fuse or some other way to protect the controller.)

I probably wouldn't buy the 7-21 MHz MFJ-1788 because of the poor
efficiency at 7 MHz. I think you'd have a better signal just using the
coax as a random end-fed wire (with a tuner).

Alan N1AL

On 1/18/2021 8:17 PM, David Gilbert wrote:

>
> You are neglecting the losses in various connections in the system ...
> including possibly the construction of the capacitor itself. I don't
> believe that they are insignificant. There is a reason why top
> quality variable capacitors often use welded plates.
>
> I would also guess that contact resistance is worse for dissimilar
> materials, such as a copper wire to an aluminum tube.
>
> Yours is a limited theoretical analysis ... not a practical one.
>
> Dave   AB7E
>
>
>
> On 1/18/2021 5:38 PM, Alan Bloom wrote:
>> Well let's see...
>>
>> Radiation resistance of a small loop is 31,171 * (Area / wavelength^2)^2
>>
>> For a loop with a 91cm diameter at 14 MHz, I believe that comes out
>> to 0.064 ohms.
>>
>> Assuming the loss is due to the RF resistance of the loop:
>>
>> From the internet I get the volume resistivity and skin depth for
>> 6063 aluminum is 0.03 microohms-meter and 23.3 micrometers
>> respectively, so the surface resistivity is 0.03/23.3 = 0.0013 ohms
>> per square. The outside circumference of the tubing is PI * 1.05" =
>> 3.3" and the loop length is PI * 36" = 113" so the loss resistance is
>> .0013 * 113/3.3 = 0.045 ohms.
>>
>> So I calculate an efficiency of 0.064 / (0.064 + 0.045) = 59%
>>
>> So worse than AEA claimed, but in the ballpark.
>>
>> Alan N1AL
>>
>>
>>
>>
>> On 1/18/2021 3:39 PM, Wayne Burdick wrote:
>>> Hi Alan,
>>>
>>> 72% sounds a bit high. Is this number based on loop size alone ("in
>>> theory")? Or are they taking conductor geometry and other losses
>>> into account?
>>>
>>> Wayne
>>> N6KR
>>>
>>>
>>>> On Jan 18, 2021, at 2:05 PM, Alan Bloom <[hidden email]> wrote:
>>>>
>>>> MFJ makes a pair of small, remotely-tuned loop antennas, the
>>>> MFJ-1786 that covers 10-30 MHz and the MFJ-1788 that covers 7 to
>>>> 21+ MHz. As far as I can tell, the two antennas are identical
>>>> except for the size of the tuning capacitor. Each consists of a 3
>>>> foot (91 cm) diameter loop made of aluminum tubing and a plastic
>>>> housing that contains the tuning capacitor, motor, and coupling
>>>> loop. No control cable is required since the control voltage is
>>>> sent from the control box in the shack to the motor in the antenna
>>>> via the coaxial cable.
>>>>
>>>> Before I purchase one of these I wanted to get an idea of the
>>>> efficiency of such a small loop. MFJ is silent on the subject so I
>>>> did my own calculations. The calculations and results are on a
>>>> 1-page document that I uploaded to Dropbox and can be downloaded here:
>>>>
>>>> https://www.dropbox.com/s/l8mv67cjrck2ssn/MFJ-1786-1788.pdf?dl=0
>>>>
>>>> My calculations are based on the assumption that the efficiency of
>>>> the MFJ antennas is similar to the (no longer manufactured) AEA
>>>> Isoloop (my reasoning for that is in the document) and that AEA's
>>>> specification of 72% efficiency at 14 MHz is correct. From that
>>>> number I can calculate the efficiency and gain on all the other bands.
>>>>
>>>> If you don't want to download the document, here is a summary of
>>>> the results:
>>>>
>>>> Freq    Eff    Gain with respect to a half-wave dipole
>>>> MHz    dB    dBd
>>>> 7.0    -7.3    -7.7
>>>> 10.1    -3.5    -3.9
>>>> 14.0    -1.4    -1.8
>>>> 18.068    -0.6    -1.0
>>>> 21.0    -0.4    -0.8
>>>> 24.89    -0.2    -0.6
>>>> 28.0    -0.15    -0.5
>>>>
>>>> I'd be interested in any comments people may have on the accuracy of
>>>> my assumptions and calculations in the document.
>>>>
>>>> Alan N1AL
>>>> ______________________________________________________________
>>>> Elecraft mailing list
>>>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>>>> Help: http://mailman.qth.net/mmfaq.htm
>>>> Post: mailto:[hidden email]
>>>>
>>>> This list hosted by: http://www.qsl.net
>>>> Please help support this email list: http://www.qsl.net/donate.html
>>>> Message delivered to [hidden email]
>>>
>>
>> ______________________________________________________________
>> Elecraft mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:[hidden email]
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>> Message delivered to [hidden email]
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

David Gilbert-2

Re: Efficiency of MFJ remotely-tuned loop antennas

According to another ham who recently posted here, he had to "tighten"
the plates on the MFJ capacitor to get it to work properly. That
doesn't sound like they are welded, and given the cost difference for
welded air variables I doubt MFJ used them.

I sincerely doubt that an actual practical small loop is only down 3 dB
from a full size antenna. That makes no sense to me at all. If that
were the case everyone would be using one, because they are not that
difficult to make ... at least for manually tuned ones.

But you seem determined to believe differently, and it's not my place to
convince you otherwise. You asked for inputs and I have made mine.
Hopefully you are right and I am wrong.

73,
Dave AB7E

On 1/18/2021 9:54 PM, Alan Bloom wrote:

> > There is a reason why top quality variable capacitors often use
> welded plates.
>
> I believe they do weld the capacitor plates and also weld the loop to
> the capacitor. (I don't have one, but that's what I've read.)
>
> > Yours is a limited theoretical analysis ... not a practical one.
>
> A number of reviews I have read (including the QST review of August
> 1994) have reported comparable performance to full-sized wire antennas
> located on the same site. If the loop is down by, say, 3 dB, that's
> only half an S unit, which would hardly be noticeable in the QSB of a
> typical amateur band.
>
>
> As I see it, the advantages of the MFJ-1786 10-30 MHz loop are:
>
> - Continuous coverage on 6 amateur bands. A convenient way to cover
> all the WARC bands.
> - Small and light.
> - Omni-directional (when mounted horizontally) so does not need a rotor.
> - No control cable required - control voltage is fed through the coax.
> - Narrow bandwidth provides excellent RF selectivity. Might be good
> on Field Day to reduce inter-station QRM.
> - Users have reported lower receiver noise compared to wire antennas.
> No doubt that is because the isolated pickup loop prevents feedline
> radiation/pickup.
>
> And the disadvantages:
>
> - Expensive ($500 list price)
> - Less gain than a simple dipole (although you would theoretically
> need 6 of them).
> - Fiddly to tune. If you QSY too far you have to re-tune.
> - MFJ quality control leaves something to be desired. (You may have
> to open it up when you get it and make minor repairs.)
> - You have to pay attention to the problem of entry of water and/or
> bugs into the housing.
> - The controller can be damaged by a DC short in the coax e.g. from an
> shorting-type antenna switch. (I don't understand why MFJ didn't
> include a fuse or some other way to protect the controller.)
>
> I probably wouldn't buy the 7-21 MHz MFJ-1788 because of the poor
> efficiency at 7 MHz. I think you'd have a better signal just using
> the coax as a random end-fed wire (with a tuner).
>
> Alan N1AL
>
>
> On 1/18/2021 8:17 PM, David Gilbert wrote:
>>
>> You are neglecting the losses in various connections in the system
>> ... including possibly the construction of the capacitor itself. I
>> don't believe that they are insignificant. There is a reason why top
>> quality variable capacitors often use welded plates.
>>
>> I would also guess that contact resistance is worse for dissimilar
>> materials, such as a copper wire to an aluminum tube.
>>
>> Yours is a limited theoretical analysis ... not a practical one.
>>
>> Dave   AB7E
>>
>>
>>
>> On 1/18/2021 5:38 PM, Alan Bloom wrote:
>>> Well let's see...
>>>
>>> Radiation resistance of a small loop is 31,171 * (Area /
>>> wavelength^2)^2
>>>
>>> For a loop with a 91cm diameter at 14 MHz, I believe that comes out
>>> to 0.064 ohms.
>>>
>>> Assuming the loss is due to the RF resistance of the loop:
>>>
>>> From the internet I get the volume resistivity and skin depth for
>>> 6063 aluminum is 0.03 microohms-meter and 23.3 micrometers
>>> respectively, so the surface resistivity is 0.03/23.3 = 0.0013 ohms
>>> per square. The outside circumference of the tubing is PI * 1.05" =
>>> 3.3" and the loop length is PI * 36" = 113" so the loss resistance
>>> is .0013 * 113/3.3 = 0.045 ohms.
>>>
>>> So I calculate an efficiency of 0.064 / (0.064 + 0.045) = 59%
>>>
>>> So worse than AEA claimed, but in the ballpark.
>>>
>>> Alan N1AL
>>>
>>>
>>>
>>>
>>> On 1/18/2021 3:39 PM, Wayne Burdick wrote:
>>>> Hi Alan,
>>>>
>>>> 72% sounds a bit high. Is this number based on loop size alone ("in
>>>> theory")? Or are they taking conductor geometry and other losses
>>>> into account?
>>>>
>>>> Wayne
>>>> N6KR
>>>>
>>>>
>>>>> On Jan 18, 2021, at 2:05 PM, Alan Bloom <[hidden email]> wrote:
>>>>>
>>>>> MFJ makes a pair of small, remotely-tuned loop antennas, the
>>>>> MFJ-1786 that covers 10-30 MHz and the MFJ-1788 that covers 7 to
>>>>> 21+ MHz. As far as I can tell, the two antennas are identical
>>>>> except for the size of the tuning capacitor. Each consists of a 3
>>>>> foot (91 cm) diameter loop made of aluminum tubing and a plastic
>>>>> housing that contains the tuning capacitor, motor, and coupling
>>>>> loop. No control cable is required since the control voltage is
>>>>> sent from the control box in the shack to the motor in the antenna
>>>>> via the coaxial cable.
>>>>>
>>>>> Before I purchase one of these I wanted to get an idea of the
>>>>> efficiency of such a small loop. MFJ is silent on the subject so
>>>>> I did my own calculations. The calculations and results are on a
>>>>> 1-page document that I uploaded to Dropbox and can be downloaded
>>>>> here:
>>>>>
>>>>> https://www.dropbox.com/s/l8mv67cjrck2ssn/MFJ-1786-1788.pdf?dl=0
>>>>>
>>>>> My calculations are based on the assumption that the efficiency of
>>>>> the MFJ antennas is similar to the (no longer manufactured) AEA
>>>>> Isoloop (my reasoning for that is in the document) and that AEA's
>>>>> specification of 72% efficiency at 14 MHz is correct. From that
>>>>> number I can calculate the efficiency and gain on all the other
>>>>> bands.
>>>>>
>>>>> If you don't want to download the document, here is a summary of
>>>>> the results:
>>>>>
>>>>> Freq    Eff    Gain with respect to a half-wave dipole
>>>>> MHz    dB    dBd
>>>>> 7.0    -7.3    -7.7
>>>>> 10.1    -3.5    -3.9
>>>>> 14.0    -1.4    -1.8
>>>>> 18.068    -0.6    -1.0
>>>>> 21.0    -0.4    -0.8
>>>>> 24.89    -0.2    -0.6
>>>>> 28.0    -0.15    -0.5
>>>>>
>>>>> I'd be interested in any comments people may have on the accuracy of
>>>>> my assumptions and calculations in the document.
>>>>>
>>>>> Alan N1AL
>>>>> ______________________________________________________________
>>>>> Elecraft mailing list
>>>>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>>>>> Help: http://mailman.qth.net/mmfaq.htm
>>>>> Post: mailto:[hidden email]
>>>>>
>>>>> This list hosted by: http://www.qsl.net
>>>>> Please help support this email list: http://www.qsl.net/donate.html
>>>>> Message delivered to [hidden email]
>>>>
>>>
>>> ______________________________________________________________
>>> Elecraft mailing list
>>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>>> Help: http://mailman.qth.net/mmfaq.htm
>>> Post: mailto:[hidden email]
>>>
>>> This list hosted by: http://www.qsl.net
>>> Please help support this email list: http://www.qsl.net/donate.html
>>> Message delivered to [hidden email]
>>
>> ______________________________________________________________
>> Elecraft mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:[hidden email]
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>> Message delivered to [hidden email]
>

Alan Bloom

Re: Efficiency of MFJ remotely-tuned loop antennas

> That doesn't sound like they are welded, and given the cost
difference for welded air variables I doubt MFJ used them.

As I said, I don't have one so I can't say for sure. I got my
information from the MFJ web site: "All welded construction, no
mechanical joints, welded butterfly capacitor with no rotating contacts
... Each plate in MFJ's tuning capacitor is welded for low loss and
polished to prevent high voltage arcing, welded to the radiator ...".
https://mfjenterprises.com/products/mfj-1786

Also, for what it's worth, some of the reviews on eham.com and qrz.com
mention that it has a welded tuning capacitor.

I got the impression that one reason people often receive units with
bent capacitor plates is that they got bent in the welding process.

It would be interesting to look at one and see what they actually mean
by "welded".

Alan N1AL

On 1/18/2021 10:10 PM, David Gilbert wrote:

>
> According to another ham who recently posted here, he had to "tighten"
> the plates on the MFJ capacitor to get it to work properly. That
> doesn't sound like they are welded, and given the cost difference for
> welded air variables I doubt MFJ used them.
>
> I sincerely doubt that an actual practical small loop is only down 3
> dB from a full size antenna. That makes no sense to me at all. If
> that were the case everyone would be using one, because they are not
> that difficult to make ... at least for manually tuned ones.
>
> But you seem determined to believe differently, and it's not my place
> to convince you otherwise. You asked for inputs and I have made
> mine. Hopefully you are right and I am wrong.
>
> 73,
> Dave   AB7E
>
>
>
> On 1/18/2021 9:54 PM, Alan Bloom wrote:
>> > There is a reason why top quality variable capacitors often use
>> welded plates.
>>
>> I believe they do weld the capacitor plates and also weld the loop to
>> the capacitor. (I don't have one, but that's what I've read.)
>>
>> > Yours is a limited theoretical analysis ... not a practical one.
>>
>> A number of reviews I have read (including the QST review of August
>> 1994) have reported comparable performance to full-sized wire
>> antennas located on the same site. If the loop is down by, say, 3
>> dB, that's only half an S unit, which would hardly be noticeable in
>> the QSB of a typical amateur band.
>>
>>
>> As I see it, the advantages of the MFJ-1786 10-30 MHz loop are:
>>
>> - Continuous coverage on 6 amateur bands. A convenient way to cover
>> all the WARC bands.
>> - Small and light.
>> - Omni-directional (when mounted horizontally) so does not need a
>> rotor.
>> - No control cable required - control voltage is fed through the coax.
>> - Narrow bandwidth provides excellent RF selectivity. Might be good
>> on Field Day to reduce inter-station QRM.
>> - Users have reported lower receiver noise compared to wire
>> antennas. No doubt that is because the isolated pickup loop prevents
>> feedline radiation/pickup.
>>
>> And the disadvantages:
>>
>> - Expensive ($500 list price)
>> - Less gain than a simple dipole (although you would theoretically
>> need 6 of them).
>> - Fiddly to tune. If you QSY too far you have to re-tune.
>> - MFJ quality control leaves something to be desired. (You may have
>> to open it up when you get it and make minor repairs.)
>> - You have to pay attention to the problem of entry of water and/or
>> bugs into the housing.
>> - The controller can be damaged by a DC short in the coax e.g. from
>> an shorting-type antenna switch. (I don't understand why MFJ didn't
>> include a fuse or some other way to protect the controller.)
>>
>> I probably wouldn't buy the 7-21 MHz MFJ-1788 because of the poor
>> efficiency at 7 MHz. I think you'd have a better signal just using
>> the coax as a random end-fed wire (with a tuner).
>>
>> Alan N1AL
>>
>>
>> On 1/18/2021 8:17 PM, David Gilbert wrote:
>>>
>>> You are neglecting the losses in various connections in the system
>>> ... including possibly the construction of the capacitor itself. I
>>> don't believe that they are insignificant. There is a reason why
>>> top quality variable capacitors often use welded plates.
>>>
>>> I would also guess that contact resistance is worse for dissimilar
>>> materials, such as a copper wire to an aluminum tube.
>>>
>>> Yours is a limited theoretical analysis ... not a practical one.
>>>
>>> Dave   AB7E
>>>
>>>
>>>
>>> On 1/18/2021 5:38 PM, Alan Bloom wrote:
>>>> Well let's see...
>>>>
>>>> Radiation resistance of a small loop is 31,171 * (Area /
>>>> wavelength^2)^2
>>>>
>>>> For a loop with a 91cm diameter at 14 MHz, I believe that comes out
>>>> to 0.064 ohms.
>>>>
>>>> Assuming the loss is due to the RF resistance of the loop:
>>>>
>>>> From the internet I get the volume resistivity and skin depth for
>>>> 6063 aluminum is 0.03 microohms-meter and 23.3 micrometers
>>>> respectively, so the surface resistivity is 0.03/23.3 = 0.0013 ohms
>>>> per square. The outside circumference of the tubing is PI * 1.05"
>>>> = 3.3" and the loop length is PI * 36" = 113" so the loss
>>>> resistance is .0013 * 113/3.3 = 0.045 ohms.
>>>>
>>>> So I calculate an efficiency of 0.064 / (0.064 + 0.045) = 59%
>>>>
>>>> So worse than AEA claimed, but in the ballpark.
>>>>
>>>> Alan N1AL
>>>>
>>>>
>>>>
>>>>
>>>> On 1/18/2021 3:39 PM, Wayne Burdick wrote:
>>>>> Hi Alan,
>>>>>
>>>>> 72% sounds a bit high. Is this number based on loop size alone
>>>>> ("in theory")? Or are they taking conductor geometry and other
>>>>> losses into account?
>>>>>
>>>>> Wayne
>>>>> N6KR
>>>>>
>>>>>
>>>>>> On Jan 18, 2021, at 2:05 PM, Alan Bloom <[hidden email]> wrote:
>>>>>>
>>>>>> MFJ makes a pair of small, remotely-tuned loop antennas, the
>>>>>> MFJ-1786 that covers 10-30 MHz and the MFJ-1788 that covers 7 to
>>>>>> 21+ MHz. As far as I can tell, the two antennas are identical
>>>>>> except for the size of the tuning capacitor. Each consists of a
>>>>>> 3 foot (91 cm) diameter loop made of aluminum tubing and a
>>>>>> plastic housing that contains the tuning capacitor, motor, and
>>>>>> coupling loop. No control cable is required since the control
>>>>>> voltage is sent from the control box in the shack to the motor in
>>>>>> the antenna via the coaxial cable.
>>>>>>
>>>>>> Before I purchase one of these I wanted to get an idea of the
>>>>>> efficiency of such a small loop. MFJ is silent on the subject so
>>>>>> I did my own calculations. The calculations and results are on a
>>>>>> 1-page document that I uploaded to Dropbox and can be downloaded
>>>>>> here:
>>>>>>
>>>>>> https://www.dropbox.com/s/l8mv67cjrck2ssn/MFJ-1786-1788.pdf?dl=0
>>>>>>
>>>>>> My calculations are based on the assumption that the efficiency
>>>>>> of the MFJ antennas is similar to the (no longer manufactured)
>>>>>> AEA Isoloop (my reasoning for that is in the document) and that
>>>>>> AEA's specification of 72% efficiency at 14 MHz is correct. From
>>>>>> that number I can calculate the efficiency and gain on all the
>>>>>> other bands.
>>>>>>
>>>>>> If you don't want to download the document, here is a summary of
>>>>>> the results:
>>>>>>
>>>>>> Freq    Eff    Gain with respect to a half-wave dipole
>>>>>> MHz    dB    dBd
>>>>>> 7.0    -7.3    -7.7
>>>>>> 10.1    -3.5    -3.9
>>>>>> 14.0    -1.4    -1.8
>>>>>> 18.068    -0.6    -1.0
>>>>>> 21.0    -0.4    -0.8
>>>>>> 24.89    -0.2    -0.6
>>>>>> 28.0    -0.15    -0.5
>>>>>>
>>>>>> I'd be interested in any comments people may have on the accuracy of
>>>>>> my assumptions and calculations in the document.
>>>>>>
>>>>>> Alan N1AL
>>>>>> ______________________________________________________________
>>>>>> Elecraft mailing list
>>>>>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>>>>>> Help: http://mailman.qth.net/mmfaq.htm
>>>>>> Post: mailto:[hidden email]
>>>>>>
>>>>>> This list hosted by: http://www.qsl.net
>>>>>> Please help support this email list: http://www.qsl.net/donate.html
>>>>>> Message delivered to [hidden email]
>>>>>
>>>>
>>>> ______________________________________________________________
>>>> Elecraft mailing list
>>>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>>>> Help: http://mailman.qth.net/mmfaq.htm
>>>> Post: mailto:[hidden email]
>>>>
>>>> This list hosted by: http://www.qsl.net
>>>> Please help support this email list: http://www.qsl.net/donate.html
>>>> Message delivered to [hidden email]
>>>
>>> ______________________________________________________________
>>> Elecraft mailing list
>>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>>> Help: http://mailman.qth.net/mmfaq.htm
>>> Post: mailto:[hidden email]
>>>
>>> This list hosted by: http://www.qsl.net
>>> Please help support this email list: http://www.qsl.net/donate.html
>>> Message delivered to [hidden email]
>>
>

Warren Merkel

Re: Efficiency of MFJ remotely-tuned loop antennas

In reply to this post by Alan Bloom

Resistive Losses around the loop are the biggest factor limiting
efficiency.

My friend, Paul Casper, K4HKX has done very extensive investigation and
documentation of Mag Loop performance. His loops exhibit excellence in
both Art and Science. I'd recommend taking in his findings posted on
his QRZ page.

https://www.qrz.com/db/k4hkx

You might get distracted looking at his other antenna creations. I
wrote the code running in the custom antenna control head.

The Mag Loops start way down at Section 7. The progression of loop
designs is staggering. You might find his HF mobile loop quite
interesting too. He uses a KX3 + KXPA100 with his mobile loop.

Warren Merkel, KD4Z

On 1/19/2021 12:17 AM, [hidden email] wrote:

> Date: Mon, 18 Jan 2021 20:17:38 -0700
> From: David Gilbert<[hidden email]>
> To:[hidden email]
> Subject: Re: [Elecraft] Efficiency of MFJ remotely-tuned loop antennas
> Message-ID:<[hidden email]>
> Content-Type: text/plain; charset=utf-8; format=flowed
>
>
> You are neglecting the losses in various connections in the system ...
> including possibly the construction of the capacitor itself.? I don't
> believe that they are insignificant.? There is a reason why top quality
> variable capacitors often use welded plates.
>
> I would also guess that contact resistance is worse for dissimilar
> materials, such as a copper wire to an aluminum tube.
>
> Yours is a limited theoretical analysis ... not a practical one.
>
> Dave?? AB7E

Brian D

Re: Efficiency of MFJ remotely-tuned loop antennas

In reply to this post by David Gilbert-2

I got a MFJ 10-30m loop at a SK sale. The original owner had never got it to
work. The rotor vanes had fallen apart. The fixed vanes were welded to the
loop. After re-assembly it worked well, but was difficult to tune as I
couldn't hear the buzzer for finding the tune point, it was too high in
audio frequency for my old ears, cured by adding a capacitor to the circuit.

David Gilbert <[hidden email]> wrote:

> According to another ham who recently posted here, he had to "tighten" the
> plates on the MFJ capacitor to get it to work properly. That doesn't
> sound like they are welded, and given the cost difference for welded air
> variables I doubt MFJ used them.
>

--
Brian D
G3VGZ Yarm England
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]

k6dgw

Re: Efficiency of MFJ remotely-tuned loop antennas

In reply to this post by Alan Bloom

The plates in the two air variables in the [now ancient] ARC-5 command
TX from WW2 were very securely welded in place to the rotor axle. We'd
remove many of the rotor plates and re-pad it with a fixed capacitor to
spread out the ham band on the dial. It took a lot of bending back and
forth to break the welds. Lower power loops often use butterfly caps
since there is no resistive loss through the rotor connection. Higher
powered loops often use vacuum variables because of the high voltages,
although I saw a design not long ago that used two coaxial copper pipes
with a PVC pipe as the insulator. The inside conductor was mounted to a
long threaded rod moving it in and out of the outer conductor.

In your list of pros, you might note that while the bi-directional
primary lobes of the loop when mounted vertically are very broad, the
null perpendicular to the plane of the loop is extremely narrow and
deep. You can use it to null out noise or even another signal without
sacrificing much of anything in the forward direction.

My Alexloop works ok on 30, poorly on 40, and really seems to come into
its own on 20 and up.

73,

Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County

On 1/18/2021 8:54 PM, Alan Bloom wrote:

Alan Bloom

Re: Efficiency of MFJ remotely-tuned loop antennas

In reply to this post by Alan Bloom

I've been convinced that AEA's specification of 72% efficiency for the
Isoloop at 14 MHz is too high, certainly too high for the MFJ antennas.
So I re-did the calculations using the 59% efficiency figure calculated
below. The new results can be downloaded here:

https://www.dropbox.com/s/ve1v49b3gjvmt64/MFJ-1786-1788_2.pdf?dl=0

If you don't want to download the (1-page) document with the pretty
graph, here's a synopsis of the results:

Freq Eff.     Gain with respect to a 1/2-wavelength dipole
MHz      dB      dBd
7.0       -9.5    -9.9
10.1   -5.1    -5.5
14.0   -2.3    -2.7
18.068 -1.1    -1.5
21.0       -0.7    -1.1
24.89     -0.4    -.8
28.0       -0.26 -0.65

My basic conclusions still stand. With almost minus 10 dBd of gain on 7
MHz, the 40 meter coverage of the MFJ-1788 doesn't seem very useful.
That is confirmed by some of the reviews I have seen. I think you'd get
better results by just loading up the coax feedline as a random-wire
antenna with a tuner.

The 10 MHz performance is a little better. Good enough to at least
allow you to get on the 30 meter band.

On the higher bands, the gain is within less than 3 dB of a full-sized
dipole, which seems a useful trade-off for its small size and wide-band
continuous coverage.

Disclaimer: Again, I have never seen one of these things so this is all
based on theory and on the many reviews I have read. Even if my figures
are off a bit, at least this gives an idea of the relative performance
on the various bands.

Alan N1AL

On 1/18/2021 5:38 PM, Alan Bloom wrote:

> Well let's see...
>
> Radiation resistance of a small loop is 31,171 * (Area / wavelength^2)^2
>
> For a loop with a 91cm diameter at 14 MHz, I believe that comes out to
> 0.064 ohms.
>
> Assuming the loss is due to the RF resistance of the loop:
>
> From the internet I get the volume resistivity and skin depth for 6063
> aluminum is 0.03 microohms-meter and 23.3 micrometers respectively, so
> the surface resistivity is 0.03/23.3 = 0.0013 ohms per square. The
> outside circumference of the tubing is PI * 1.05" = 3.3" and the loop
> length is PI * 36" = 113" so the loss resistance is .0013 * 113/3.3 =
> 0.045 ohms.
>
> So I calculate an efficiency of 0.064 / (0.064 + 0.045) = 59%
>
> So worse than AEA claimed, but in the ballpark.
>
> Alan N1AL
>
>
>
>
> On 1/18/2021 3:39 PM, Wayne Burdick wrote:
>> Hi Alan,
>>
>> 72% sounds a bit high. Is this number based on loop size alone ("in
>> theory")? Or are they taking conductor geometry and other losses into
>> account?
>>
>> Wayne
>> N6KR
>>
>>
>>> On Jan 18, 2021, at 2:05 PM, Alan Bloom <[hidden email]> wrote:
>>>
>>> MFJ makes a pair of small, remotely-tuned loop antennas, the
>>> MFJ-1786 that covers 10-30 MHz and the MFJ-1788 that covers 7 to 21+
>>> MHz. As far as I can tell, the two antennas are identical except
>>> for the size of the tuning capacitor. Each consists of a 3 foot (91
>>> cm) diameter loop made of aluminum tubing and a plastic housing that
>>> contains the tuning capacitor, motor, and coupling loop. No control
>>> cable is required since the control voltage is sent from the control
>>> box in the shack to the motor in the antenna via the coaxial cable.
>>>
>>> Before I purchase one of these I wanted to get an idea of the
>>> efficiency of such a small loop. MFJ is silent on the subject so I
>>> did my own calculations. The calculations and results are on a
>>> 1-page document that I uploaded to Dropbox and can be downloaded here:
>>>
>>> https://www.dropbox.com/s/l8mv67cjrck2ssn/MFJ-1786-1788.pdf?dl=0
>>>
>>> My calculations are based on the assumption that the efficiency of
>>> the MFJ antennas is similar to the (no longer manufactured) AEA
>>> Isoloop (my reasoning for that is in the document) and that AEA's
>>> specification of 72% efficiency at 14 MHz is correct. From that
>>> number I can calculate the efficiency and gain on all the other bands.
>>>
>>> If you don't want to download the document, here is a summary of the
>>> results:
>>>
>>> Freq      Eff      Gain with respect to a half-wave dipole
>>> MHz      dB      dBd
>>> 7.0        -7.3    -7.7
>>> 10.1 -3.5    -3.9
>>> 14.0      -1.4    -1.8
>>> 18.068 -0.6    -1.0
>>> 21.0      -0.4    -0.8
>>> 24.89    -0.2    -0.6
>>> 28.0      -0.15 -0.5
>>>
>>> I'd be interested in any comments people may have on the accuracy of
>>> my assumptions and calculations in the document.
>>>
>>> Alan N1AL

Elecraft mailing list

Re: Efficiency of MFJ remotely-tuned loop antennas

Certainly don't want to throw away power in wasted heat, but when I turn down my 200W rig to 5W for QRP, it's still useful and that's 16 dB down.

With ant restrictions, I'm looking at building a small horizontal loop for 6m. It'd be a little more than 4' in circumference, 16" dia, and the capacitor could be be two 4 cm diameter plates ~2mm apart. That should be reasonably easy to make relatively efficent as there are not a lot of plates that need to be connected with very low resistance. I calculate ~85% with 1/2" copper. Should be good for 200W. I'd orient it horizontally for horizontal polarization (weak sigs) and local noise rejection.

I have a squalo made from a lawn chair, but technically that's not a small loop and a little big to be stealth.

On Tuesday, January 19, 2021, 3:58:09 PM MST, Alan Bloom <[hidden email]> wrote:

I've been convinced that AEA's specification of 72% efficiency for the
Isoloop at 14 MHz is too high, certainly too high for the MFJ antennas.
So I re-did the calculations using the 59% efficiency figure calculated
below. The new results can be downloaded here:

https://www.dropbox.com/s/ve1v49b3gjvmt64/MFJ-1786-1788_2.pdf?dl=0

If you don't want to download the (1-page) document with the pretty
graph, here's a synopsis of the results:

Freq Eff.     Gain with respect to a 1/2-wavelength dipole
MHz      dB      dBd
7.0       -9.5    -9.9
10.1   -5.1    -5.5
14.0   -2.3    -2.7
18.068 -1.1    -1.5
21.0       -0.7    -1.1
24.89     -0.4    -.8
28.0       -0.26 -0.65

My basic conclusions still stand. With almost minus 10 dBd of gain on 7
MHz, the 40 meter coverage of the MFJ-1788 doesn't seem very useful.
That is confirmed by some of the reviews I have seen. I think you'd get
better results by just loading up the coax feedline as a random-wire
antenna with a tuner.

The 10 MHz performance is a little better. Good enough to at least
allow you to get on the 30 meter band.

On the higher bands, the gain is within less than 3 dB of a full-sized
dipole, which seems a useful trade-off for its small size and wide-band
continuous coverage.

Disclaimer: Again, I have never seen one of these things so this is all
based on theory and on the many reviews I have read. Even if my figures
are off a bit, at least this gives an idea of the relative performance
on the various bands.

Alan N1AL

On 1/18/2021 5:38 PM, Alan Bloom wrote:

> Well let's see...
>
> Radiation resistance of a small loop is 31,171 * (Area / wavelength^2)^2
>
> For a loop with a 91cm diameter at 14 MHz, I believe that comes out to
> 0.064 ohms.
>
> Assuming the loss is due to the RF resistance of the loop:
>
> From the internet I get the volume resistivity and skin depth for 6063
> aluminum is 0.03 microohms-meter and 23.3 micrometers respectively, so
> the surface resistivity is 0.03/23.3 = 0.0013 ohms per square. The
> outside circumference of the tubing is PI * 1.05" = 3.3" and the loop
> length is PI * 36" = 113" so the loss resistance is .0013 * 113/3.3 =
> 0.045 ohms.
>
> So I calculate an efficiency of 0.064 / (0.064 + 0.045) = 59%
>
> So worse than AEA claimed, but in the ballpark.
>
> Alan N1AL
>
>
>
>
> On 1/18/2021 3:39 PM, Wayne Burdick wrote:
>> Hi Alan,
>>
>> 72% sounds a bit high. Is this number based on loop size alone ("in
>> theory")? Or are they taking conductor geometry and other losses into
>> account?
>>
>> Wayne
>> N6KR
>>
>>
>>> On Jan 18, 2021, at 2:05 PM, Alan Bloom <[hidden email]> wrote:
>>>
>>> MFJ makes a pair of small, remotely-tuned loop antennas, the
>>> MFJ-1786 that covers 10-30 MHz and the MFJ-1788 that covers 7 to 21+
>>> MHz. As far as I can tell, the two antennas are identical except
>>> for the size of the tuning capacitor. Each consists of a 3 foot (91
>>> cm) diameter loop made of aluminum tubing and a plastic housing that
>>> contains the tuning capacitor, motor, and coupling loop. No control
>>> cable is required since the control voltage is sent from the control
>>> box in the shack to the motor in the antenna via the coaxial cable.
>>>
>>> Before I purchase one of these I wanted to get an idea of the
>>> efficiency of such a small loop. MFJ is silent on the subject so I
>>> did my own calculations. The calculations and results are on a
>>> 1-page document that I uploaded to Dropbox and can be downloaded here:
>>>
>>> https://www.dropbox.com/s/l8mv67cjrck2ssn/MFJ-1786-1788.pdf?dl=0
>>>
>>> My calculations are based on the assumption that the efficiency of
>>> the MFJ antennas is similar to the (no longer manufactured) AEA
>>> Isoloop (my reasoning for that is in the document) and that AEA's
>>> specification of 72% efficiency at 14 MHz is correct. From that
>>> number I can calculate the efficiency and gain on all the other bands.
>>>
>>> If you don't want to download the document, here is a summary of the
>>> results:
>>>
>>> Freq      Eff      Gain with respect to a half-wave dipole
>>> MHz      dB      dBd
>>> 7.0        -7.3    -7.7
>>> 10.1 -3.5    -3.9
>>> 14.0      -1.4    -1.8
>>> 18.068 -0.6    -1.0
>>> 21.0      -0.4    -0.8
>>> 24.89    -0.2    -0.6
>>> 28.0      -0.15 -0.5
>>>
>>> I'd be interested in any comments people may have on the accuracy of
>>> my assumptions and calculations in the document.
>>>
>>> Alan N1AL

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Eric Garner

Re: Efficiency of MFJ remotely-tuned loop antennas

In reply to this post by Alan Bloom

Here's an interior shot of the tuning capacitor

https://imgur.com/a/sYdvgzF

-Eric KI7LTT

On Mon, Jan 18, 2021 at 10:59 PM Alan Bloom <[hidden email]> wrote:

> > That doesn't sound like they are welded, and given the cost
> difference for welded air variables I doubt MFJ used them.
>
> As I said, I don't have one so I can't say for sure. I got my
> information from the MFJ web site: "All welded construction, no
> mechanical joints, welded butterfly capacitor with no rotating contacts
> ... Each plate in MFJ's tuning capacitor is welded for low loss and
> polished to prevent high voltage arcing, welded to the radiator ...".
> https://mfjenterprises.com/products/mfj-1786
>
> Also, for what it's worth, some of the reviews on eham.com and qrz.com
> mention that it has a welded tuning capacitor.
>
> I got the impression that one reason people often receive units with
> bent capacitor plates is that they got bent in the welding process.
>
> It would be interesting to look at one and see what they actually mean
> by "welded".
>
> Alan N1AL
>
>
>
> On 1/18/2021 10:10 PM, David Gilbert wrote:
> >
> > According to another ham who recently posted here, he had to "tighten"
> > the plates on the MFJ capacitor to get it to work properly. That
> > doesn't sound like they are welded, and given the cost difference for
> > welded air variables I doubt MFJ used them.
> >
> > I sincerely doubt that an actual practical small loop is only down 3
> > dB from a full size antenna. That makes no sense to me at all. If
> > that were the case everyone would be using one, because they are not
> > that difficult to make ... at least for manually tuned ones.
> >
> > But you seem determined to believe differently, and it's not my place
> > to convince you otherwise. You asked for inputs and I have made
> > mine. Hopefully you are right and I am wrong.
> >
> > 73,
> > Dave AB7E
> >
> >
> >
> > On 1/18/2021 9:54 PM, Alan Bloom wrote:
> >> > There is a reason why top quality variable capacitors often use
> >> welded plates.
> >>
> >> I believe they do weld the capacitor plates and also weld the loop to
> >> the capacitor. (I don't have one, but that's what I've read.)
> >>
> >> > Yours is a limited theoretical analysis ... not a practical one.
> >>
> >> A number of reviews I have read (including the QST review of August
> >> 1994) have reported comparable performance to full-sized wire
> >> antennas located on the same site. If the loop is down by, say, 3
> >> dB, that's only half an S unit, which would hardly be noticeable in
> >> the QSB of a typical amateur band.
> >>
> >>
> >> As I see it, the advantages of the MFJ-1786 10-30 MHz loop are:
> >>
> >> - Continuous coverage on 6 amateur bands. A convenient way to cover
> >> all the WARC bands.
> >> - Small and light.
> >> - Omni-directional (when mounted horizontally) so does not need a
> >> rotor.
> >> - No control cable required - control voltage is fed through the coax.
> >> - Narrow bandwidth provides excellent RF selectivity. Might be good
> >> on Field Day to reduce inter-station QRM.
> >> - Users have reported lower receiver noise compared to wire
> >> antennas. No doubt that is because the isolated pickup loop prevents
> >> feedline radiation/pickup.
> >>
> >> And the disadvantages:
> >>
> >> - Expensive ($500 list price)
> >> - Less gain than a simple dipole (although you would theoretically
> >> need 6 of them).
> >> - Fiddly to tune. If you QSY too far you have to re-tune.
> >> - MFJ quality control leaves something to be desired. (You may have
> >> to open it up when you get it and make minor repairs.)
> >> - You have to pay attention to the problem of entry of water and/or
> >> bugs into the housing.
> >> - The controller can be damaged by a DC short in the coax e.g. from
> >> an shorting-type antenna switch. (I don't understand why MFJ didn't
> >> include a fuse or some other way to protect the controller.)
> >>
> >> I probably wouldn't buy the 7-21 MHz MFJ-1788 because of the poor
> >> efficiency at 7 MHz. I think you'd have a better signal just using
> >> the coax as a random end-fed wire (with a tuner).
> >>
> >> Alan N1AL
> >>
> >>
> >> On 1/18/2021 8:17 PM, David Gilbert wrote:
> >>>
> >>> You are neglecting the losses in various connections in the system
> >>> ... including possibly the construction of the capacitor itself. I
> >>> don't believe that they are insignificant. There is a reason why
> >>> top quality variable capacitors often use welded plates.
> >>>
> >>> I would also guess that contact resistance is worse for dissimilar
> >>> materials, such as a copper wire to an aluminum tube.
> >>>
> >>> Yours is a limited theoretical analysis ... not a practical one.
> >>>
> >>> Dave AB7E
> >>>
> >>>
> >>>
> >>> On 1/18/2021 5:38 PM, Alan Bloom wrote:
> >>>> Well let's see...
> >>>>
> >>>> Radiation resistance of a small loop is 31,171 * (Area /
> >>>> wavelength^2)^2
> >>>>
> >>>> For a loop with a 91cm diameter at 14 MHz, I believe that comes out
> >>>> to 0.064 ohms.
> >>>>
> >>>> Assuming the loss is due to the RF resistance of the loop:
> >>>>
> >>>> From the internet I get the volume resistivity and skin depth for
> >>>> 6063 aluminum is 0.03 microohms-meter and 23.3 micrometers
> >>>> respectively, so the surface resistivity is 0.03/23.3 = 0.0013 ohms
> >>>> per square. The outside circumference of the tubing is PI * 1.05"
> >>>> = 3.3" and the loop length is PI * 36" = 113" so the loss
> >>>> resistance is .0013 * 113/3.3 = 0.045 ohms.
> >>>>
> >>>> So I calculate an efficiency of 0.064 / (0.064 + 0.045) = 59%
> >>>>
> >>>> So worse than AEA claimed, but in the ballpark.
> >>>>
> >>>> Alan N1AL
> >>>>
> >>>>
> >>>>
> >>>>
> >>>> On 1/18/2021 3:39 PM, Wayne Burdick wrote:
> >>>>> Hi Alan,
> >>>>>
> >>>>> 72% sounds a bit high. Is this number based on loop size alone
> >>>>> ("in theory")? Or are they taking conductor geometry and other
> >>>>> losses into account?
> >>>>>
> >>>>> Wayne
> >>>>> N6KR
> >>>>>
> >>>>>
> >>>>>> On Jan 18, 2021, at 2:05 PM, Alan Bloom <[hidden email]> wrote:
> >>>>>>
> >>>>>> MFJ makes a pair of small, remotely-tuned loop antennas, the
> >>>>>> MFJ-1786 that covers 10-30 MHz and the MFJ-1788 that covers 7 to
> >>>>>> 21+ MHz. As far as I can tell, the two antennas are identical
> >>>>>> except for the size of the tuning capacitor. Each consists of a
> >>>>>> 3 foot (91 cm) diameter loop made of aluminum tubing and a
> >>>>>> plastic housing that contains the tuning capacitor, motor, and
> >>>>>> coupling loop. No control cable is required since the control
> >>>>>> voltage is sent from the control box in the shack to the motor in
> >>>>>> the antenna via the coaxial cable.
> >>>>>>
> >>>>>> Before I purchase one of these I wanted to get an idea of the
> >>>>>> efficiency of such a small loop. MFJ is silent on the subject so
> >>>>>> I did my own calculations. The calculations and results are on a
> >>>>>> 1-page document that I uploaded to Dropbox and can be downloaded
> >>>>>> here:
> >>>>>>
> >>>>>> https://www.dropbox.com/s/l8mv67cjrck2ssn/MFJ-1786-1788.pdf?dl=0
> >>>>>>
> >>>>>> My calculations are based on the assumption that the efficiency
> >>>>>> of the MFJ antennas is similar to the (no longer manufactured)
> >>>>>> AEA Isoloop (my reasoning for that is in the document) and that
> >>>>>> AEA's specification of 72% efficiency at 14 MHz is correct. From
> >>>>>> that number I can calculate the efficiency and gain on all the
> >>>>>> other bands.
> >>>>>>
> >>>>>> If you don't want to download the document, here is a summary of
> >>>>>> the results:
> >>>>>>
> >>>>>> Freq Eff Gain with respect to a half-wave dipole
> >>>>>> MHz dB dBd
> >>>>>> 7.0 -7.3 -7.7
> >>>>>> 10.1 -3.5 -3.9
> >>>>>> 14.0 -1.4 -1.8
> >>>>>> 18.068 -0.6 -1.0
> >>>>>> 21.0 -0.4 -0.8
> >>>>>> 24.89 -0.2 -0.6
> >>>>>> 28.0 -0.15 -0.5
> >>>>>>
> >>>>>> I'd be interested in any comments people may have on the accuracy of
> >>>>>> my assumptions and calculations in the document.
> >>>>>>
> >>>>>> Alan N1AL
> >>>>>> ______________________________________________________________
> >>>>>> Elecraft mailing list
> >>>>>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> >>>>>> Help: http://mailman.qth.net/mmfaq.htm
> >>>>>> Post: mailto:[hidden email]
> >>>>>>
> >>>>>> This list hosted by: http://www.qsl.net
> >>>>>> Please help support this email list: http://www.qsl.net/donate.html
> >>>>>> Message delivered to [hidden email]
> >>>>>
> >>>>
> >>>> ______________________________________________________________
> >>>> Elecraft mailing list
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> >>>
> >>> ______________________________________________________________
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> >>
> >
>
> ______________________________________________________________
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--
--Eric
_________________________________________
Eric Garner
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ANDY DURBIN

Efficiency of MFJ remotely-tuned loop antennas

In reply to this post by Alan Bloom

"although I saw a design not long ago that used two coaxial copper pipes with a PVC pipe as the insulator. The inside conductor was mounted to a long threaded rod moving it in and out of the outer conductor."

Not my original idea but that's what I have on my loop (PEX insulator not PVC). Works fine up to 100 W on 30 m but has SWR runaway at 150 W. Capacitor is driven by a geared DC motor and is tuned from the shack. Home brew is far more fun than buying commercial.

I'll post a link to a presentation if there is any interest.

Andy, k3wyc
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k6dgw

Re: Efficiency of MFJ remotely-tuned loop antennas

1 vote for interest!

73,

Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County

On 1/19/2021 4:39 PM, Andy Durbin wrote:
> "although I saw a design not long ago that used two coaxial copper pipes with a PVC pipe as the insulator. The inside conductor was mounted to a long threaded rod moving it in and out of the outer conductor."
>
> Not my original idea but that's what I have on my loop (PEX insulator not PVC). Works fine up to 100 W on 30 m but has SWR runaway at 150 W. Capacitor is driven by a geared DC motor and is tuned from the shack. Home brew is far more fun than buying commercial.
>
> I'll post a link to a presentation if there is any interest.
>
> Andy, k3wyc
>

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ANDY DURBIN

Re: Efficiency of MFJ remotely-tuned loop antennas

In reply to this post by ANDY DURBIN

A couple of people expressed an interest so here is a link to a presentation on my mag loop. It's a while since the presentation was given but the loop is still in service in the gamma match configuration.

https://www.dropbox.com/s/5va4ygzbd410le4/Prototype%20Magnetic%20Loop%20rev%20-.pdf?dl=0

Andy, k3wyc

________________________________
From: Andy Durbin
Sent: Tuesday, January 19, 2021 5:39 PM
To: [hidden email] <[hidden email]>
Subject: Efficiency of MFJ remotely-tuned loop antennas

"although I saw a design not long ago that used two coaxial copper pipes with a PVC pipe as the insulator. The inside conductor was mounted to a long threaded rod moving it in and out of the outer conductor."

Not my original idea but that's what I have on my loop (PEX insulator not PVC). Works fine up to 100 W on 30 m but has SWR runaway at 150 W. Capacitor is driven by a geared DC motor and is tuned from the shack. Home brew is far more fun than buying commercial.

I'll post a link to a presentation if there is any interest.

Andy, k3wyc
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Alan Bloom

Re: Efficiency of MFJ remotely-tuned loop antennas

In reply to this post by ANDY DURBIN

I love to see people doing serious homebrew. Bravo!

I wonder if the SWR runaway at 150W could be due to the PVC pipe used as
the insulator for the variable capacitor? PVC is quite lossy at RF.
Perhaps when it gets hot it detunes the capacitor. It would be easy to
check by simply feeling the cap to see if it gets warm.

Alan N1AL

On 1/19/2021 5:39 PM, Andy Durbin wrote:

> "although I saw a design not long ago that used two coaxial copper pipes with a PVC pipe as the insulator. The inside conductor was mounted to a long threaded rod moving it in and out of the outer conductor."
>
> Not my original idea but that's what I have on my loop (PEX insulator not PVC). Works fine up to 100 W on 30 m but has SWR runaway at 150 W. Capacitor is driven by a geared DC motor and is tuned from the shack. Home brew is far more fun than buying commercial.
>
> I'll post a link to a presentation if there is any interest.
>
> Andy, k3wyc
> ______________________________________________________________
> Elecraft mailing list
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