http://elecraft.85.s1.nabble.com/Efficiency-of-MFJ-remotely-tuned-loop-antennas-tp7667578p7667630.html
> > 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
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