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Loaded Full-Wave Loop Antenna?




 
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WA2SQQ
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« on: November 19, 2019, 03:12:14 PM »

Ideally, Id like to put up a full wave loop on 75m, but I just dont have the space. Has anyone ever built a loop and incorporated a loading coil(s) to decrease the overall size? Id assume that if it works with a loop, it might also work with a triangular delta loop? Id imagine it would decrease the overall bandwidth? I can fit a 40m loop.
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« Reply #1 on: November 19, 2019, 04:39:11 PM »

I try to avoid loading coils, if I can. I haven't used them. But...

For a horizontal 75M loop just about any configuration will work. It can go back and forth, zig and zag, up and down, whichever way gets all the wire out there. Think in three dimensions. Call it "linear loading". It's not optimum, but neither are loading coils.

Try to keep the current nodes (the feed-point and the point opposite) as high in the air as you can. If you use insulated wire it will probably shorten a 75M loop by about 10 feet in overall length. I use 14ga THHN stranded copper wire from the local home center.

Feed it with an electrical quarter-wave length (about 50 feet for 75M) of 70 ohm cable as a matching section. I use RG-59 for this, and wrap a bunch of turns around ferrite cores at each end to keep noise from inside the house getting up to the antenna via common mode.


Don
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« Reply #2 on: November 19, 2019, 05:39:04 PM »

It's been a third of a century since I've had a loop in the air and it was full-sized, so I speak only from theory.  My suggestion would be to, if you use inductance(s),  keep them at a current loop, rather than at a voltage point.
I'd start with two coils, one on either side of the coax connection and tap down symmetrically on them with alligator clips to find out where you are in the band.  Another, more complicated way would be to put an inductance in series with both of the "near" ends of the loop and feed it with a link coupled configuration.  However, it's winter and that will take a bit of time and trial and error that you just might want to avoid.  Weatherproofing that will be difficult as well.

Yet another trick, and probably my preference, would be to make the loop have the largest perimeter that you can manage, then make up the length with a shorted stub of open wire line hanging down from the point that is exactly opposite the feedpoint.  Depending on yard configuration, you could even pull that shorted stub back toward the center of the loop.  With a couple of predetermined tap points, you could even have two or three taps to chose from going from CW to Phone.
It will all work and your way-cool NanoVNA will help you adjust it.  Remember to sweep fairly wide to find the resonances initially then zoom in to the area of interest, but remember that the Nano only has 101 measurement points so if you sweep way too wide you may miss something.
73 de Norm W1ITT
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« Reply #3 on: November 19, 2019, 05:44:17 PM »

I try to avoid loading coils, if I can. I haven't used them. But...

For a horizontal 75M loop just about any configuration will work. It can go back and forth, zig and zag, up and down, whichever way gets all the wire out there. Think in three dimensions. Call it "linear loading". It's not optimum, but neither are loading coils.

Try to keep the current nodes (the feed-point and the point opposite) as high in the air as you can. If you use insulated wire it will probably shorten a 75M loop by about 10 feet in overall length. I use 14ga THHN stranded copper wire from the local home center.

Feed it with an electrical quarter-wave length (about 50 feet for 75M) of 70 ohm cable as a matching section. I use RG-59 for this, and wrap a bunch of turns around ferrite cores at each end to keep noise from inside the house getting up to the antenna via common mode.


Don

lnear loading does work .... Hy gain's 40 mtr beam used it for both driven element and reflector .....  some window line could be added to the voltage max points and then dressed along the sides or just possibly let hang down with the far end shorted .... it will have to be longer than the element length (guessing 35%) or so .... this is unproved supposition and will require an antenna bridge and considerable cut-and-try ... 73 and gud luck ...John
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Steve - K4HX
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« Reply #4 on: November 19, 2019, 07:50:24 PM »

Why do you want to put up a loop?

Horizontal or vertical?
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« Reply #5 on: November 20, 2019, 01:43:08 AM »

ON4UNs book Low Band Dxing is invaluable for this situation.
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« Reply #6 on: November 20, 2019, 08:58:21 AM »

Why do you want to put up a loop?

Horizontal or vertical?

A few reasons
Every winter season I like installing a new antenna, to experiment with. Hands on learning
I'm told that loops are "quieter" - I live within 10 miles of the marsh land where most of the major NY AM broadcast stations are.
Lop would be primarily horizontal.
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« Reply #7 on: November 20, 2019, 09:03:31 AM »

Yet another trick, and probably my preference, would be to make the loop have the largest perimeter that you can manage, then make up the length with a shorted stub of open wire line hanging down from the point that is exactly opposite the feedpoint.  Depending on yard configuration, you could even pull that shorted stub back toward the center of the loop.  With a couple of predetermined tap points, you could even have two or three taps to chose from going from CW to Phone.
73 de Norm W1ITT

That's a great idea that I did not think of. So hypothetically. if I needed to add 20 ft, would the "stub" be 10ft, or would the two parallel conductors cancel each other and effectively only offer the 10ft? Yes, the NanoVNA has been used a few times. Between it and by antenna analyzer it really makes antenna installations so much easier.
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« Reply #8 on: November 20, 2019, 10:05:20 AM »

I think the "loops are quieter" thing is a myth. I not sure how a loop would be of any advantage with regard to having nearby BC transmitters except that *maybe* it might be less responsive to those lower frequencies. What issues you are having with proximity to those BC stations?

An advantage loops do have over a dipoles is that they can be current fed on all harmonics, whereas a dipole is limited to odd harmonics. If you'd like to work 75M and 40M (where the bulk of AM activity is) a coax fed 75M loop will do it. A coax fed 75M dipole, not so much. At my camp, where I have lots or room, I have 75M and 40M crossed dipoles with a common coax feed. That antenna will take power on 75M, 40M, and 15M without a fuss. At my home station I use a 75M loop because the size of the lot and the layout of practical supports don't favor the installation of dipoles. The loop I installed here is very close to being square. It was a good fit and works well.

I don't tout one type over the other. I like, and use, them both. They each have their own place in the world.


Don
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« Reply #9 on: November 20, 2019, 10:10:49 AM »

-SQQ...   A shorted stub is an inductor.  It's just not curly like the usual coil.   In typical off the top of the head ham radio pseudo engineering fashion, if I thought I needed to make up 20 feet of wire length, I'd take about 30 feet of wire and make some open wire line about 15 feet long, short circuited.  Then you could take a clip lead and make it even shorter if that's what is needed to bring it into your favorite part of the spectrum.  When you find the sweet spot you can make it all tidy, although the use of clip leads as a permanent fixture has some historical credibility among many users of this site.  
73 de Norm W1ITT
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« Reply #10 on: November 20, 2019, 02:55:22 PM »

I think the "loops are quieter" thing is a myth. I not sure how a loop would be of any advantage with regard to having nearby BC transmitters except that *maybe* it might be less responsive to those lower frequencies. What issues you are having with proximity to those BC stations?

Don
Sorry for the confusion
Combined, all the nearby broadcast stations just raise the overall noise floor. I once had advanced notice that WABC (50KW 770 khz - 2 miles away) was going off for emergency maintenance. I watched the noise level on 160 / 75 drop almost 3 S-units.

The belief / hope that the loop might be quieter wasa aimed more at the other residual noiuse, likely comprised with automobile ignition noise, and other extraneous local noise. All things considered, I have two BCB brute force filters that clean up an incredible amount of noise from that part of the spectrum. Just trying to experiment,ent with different techniques to decrease an existing condition.
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« Reply #11 on: November 20, 2019, 02:58:49 PM »

-SQQ...   A shorted stub is an inductor.  It's just not curly like the usual coil.   In typical off the top of the head ham radio pseudo engineering fashion, if I thought I needed to make up 20 feet of wire length, I'd take about 30 feet of wire and make some open wire line about 15 feet long, short circuited.  
73 de Norm W1ITT

OK, noted.
So in my mind I was envisioning the hanging "stub" as the equivalent to allowing excess wire to hang in a loop, rather than a taught point-to-point run.
Thanks for educating me ;-)
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« Reply #12 on: November 20, 2019, 03:27:10 PM »

To be more precise, I should have said that a shorted stub that is less than an odd number of quarter wavelengths long is an inductor.  It's all due to things running around the circumference of the Smith Chart.  For instance a shorted stub that's 3/8 wavelength will be a capacitor, of sorts.  When we go up higher in frequency what was a coil shape-shifts into a capacitor.  Ain't radio cool?  But this is why multiband operation of a stub loaded loop may get curiouser and curiouser.
de Norm W1ITT
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« Reply #13 on: November 20, 2019, 08:54:57 PM »

The loop is not likely to help you with your BC interference problem. That is either a receiver problem or a problem at the BC transmitter end. Have you tried a notch on the offending frequency. You can make one with a coax stub (hey we're back to that again).

http://www.hb9amo.net/hfcoaxialfilter.php

Loop loading options in attached diagram.

* loop loading options.pdf (103.21 KB - downloaded 24 times.)
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« Reply #14 on: November 21, 2019, 03:46:14 AM »

The other way to filter out local interference would be to use an antenna with a null in the direction of the interference source. A small receiving loop would probably work well on HF.

I did this some years ago on 2M FM when a local operator decided to relentlessly jam my simplex communications. I put up a 4 element yagi which had a deep, sharp null in its pattern and carefully steered that null in his direction. I couldn't hear him and he couldn't hear me, but I could hear everyone else. His jamming modus operandi was to transmit when I was receiving but not when I was transmitting. Realizing this, I had some fun transmitting on an omni directional antenna (so he could hear me) then switching to the yagi to receive. Of course, he didn't know I had this ability and must have wondered why him jamming attempts were futile. Fun times.


Don
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« Reply #15 on: November 21, 2019, 10:24:56 AM »

The other way to filter out local interference would be to use an antenna with a null in the direction of the interference source. A small receiving loop would probably work well on HF.

I did this some years ago on 2M FM when a local operator decided to relentlessly jam my simplex communications. I put up a 4 element yagi which had a deep, sharp null in its pattern and carefully steered that null in his direction. I couldn't hear him and he couldn't hear me, but I could hear everyone else. His jamming modus operandi was to transmit when I was receiving but not when I was transmitting. Realizing this, I had some fun transmitting on an omni directional antenna (so he could hear me) then switching to the yagi to receive. Of course, he didn't know I had this ability and must have wondered why him jamming attempts were futile. Fun times.


Don

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« Reply #16 on: November 21, 2019, 11:14:07 AM »

I have a 500' horizontal loop strung up in the shape of a rectangle. Average height above ground is under 20'.   My SWR is under 2.0 from 160m through the low end of the 6m band.  I don't know why loading coils would not let you use a physically shorter length of wire.  But I am admittedly not as well versed in antenna theory as others either.  On the lower frequencies at least I do consistently get better signal reports than the other station I'm working.

Whether from the receiver, lack of height or the antenna type itself I do hear a LOT of noise though.
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« Reply #17 on: November 21, 2019, 03:21:19 PM »

The loop is not likely to help you with your BC interference problem. That is either a receiver problem or a problem at the BC transmitter end. Have you tried a notch on the offending frequency. You can make one with a coax stub (hey we're back to that again).

http://www.hb9amo.net/hfcoaxialfilter.php

Loop loading options in attached diagram.

Steve - it sucks getting old! I probably was not as clear as I should have been.

My noise is a mixture of urban noise, power lines, inverters and God knows what. The BCB portion is well under control. I use a magnetic loop as a receiving antenna along with two different brute force filters. With the loop I can really null out any local offensive noise. I find that in the evening, the loop becomes almost omnidirectional with skywave propagation.  I've also been having some very good results with this "Loop on Ground"receiving antenna.
http://www.kk5jy.net/LoG/

The 75m loop is just my yearly lets experiment antenna for the winter season.
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« Reply #18 on: November 21, 2019, 10:15:00 PM »

Yea, urban noise sucks. In a previous location, I had to deal with power line noise, leaky cable TV lines, and tons of home appliances, computer and the like from my dozens of neighbors.

Yes, the small receiving loop will be more or less omni-directional for higher angle signals. I tried on at the at previous noisy location. I could null out some noises, but there were so many others coming from other directions, it didn't do much good.

Good luck with the loop experiments. I've had two loops up here on 40 meters since 2007. So, I am a fan of loops for some applications. Had one up for 10 meters and another for 20 meters for a while too.

http://amfone.net/Amforum/index.php?topic=21534.0

http://amfone.net/Amforum/index.php?topic=30353.0
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« Reply #19 on: November 22, 2019, 12:32:52 AM »

Actually, G6XN's book "HF Antennas for all locations" is a must-read on this subject.
It goes into details about loading loops. An excellent reference.
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