I have made my first PVC loop antenna. This is much more sensitive than the smaller ones that I had previously made on cardboard boxes. It is also high-Q (sharp tuning). This posting is not intended to go into loop theory, calculations, and the like. That info is readily available on the WWW. Instead, I thought I'd share the construction details that I came up with. I knocked this together in ~4-5 hours with no previous plans and just a few cold 807s. I am particularly dissatisfied with the upright support's stability. Right now, they are reinforced by having been stuffed with 3/4" wooden dowel stock. The support mechanism will be replaced soon. I am particularly happy with the way that I stabilized the circumference of the loop. I have been using it by simply coupling the radios' antennae to the loop's field. In the very near future I will add a single-turn coupling turn for direct connection. I will also have a signal strength limiter in that application so that that I minimize the chance of blowing the sensitive front ends of my radio in use on local stations. This thing also really helps cut the local QRM generated by our (stupid) compact fluorescent lamps and ceiling fan speed controls!

Here is a shot of the whole thing:

http://antiqueradios.com/gallery/v/temp/Antenna_003.JPG.html

Don't pay too much attention to the room. See if you can find the Fisher FM-40 and Zenith A-600 in these pictures. And the clock? It is a Seth Thomas, early brass works #6 3/4, dating from between 1862 - 1865. Yes, it functions - very well! This is my son Seth's stuff. He's a lucky boy!
So, here is a shot of how the circumference is stabilized. This method also allows for room to run the coupling turn:

http://antiqueradios.com/gallery/v/temp/Oblique+View.JPG.html

OK, so, this outer ring allows the whole thing to mount to the central hub and the weight really is held pretty well by the spokes at the 12, 3, and 9 o'clock positions via the use of a "T" fitting inside the hub. The rest of the spokes sort of self-center. The rings and glue are just for added security. I drilled all of the holes with a drill press, Forstner bits and a "V" shaped center-finding fixture for the drill press table. The hub detail:

http://antiqueradios.com/gallery/v/temp/Hub+Closeup.JPG.html

The coil can adjust for alt/azimuth because it is mounted in a yoke made with "split" "T" fittings. Actually, I cut a pretty good swath out of them to allow for a tight fit yet I can actually pop the coil in and out of the support yoke. Detail:

http://antiqueradios.com/gallery/v/temp/Split+Yoke.JPG.html

This yoke allows for the "full-tilt" of the coil. I can null out all signals by doing this:

http://antiqueradios.com/gallery/v/temp/Cap+detail+2.JPG.html

The above picture also shows the capacitor connection. It is screwed to the coil form and tuning is very sharp! Also, did you see that little "dog-leg" coming off of the coil form? When I built this, I wanted the coil to have a positive stop when it was fully upright. This strikes the upright and prevents it from tilting if I don't (which I don't - yet) have a counter-balance installed. The dog-leg was added just in case the coil form didn't strike the support squarely. I can also hang a radio from here if I'm so inclined. See detail:

http://antiqueradios.com/gallery/v/temp/Catch+Detail.JPG.html

Well, that's it. I hope that this shows you that even a rank amateur can do it. All it took was an afternoon and some 807s!

I can't wait until the Winter DX season..

Edit - re-pointed pictures for newly-loaded forum software. It would not determine size of picture from Gallery, so I put url for now. Error message cut/paste follows:
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Nice work! I particularly like the ability to rotate as well as tilt, giving you the opportunity to work in three dimensions.

cheers

John

AWESOME !

K

Looks great! Man, thats a lot of fittings.

It's real sci-fi looking

How big across is it? Did you measure the inductance?

Really KOOL!

189uH and it is 50" at the spokes, 48" at the "flats"

Edit: I was shooting for ~180uH. So, the end result is not so bad at +5%. I believe that I mentioned that it was eight turns. It is #22 ga. stranded wire. I happened to have a 1000' spool. I also have enough 3/16" dia. Litz wire to make one of these. Hmmmmm..

A handsome job indeed! Happy DX'ing and keep up the good work!



Larry

Make another one with the litz, and compare the two and let us know what happens
Being the cheapskate I am, I think I would use 12 ga thhn house wiring, large diameter wire instead of Litz.

Just for the heck of it, I used a couple of on-line inductance calculators and got around 153-159 uh. I used 49" across, 8 turns, and "length" of 3"

The turns are 1/2" apart. Thus, the approximate length is 4"

Oh ok; I tried that in proffessor coyle and got 147.5. But theory isn't always in line with reality. The calculators I used are ideal for small selenoids, not loops. Coil Maestro gave 78. Way off. This one: http://www.66pacific.com/calculators/coil_calc.aspx also gave 147

Go figure. I measured it with two different inductance meters. I have found, in my past xfmr winding experience, that it helps to wind a few extra turns, measure, and just take a few off if needed. I'll try to put the pictures back up when I get a chance. Today is do things for the wife day.

Sounds like wise moves.

Both- adding a few extra windings, and doing things for the wife

Seems like most of my time is spent doing things for wifey

Excellent! It was clearly a time consuming design in cutting the pieces. Here's some things that you may want to consider as you continue in loops:

First might be adding a low impedance coupling output (for direct connect to a receiver) that does not load down the circuit and kill the Q. On the wire to the tuning cap I wound one turn around a ferrite torroid ring (NOT powdered iron) and a secondary winding of roughly 10 turns for the low Z output connection. An HP spectrum analyzer confirmed that this coupling has a negligible effect on the circuit Q and resonant voltage, but coupling was excellent.

Various Litz wires had poor results compared to plain 20ga. solid/stranded wire. Litz is delicate, easily broken, and is much higher in resistance. For these measurements a Boonton Q meter was used.

My experience with the non-lab L/C meters is that they are great for C but lousy for L. If you can measure your C accurately, you can always calculate the L easily.

Thanks for the coupling tip. Right now, I have a single loop connected to an AK 55-C. Sounds good when not overloading the AK.

Cutting of the PVC was done on a compound miter saw and, once the cuts were set up, it really was as quick and easy as 1-2-3! The two LCR meters used were a cheap variable (device-under-test-controlled) frequency oscillator type made by "Mostly Digital Electronics" and a BK Precision model 886 that allows me to use various test frequencies. I think that I set the BK 886 to 100kHz. They both agreed to a small (trivial) percentage. With a 1/2" turn-turn spacing, I didn't expect distributed capacitance to be playing a significant role, but, I didn't calculate it either. The bottom line? I can tune from the bottom of the band to the top with either one or two sections of the capacitor switched in.
But, because it is bugging me, I'll bring the BK meter back home and check the coil inductance again. This time I will make sure that I am testing at 100kHz. That might not be the middle of the broadcast band, but, it is the best that I can do since my Boonton Z meter found new pastures. Marriage will do things like that sometimes..


Edit - OK, now I'm curious. I drilled enough holes for 9 turns and 9 turns calculates out to be ~187uH. I think I'd better measure it again. I'll post back later today.

Think of how strong and sturdy it'll be if you were to use black iron plumbing pipe . . . .

Laughed so hard the dog wanted to know the joke..

What size litz are you referring to? If its the 5 or 7 strand wire then I agree whole-heartedly but in my experience by getting up into the 50+ strand (44-46 ga strands) material then it would be noticeably superior to 20 gauge. 660-strand is available for those with deep pockets and its neither fragile or higher in RF resistance.
I would love to try it on a loop!

The whole Idea behind litz is to have lower resistance to RF, but large wire like 12awg (THHN stranded) will have low Resistance, and is for the folks like me with shallow pockets

AT some point as you go up in frequency, litz becomes less and less relevant . . . .I'm not sure when that is, but I've read various ideas about litz on the short waves.