Friday, August 14, 2009

Update on the multi-turn loop effort

In my first post on the subject, I was flushed with the success of actually making two QSOs with the loop. But on further examination, I was having trouble keeping a good stable match or even getting back to where I had been. Now I was seeing minimum SWRs (at resonance) of 3:1 or higher.
I thought the problem might be my use of single conductor wire as opposed to Newkirk's zip cord. I taped and tied my conductors to increase coupling but without much effect. So I decided to rebuild the antenna with zip cord, in my case 2c/#18 speaker wire with clear plastic insulation. I loosely wove the two lengths of cord together as the author had reported that his were "entwined" (sounds romantic - he's a flowery writer).
My initial trails with the new version were disappointing. I tried a number of different fixed capacitors, including 20 pF dipped silver micas, 33 pF postage stamp micas, and 25 pF "doorknob" HV capacitors. In each case I could tune to a resonance but could not get to a low SWR. I even installed a compression mica trimmer in place of the split stator variable with no improvement. Sometimes I could get a low SWR at some point, but tuning to resonance on my desired 40 meter frequency gave a much higher SWR value.
One tangent I got off on concerned the existence of the fourth wire. Mr. Newkirk said he used two loops of zip cord but on 40 meters he just used three wires (turns), sparing the fourth for a future conversion to 80 meters. The article didn't describe whether the spared conductor was broken only at the top or in two places (top and at the matching capacitors at the bottom). Would it make any difference? I had originally cut the condutor in both places, so I jumpered it together at the bottom and found that it made a big difference in the resonance point. The resonant frequency dropped about 1 MHz and the SWR at resonance was well under 2:1. So the spare isn't just hanging around minding its own business.
Too many variables - the quality of the capacitors, the disposition of the spare wire, -- what about balance, which Mr. Newkirk emphasized? Currently I'm having to tune my variable to minimum capacitance to reach 40 meter resonance. That might mean my fixed capacitors (lowest tried - 20 pF) are too large. So I put in two 14.7 pF silver micas. Much better! Now my variable is partially meshed for resonance at 7040 kHz and my SWR is well under 2:1. It seems stable too, and not sensitive to body capacitance.
So that's where I am at this point. No more QSOs yet -- not much activity on 40 and no contests to exploit. I think I'd like to put the antenna on the second floor or even outside to give it a better chance.
It's interesting to note the big difference in capacitor values I wound up with as opposed to those in the article: 14.7 pF for me versus 40 pF for Rod Newkirk.
So now I'm happy with it again. I think it might make a good hotel room antenna. And it's now much easier to fold up due to the superior flexibility of the zip cord.

Tuesday, August 4, 2009

Multi-turn small 40 meter loop antenna

Here's the loop hung from the ceiling with string.
The feed is at top, capacitors bottom center.
I've had an interest both in compact loops and in in-room hotel antennas for a while, but really hadn't gotten far. So I finally got around to trying a multi-turn design by Rod Newkirk, W9BRD in the July 1993 QST article called, "Honey, I Shrunk the Antenna". (Guess what movie was new at the time.) According to the author, the multi-turn design raises radiation resistance significantly, and a resonating capacitor in each turn, along with the multi-turn design, calms down the extremes of voltage and current seen in single turn loops.

The antenna is a rectangle 4.5 feet by 3.5 feet, three turns, each turn cut at the bottom (one 4.5 foot side) with a 40 pF capacitor inserted, one of them variable, and fed at the center of the opposite side, which is the top in my case.

I used #14 AWG THWN house wire, stranded (it's a little springy). I made a little circuit board for inserting the three capacitors and another one for the feed line connection "insulator" at the top. Instead of etching, I just scored traces with my Dremel tool abrasive wheel. I mounted the variable capacitor on the opposite side of the (singled sided) board, using nylon screws so's not so short anything out. The capacitor is a dual section, unequal values (80 pF and 150 pF or so) in series. This is standard practice so current doesn't have to flow through the shaft to frame bearings. In the photo below, the variable cap is on the back side and connected to the center traces.

Circuit board material for series capacitor connections

The wire generally is bunched up, even tie-wrapped or taped in a few places, in keeping with how the author did it. In fact, he used a couple turns of 2/c zip cord and spared one pass of the resulting four.

The article called for 40 pF capacitors with the variable used for fine tuning. I put two 20.5 pF silver micas in parallel to get 41 pF with lower current in each. In the photo, I'm experimenting with just one capacitor. I'm publishing this a little prematurely as I'm still not satisfied with my values and possibly the quality of the capacitors. I got the SWR down to a fairly stable 2:1 and let the K3's ATU finish the job. I actually made two QSOs at 5 watts with the top of the antenna barely above head height. Note that my shack is on the ground floor.

More to come, maybe, if I make some improvements.

Nick, WA5BDU
August 4, 2009