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Magnetic Loop Antenna Question

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Terry

Magnetic Loop Antenna Question

Hello Group,

I am looking for some feedback from those using magnetic loop antennas with
the current band conditions. I use dipoles or end fed random wires when I
am in the field with my KX3. When I am at my home, I use a horizontal loop
that is a close to a full wavelength on 80, and a 10-15-20 meter beam. In
the field I a running max 15w with my KX3 and at home 100W with my KXPA100.

I know that mag loops are not as efficient as a dipole and will never
receive or transmit as well because of their compact size as my field
antennas. I have been toying with the idea of building/buying one for times
when I need a compact antenna for 40-10 meters.

When I have looked at YouTube videos of folks using magnetic loops, they are
all from a time when band conditions were far better than they are now.

Can any of you current magnetic loop antenna users in the group tell me how
they are performing now and what is a realistic expectation as the cycle
continues to drop. I should also say that I am almost exclusively a CW
operator. I can envision using a mag loop from hotel rooms or other travel
times when I can't get a long wire or a dipole in a tree.

Thanks,

Terry, N7TB

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k6dgw

Re: Magnetic Loop Antenna Question

All the antennas you say you use are E-field antennas, they couple to
the E-component of the EM wave [which includes your horizontal loop].
Mag loops couple to the H-component. Away from the antenna [far-field],
the EM-wave has both E and H components orthogonal to each other, and it
doesn't matter how the wave was launched.

Antenna efficiency is complex. E-field antennas are strongly affected
by surroundings, imbalance, and the like because objects in their
environment have varying conductivities which contributes to their
inefficiencies. Mag loops are mostly insensitive to surroundings
because most of the objects [including me sitting under my Alexloop]
exhibit a permeability of 1.0 or very close [ferromagnetic metal
buildings, towers, and railroad tracks excepted].

The radiation resistance of most E-field antennas that hams use is
fairly high compared to the loss resistance in the wires. Radiation
resistance for mag loops is very low, usually measured in the milliohm
range, and in the general range of the loss resistance in the loop
conductor. Thus, loop resistance is a major factor in efficiency or the
lack of it.

In QRP field settings, my Alexloop [H-field] regularly outperforms the
E-field Buddipole I had in all the configurations I tried for the BP.
The resonant loop has a very high Q and suffers from very narrow
bandwidth [4-10 KHz depending on the band] and must be very carefully
adjusted after each QSY. On balance, my loop is quieter than the
end-fed I also use outside and it has a pronounced null orthogonal to
the loop plane which can be useful. The KAT2 takes care of matching the
end-fed, it's bypassed for the loop and tuning is critical.

As always, life is a series of trade-offs. I have not found any
correlation between the solar cycle and antenna choice. [:-)

You mentioned a KXPA100: Beware, mag loops exhibit some surprisingly
high voltages and circulating currents, even at 10-15 W. There's a
nifty calculator at

www.66pacific.com/calculators/small_tx_loop_calc.aspx

73,

Fred K6DGW
- Sparks NV DM09dn

- Northern California Contest Club
- CU in the Cal QSO Party 7-8 Oct 2017
- www.cqp.org

On 11/12/2016 11:23 AM, Terry Brown wrote:

> I am looking for some feedback from those using magnetic loop antennas with
> the current band conditions. I use dipoles or end fed random wires when I
> am in the field with my KX3. When I am at my home, I use a horizontal loop
> that is a close to a full wavelength on 80, and a 10-15-20 meter beam. In
> the field I a running max 15w with my KX3 and at home 100W with my KXPA100.

> I know that mag loops are not as efficient as a dipole and will never
> receive or transmit as well because of their compact size as my field
> antennas. I have been toying with the idea of building/buying one for times
> when I need a compact antenna for 40-10 meters.

> When I have looked at YouTube videos of folks using magnetic loops, they are
> all from a time when band conditions were far better than they are now.

> Can any of you current magnetic loop antenna users in the group tell me how
> they are performing now and what is a realistic expectation as the cycle
> continues to drop. I should also say that I am almost exclusively a CW
> operator. I can envision using a mag loop from hotel rooms or other travel
> times when I can't get a long wire or a dipole in a tree.
>
>
>
> Thanks,
>
>
>
> Terry, N7TB
>
>
>
>
>
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Elecraft mailing list

Re: Magnetic Loop Antenna Question

In reply to this post by Terry

Sorry, this ended up being quite long winded! I'm currently building one from a Russian Vacuum Variable Cap from ebay. Pure DC resistance is the biggest challenge as I see it. Even little additional resistance just turns into useless heat in the loop so R needs to be as low as possible. The MFJ mag loop helps in that dept by welding the loop to the variable cap. Since that is out of reach I am going to use one solid piece of copper pipe bent into a circle and then a length of 1" solid copper strap will be soldered to each pipe end and wrap around the ends of the vacuum cap. I figure once a year I should disassemble the copper/vacuum cap connection and clean any oxidation off. I found 6cm aluminum automotive fuel pump mounts on ebay that fit onto the ends of the vacuum caps perfectly providing a mount and allowing the copper strap connection to take place. Email me directly if you are going the vacuum cap route and want to see how I'm doing it. There are two lessons I have learned so far: 1). That Russian vacuum caps are way heaver then I thought. 2): The screw device needs more torque then I originally anticipated. The weight factor just needs to be addressed with a beefier mount. Since it will reside at the top of the loop it will have a high center of gravity and I don't want the wind to knock over a >$200 glass cap! The torque issue just means I need a stronger NEMA17 stepping motor and a 2 amp easy driver board to give it extra current. I imagine when it is cold the grease will stiffen up requiring even more torque. It may come to pass that I will use a gear reduction stepper motor to get the torque I need. I'm tending to figure out the little problems as they crop up! I'm building this one for 100 watt operation as the vacuum cap should be able to handle the voltage. For QRP operation I have found a 2Kv air variable on ebay that (according to the 99pacific calculator) should handle the 10 watt KX2 for portable operation. I'm already starting rev.2 and I haven't even finished the first one yet! Anyway I'm having a hard time finding someone to bend the copper pipe. It takes a rolling tube bender and I don't want to invest $150 at Harbor Freight to use it a few times! Most local garages can't bend 1" pipe. Too small. I need to find a fabrication shop. I could use soft copper and try to make a good circle but I'm a little too OCD for that! Well that's about all I can say so far. I'm sure I'll have more advice as I finally get this thing actually working!
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k6dgw

Re: Magnetic Loop Antenna Question

Try muffler repair shops. I think most new ones get the pipe pre-formed
from the suppliers, but an older one might have the equipment to do it.
Or, make it octagonal with soldered fittings, there really is no
discernible difference.

An 18 ft circumference loop with 1" copper pipe will be about 50%
efficient on 40, with a BW of ~5-6 KHz [Q is about 1,000]. That's close
to as small as you'd want for 40, just outside the upper limit for 20
and it will transition to an E-field antenna above 14 MHz, basically it
becomes "an elevated conductor." About 5KV across the capacitor @ 100
W, resonant current will be a bit north of 20 A.

The RF loss-resistance of the pipe will be higher than the DC resistance
since the RF sees only the outside surface of the pipe. I'd look into
some way of getting that cap to the bottom of the loop. On a 5 ft
tripod, the top will be close to 11 ft in the air with a big
"falling-over moment" in the wind.

73,

Fred K6DGW
- Sparks NV DM09dn

- Northern California Contest Club
- CU in the Cal QSO Party 7-8 Oct 2017
- www.cqp.org

On 11/12/2016 1:36 PM, rick jones via Elecraft wrote:

> Anyway I'm
> having a hard time finding someone to bend the copper pipe. It takes
> a rolling tube bender and I don't want to invest $150 at Harbor
> Freight to use it a few times! Most local garages can't bend 1" pipe.
> Too small. I need to find a fabrication shop. I could use soft copper
> and try to make a good circle but I'm a little too OCD for that!
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Thomas Beltran

Re: Magnetic Loop Antenna Question

In reply to this post by Terry

Hi Terry - I have two loops: the MFJ 1788 (40-15 meters) and the Ciro
Mazzoni Midi loop (80-20 meters).

I use the MFJ portable, on a MFJ tripod, and it works well, but better to
answer someone's CQ than trying to call. The downside is it is limited to
places you can drive up to - it is pretty heavy for its small size.
Interestingly, I use it on 40 meters, where I seem to do better than on the
higher bands. I used this loop extensively during the last low cycle,
2008-2010, and checked into a 40 meter net every Sunday with good results
most of the time. The stations were located all over the state.

The Midi is for permanent operation. It weighs 40 pounds, the loop is made
of 3" diameter thick wall tubing, 6.5 feet in diameter. I had troubles at
first with the KX3's cat system, but it is working fine with a work-around
(I use the KX3PA100 as a generic amp). For the past week, I have been
comparing the Midi with my regular antenna, a vertical delta loop with the
apex at 50'. I've only used the Midi on 40 meters so far, where the Midi is
is typically lower than the loop by less than an S-Unit. On receive, they
for the most part, identical. Tom W2CRN

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w7aqk

Re: Magnetic Loop Antenna Question

In reply to this post by Terry

Terry,

If you have the inclination at all, why not build one! It's really not all
that hard to do. Quite a few years ago I did that, and the thing worked
pretty well. I got the info from a QST article from back in the 90's. The
hardest part might be finding a proper capacitor for tuning, but if your
junk box has much in it, you may have that problem solved. I complicated
things a bit by adding a small low voltage motor to turn the capacitor. You
need to use one that has a very low rpm rate for that, but they are
available. Not counting the motor, the whole thing cost lest than $10 to
build. I used copper tubing, but you could use coax shield. I recommend
copper tubing.

Even though band conditions are rather poor these days, you can still do
good things with a mag loop--especially on 20 meters. You may not be able
to run that amp through it, unless you get a pretty husky capacitor, but 15
watts is not usually a problem. You just might be surprised at what you can
do with one of these things. They are fun to play with.

73,

Dave W7AQK

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stan levandowski

Re: Magnetic Loop Antenna Question

My first loop was because of an article I read online that said, "Look
no further! Here is all you need: 10 feet 3 inches of 3/4-inch hard
line, 2 hose clamps, a chassis connector, a little bit of house wire,
and you could be in business as shown in the photo below"

The darn thing worked incredibly well! It was for 40 meters. Here's the
"starter article" at http://www.antennex.com/hws/ws0902/hardline.html

This inspired me to build a square loop made of two inch copper pipe
with a vacuum variable and a 1 rpm 12VDC motor which I switched by
reversing polarity. It was for 20 meters and worked great with QRP.

I finally bought an Alexloop for true portable work.

73, Stan WB2LQF

On Sun, Nov 13, 2016 at 04:41 AM, w7aqk wrote:

> Terry,
>
> If you have the inclination at all, why not build one! It's really
> not all that hard to do. Quite a few years ago I did that, and the
> thing worked pretty well. I got the info from a QST article from back
> in the 90's. The hardest part might be finding a proper capacitor for
> tuning, but if your junk box has much in it, you may have that problem
> solved. I complicated things a bit by adding a small low voltage
> motor to turn the capacitor. You need to use one that has a very low
> rpm rate for that, but they are available. Not counting the motor,
> the whole thing cost lest than $10 to build. I used copper tubing,
> but you could use coax shield. I recommend copper tubing.
>
> Even though band conditions are rather poor these days, you can still
> do good things with a mag loop--especially on 20 meters. You may not
> be able to run that amp through it, unless you get a pretty husky
> capacitor, but 15 watts is not usually a problem. You just might be
> surprised at what you can do with one of these things. They are fun
> to play with.
>
> 73,
>
> Dave W7AQK
>
>
> ______________________________________________________________
> 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
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> Message delivered to [hidden email]
>

w7aqk

Re: Magnetic Loop Antenna Question

In reply to this post by w7aqk

Stan,

Well, my first question was "how about the capacitor?". I looked at the
article, and it says no capacitor needed, but that it is mono band only. I
would have to guess that it is "mono-frequency" as well, or just about!!!

Dave W7AQK

-----Original Message-----
From: stan levandowski
Sent: Sunday, November 13, 2016 7:01 AM
To: w7aqk
Cc: Elecraft Reflector
Subject: Re: [Elecraft] Magnetic Loop Antenna Question

My first loop was because of an article I read online that said, "Look
no further! Here is all you need: 10 feet 3 inches of 3/4-inch hard
line, 2 hose clamps, a chassis connector, a little bit of house wire,
and you could be in business as shown in the photo below"

The darn thing worked incredibly well! It was for 40 meters. Here's the
"starter article" at http://www.antennex.com/hws/ws0902/hardline.html

This inspired me to build a square loop made of two inch copper pipe
with a vacuum variable and a 1 rpm 12VDC motor which I switched by
reversing polarity. It was for 20 meters and worked great with QRP.

I finally bought an Alexloop for true portable work.

73, Stan WB2LQF

On Sun, Nov 13, 2016 at 04:41 AM, w7aqk wrote:

> Terry,
>
> If you have the inclination at all, why not build one! It's really not
> all that hard to do. Quite a few years ago I did that, and the thing
> worked pretty well. I got the info from a QST article from back in the
> 90's. The hardest part might be finding a proper capacitor for tuning,
> but if your junk box has much in it, you may have that problem solved. I
> complicated things a bit by adding a small low voltage motor to turn the
> capacitor. You need to use one that has a very low rpm rate for that, but
> they are available. Not counting the motor, the whole thing cost lest
> than $10 to build. I used copper tubing, but you could use coax shield.
> I recommend copper tubing.
>
> Even though band conditions are rather poor these days, you can still do
> good things with a mag loop--especially on 20 meters. You may not be able
> to run that amp through it, unless you get a pretty husky capacitor, but
> 15 watts is not usually a problem. You just might be surprised at what
> you can do with one of these things. They are fun to play with.
>
> 73,
>
> Dave W7AQK
>
>
> ______________________________________________________________
> 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]
>

stan levandowski

Re: Magnetic Loop Antenna Question

Yeah, if I remember my experience, it was rather "mono frequency" for a
few kHz. Then I had to adjust the coupling for better SWR. It wasn't
all that user-friendly! However, it was a great and cheap way to play
with a mag loop. I remember sitting on my front porch with a K1 and
working DX with it. The loop was just tie-wrapped to a broomstick stuck
into a patio umbrella stand and sitting on my driveway. It was enough to
get me interested in reading and experimenting so it accomplished the
authors' objective.

73, Stan WB2LQF

On Sun, Nov 13, 2016 at 01:57 PM, w7aqk wrote:

> Stan,
>
> Well, my first question was "how about the capacitor?". I looked at
> the article, and it says no capacitor needed, but that it is mono band
> only. I would have to guess that it is "mono-frequency" as well, or
> just about!!!
>
> Dave W7AQK
>
>
>
> -----Original Message----- From: stan levandowski
> Sent: Sunday, November 13, 2016 7:01 AM
> To: w7aqk
> Cc: Elecraft Reflector
> Subject: Re: [Elecraft] Magnetic Loop Antenna Question
>
> My first loop was because of an article I read online that said, "Look
> no further! Here is all you need: 10 feet 3 inches of 3/4-inch hard
> line, 2 hose clamps, a chassis connector, a little bit of house wire,
> and you could be in business as shown in the photo below"
>
>
> The darn thing worked incredibly well! It was for 40 meters. Here's
> the
> "starter article" at http://www.antennex.com/hws/ws0902/hardline.html
>
>
> This inspired me to build a square loop made of two inch copper pipe
> with a vacuum variable and a 1 rpm 12VDC motor which I switched by
> reversing polarity. It was for 20 meters and worked great with QRP.
>
>
> I finally bought an Alexloop for true portable work.
>
>
> 73, Stan WB2LQF
>
> On Sun, Nov 13, 2016 at 04:41 AM, w7aqk wrote:
>
>> Terry,
>>
>> If you have the inclination at all, why not build one! It's really
>> not all that hard to do. Quite a few years ago I did that, and the
>> thing worked pretty well. I got the info from a QST article from
>> back in the 90's. The hardest part might be finding a proper
>> capacitor for tuning, but if your junk box has much in it, you may
>> have that problem solved. I complicated things a bit by adding a
>> small low voltage motor to turn the capacitor. You need to use one
>> that has a very low rpm rate for that, but they are available. Not
>> counting the motor, the whole thing cost lest than $10 to build. I
>> used copper tubing, but you could use coax shield. I recommend copper
>> tubing.
>>
>> Even though band conditions are rather poor these days, you can still
>> do good things with a mag loop--especially on 20 meters. You may not
>> be able to run that amp through it, unless you get a pretty husky
>> capacitor, but 15 watts is not usually a problem. You just might be
>> surprised at what you can do with one of these things. They are fun
>> to play with.
>>
>> 73,
>>
>> Dave W7AQK
>>
>>
>> ______________________________________________________________
>> 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: Magnetic Loop Antenna Question

I've built several of those ... most before I understood how they worked
... or didn't. [:-) The #1 requirement, if you are building an antenna
and not a dummy load, is resonance. Since most loops are used fairly
close to the TX with short transmission lines, SWR doesn't matter much,
except to keep the TX happy.

It the loop is resonant, you can match the impedance at the input end of
the line with a tuner if you have to. But, the problem is folks new to
mag loops tend to get the loop close to resonance [usually because the
exact point tends to be fairly sharp on the capacitor rotation], and
then "let the ATU do the rest."

Loops will get you attention, sometimes more than you want:

A couple years ago, I set up for the FOBB at a local park on top of a
little knoll with my Alexloop. I sit under mine and can reach up to
tune it easily. Several people inquired what I was doing. Finally, one
older guy who had been walking past me several times at a bit of a
distance finally came up and said, "You're the CIA and you're spying on
me aren't you?"

73,

Fred K6DGW
- Sparks NV DM09dn

- Northern California Contest Club
- CU in the Cal QSO Party 7-8 Oct 2017
- www.cqp.org

On 11/13/2016 12:30 PM, stan levandowski wrote:
> Yeah, if I remember my experience, it was rather "mono frequency" for a
> few kHz. Then I had to adjust the coupling for better SWR. It wasn't
> all that user-friendly! However, it was a great and cheap way to play
> with a mag loop. I remember sitting on my front porch with a K1 and
> working DX with it. The loop was just tie-wrapped to a broomstick stuck
> into a patio umbrella stand and sitting on my driveway. It was enough to
> get me interested in reading and experimenting so it accomplished the
> authors' objective.

> 73, Stan WB2LQF

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Augie "Gus" Hansen

Re: Magnetic Loop Antenna Question

In reply to this post by w7aqk

The length of hardline provides both an inductor (the shield) and a
series capacitor (the center conductor to shield). It is an interesting
experimental loop antenna, but not particularly versatilefor general use.

There is a picture of my two-turn mag loop for 40-80m on my QRZ .com
page. I wrote an article for antenneX several years ago describing this
antenna, but it is behind the "pay wall" so I can't post it anywhere else.

The main loop is about 4.5 feet in diameter formed by using the shield
of 7/8-inch Heliax. The loop is remotely tuned over the coax by a DC
motor-driven vacuum variable capacitor. The antenna can handle between
500 watts and 1KW depending on frequency if the VVC is stout enough
(15kV at 60 amps). The driver loop is about 1/5 the diameter of the main
loop diameter.

73,
Gus Hansen
KB0YH

On 11/13/2016 11:57 AM, w7aqk wrote:
> Stan,
>
> Well, my first question was "how about the capacitor?". I looked at
> the article, and it says no capacitor needed, but that it is mono band
> only. I would have to guess that it is "mono-frequency" as well, or
> just about!!!
>
> Dave W7AQK

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Ignacy

Re: Magnetic Loop Antenna Question

Magnetic loop is interesting when running 100W without a tuner. I had lots of interesting contacts on 20-10 with the loop 8 ft up and an air split-stator capacitor. Altogether the loop weighted some 2 kg and was pretty large. Such a loop for 40m would be inefficient.

When your KX3 weighs < 1 kg and has ATU, end fed is likely to be as efficient and much lighter. Also it can cover many bands without extra tuning.

I made my mag loop over 20 years ago and made many contacts. Since I bought ATU, the loop was not in use. I tried to compare it to along wire, and the wire was stronger and lighter. The loop may have a purpose in quasi-stationary situation especially if there is RFI since the loop can be rotated to minimize RFI.

Ignacy, NO9E

Edward R Cole

Re: Magnetic Loop Antenna Question

In reply to this post by Terry

Another source of copper tubing is to use a length of either 1/2-inch
Heliax (LDF4-50A) or 7/8-inch (LDF5-50A). Often available in short
pieces at hamfests. I plan to use some 1/2-inch to make a 4-foot
loop one day (to use on 630m Rx).

73, Ed - KL7UW
http://www.kl7uw.com
"Kits made by KL7UW"
Dubus Mag Business e-mail:
[hidden email]

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Ken G Kopp

Re: Magnetic Loop Antenna Question

Two-way radio shops are another source of "hard-line" in short lengths.

73

K0PP

On Nov 13, 2016 16:42, "Edward R Cole" <[hidden email]> wrote:

> Another source of copper tubing is to use a length of either 1/2-inch
> Heliax (LDF4-50A) or 7/8-inch (LDF5-50A). Often available in short pieces
> at hamfests. I plan to use some 1/2-inch to make a 4-foot loop one day (to
> use on 630m Rx).
>
> 73, Ed - KL7UW
> http://www.kl7uw.com
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Augie "Gus" Hansen

Re: Magnetic Loop Antenna Question

On 11/13/2016 4:47 PM, Ken G Kopp wrote:
> Two-way radio shops are another source of "hard-line" in short lengths.

The best source I've found is a cell phone company. They usually have
freebees in the form of "pulls" of old hardline during upgrades or "cut
ends" from newly installed cable. We only care about the outer (shield)
so the cable doesn't need to be perfect.

73,
Gus

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