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Author Topic: 20m antenna that might be useable in an HOA  (Read 354 times)

Posts: 590

« on: July 18, 2019, 10:27:57 AM »

The .pdf linked above describes something I have modeled, but not actually built. It is a vertical square loop that for 20m is only 6.75ft on a side, and is small enough to hide in a HOA backyard. I will consult with anybody that would like to try building one...

Unlike "Small Transmitting Loops" which require an expensive high-voltage vacuum variable capacitor, this one uses a much smaller value capacitor which could easily be home-brewed out of tubing or parallel plates. Two 2.5"x2.5" pieces of copper-clad PCB material separated 0.3inches have a capacitance of 4.7pF.

The magnitude of the currents which circulate in this loop at 100W are much smaller than they would be in an STL, so you can forgo the copper tubing. This loop can be fed directly from 50 Ohm coax, and since its Q is lower than an STL, you can cover more of the band without having to re-tune.

« Last Edit: July 18, 2019, 10:32:52 AM by WA7ARK » Logged

Mike, WA7ARK

Posts: 204

« Reply #1 on: July 18, 2019, 01:53:03 PM »

what is the height off the ground from the bottom.

Posts: 590

« Reply #2 on: July 18, 2019, 02:39:56 PM »

what is the height off the ground from the bottom.
As stated in the paper, originally modeled at 8ft. Height can be adjusted, and remodeled at the new height. Obviously, the higher, the better to reduce ground clutter and ground losses.
« Last Edit: July 18, 2019, 02:55:18 PM by WA7ARK » Logged

Mike, WA7ARK

Posts: 80

« Reply #3 on: July 18, 2019, 06:16:56 PM »

I don't have access to my simulator right now as I'm on vacation but I would bet that an inverted-V at the same height would likely out-perform the loop and you would have less wire to hide.  You may want to run a simulation as a performance comparison. My favorite go-to "HOA friendly" antenna is a ground mounted vertical as the radials will be on, or close to, the ground surface and you will find DX transmit performance better than the loop or inverted-V assuming at least 16 radials (on the ground) or at least 4 radials if elevated above ground. Keep in mind that RX S/N will be worse than the other two choices.

BTW, if you're trying to mitigate visibility, use 0.023 AL welding wire and spray it with a flat camouflage color.  Use Kevlar 100+LB rated dark green fishing line (no insulator required) for stringers.  The AL wire will hold up quite well and the whole installation will be very pretty invisible unless you get really close to the antenna. Will work just fine up to the legal power limit.

Don / W6QW
« Last Edit: July 18, 2019, 06:25:04 PM by W6QW » Logged

Posts: 80

« Reply #4 on: July 19, 2019, 08:10:33 AM »

Also, congratulation on an excellent assessment of the miniature loop and I applaud you for feeding your curiosity by completing the simulation.

Posts: 3231

« Reply #5 on: July 19, 2019, 08:29:19 AM »

Circuit elements made out of glass epoxy FR-4 or G-10 PCB are usable but they have very low Q at VHF, as I confirmed by making and measuring bandpass filters.
I suspect they aren't much better at HF.
You may want to compare them against surplus Russian doorknob capacitors that can easily be purchased in small quantities via Ebay.

Zak W1VT
« Last Edit: July 19, 2019, 08:32:05 AM by W1VT » Logged

Posts: 590

« Reply #6 on: July 19, 2019, 09:03:47 AM »

Most folk that build loops do so for very small size (STLs) which are <0.1 wl in circumference. Regular ~1wl circumference resonant loops are built so as to minimize the jX term which makes the R term about 100 Ohms. The one I posted the link to is ~0.38wl in circumference but has a feedpoint resistance of 50 Ohms after the capacitor tunes out the inductive reactance.

For 20m, the pattern is about the same as a low-mounted 37ft dipole, but it is only 6.75ft x 6.75ft, which might be attractive to someone that has to make it fit a small backyard.

Mike, WA7ARK

Posts: 943

« Reply #7 on: July 19, 2019, 10:41:17 AM »

I easily put up an 80m horizontal loop on my house, and no one can tell it's there. Horizontal loops are great. They are very forgiving of height and shape. All of the trim around my house is white. I use some white 14 or 16 gauge wire and tack it to the trim. Can't even see it. At the point closest to the window of my shack, I complete the loop with a 4:1 current balun and about 25 feet of white LMR-400 coax into the shack. Connect the coax to my LDG AT-1000proiii and my rig/amp and all is good.

I make contacts on ALL bands 10-80m. All over the world. And no one knows I have an antenna hooked up.

Born Wild - Raised Proud: 73
Cheyenne, Wyoming

Posts: 18331

« Reply #8 on: July 19, 2019, 08:34:19 PM »

I've done some modeling of similar sized loops for portable antennas for 40/60/80m,
typically in a triangle hanging from a single center mast.  I actually had a piece of #8 wire cut
for it, and a string of mica capacitors for testing, but haven't found time to try it since
then. Some things I found (though I'll have to go back to my notes for the details):

Wire size makes a difference in the loop dimensions.  Larger wire also helped improve
efficiency, though I don't remember just how much.

I fed mine at the bottom, with equal capacitors on either side of the feedpoint for
better balance.  I don't know how critical that is.

The feedpoint resistance increases up to 1/2 wavelength, then drops back as the perimeter
approaches 1 wavelength.  That means not only is there a 50 ohm point, but also
a 200 ohm point where a 4 : 1 balun would give a good match.  For some reason I
seemed to have preferred the higher impedance, but I don't remember if it was due
to wider bandwidth, easier values for multiband operation, or what. 

I did develop some designs that worked several bands on the same loop by changing
the matching components at the feedpoint.  Banana plugs might be a good switching
method for portable use.

It certainly has good possibilities, and there is a lot of flexibility in the loop shape,
though the optimum dimensions for a good match will vary somewhat with shape.
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