TheRadioBoard

As I've said before, There's no free lunch! How much balance you need depends on several factors. In a noise-free environment you wouldn't be having the problems you're facing. On HF it's unlikely you'll need the nulling capability as much as on crowded (over here, at least) MW BC. It may be more important to be able to null noise sources. If they are close and powerful, they'll be hard to null, especially if the feedline is picking them up and isn't balanced.

The tripod is probably affecting balance some, as are the metal balcony rail and window frames. What you really need is to reduce feedline and control-line pickup, which is working against you in two directions. First, it's coming right down the feedline into the receiver. Second, it's being re-radiated into the loop, and very likely is modulating the varactor control voltage.

Doesn't your radio work on 2 M and 70 cm? At this point I'd be thinking seriously about VHF and UHF. I'll bet there is a lot of CW activity on those bands in Japan.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

Yes, my radio covers 2m and 70cm. I haven't heard many QSOs up there in casual listening with the rubber ducky antenna. Maybe a real VHF antenna would be fun to build and let me hear more of what's going on up there.

Still, I'm mostly interested in HF now and I definitely will find a way to have some HF QSOs. While the varactor version of the receiving loop still needs debugging, the variable-capacitor-tuned version works OK and isn't overly noisy. It seems to have some balance issues as I said (moving the coax cable seems to affect reception), but it is usable. Right now I'm listening to plenty of early-evening domestic 40m CW contest activity on the loop.

About receiving loop balance, it seems that if I coil up the coax (instead of leaving it in a jumble) then moving the coax around seems to have less of an effect on the reception than when the coax is in a jumble. Perhaps it's acting as some sort of a choke when it's in a coil.

Anyway, if I can't easily get the varactor version working as well (as low noise) as the variable capacitor version, I'll just keep the variable capacitor version for now.

The transmitting antenna is the real challenge, but I remain very hopeful that some sort of low loss magnetic loop will work.

Mom always said, "Neatness counts!" Yes, you're making a choke. A smaller diameter coil, wrapped on a plastic bottle or something, would be even better.

As for VHF/UHF, another way of saying "rubber ducky antenna" is, Dummy Load. No surprise you aren't hearing much. Because much FM-voice operation is done mobile, vertical polarization is the standard for FM. CW and SSB operators almost always use horizontal polarization. On the VHF/UHF bands, polarization makes a big difference.

The good news is, it's easy to make Yagi antennas for those bands. Mssrs Yagi and Uda, who developed the antenna, were, of course Japanese, so an array of them on your balcony could be talked away as celebrating your host country's rich radio heritage! If your Yagi isn't going to stay outdoors permanently, you can make it from cheap, lightweight materials. A half-wavelength at 432 MHz is only about 14 inches. You easily can make a Yagi with whopping gain that stashes in a closet when not in use. Gains of 13 dB or more are no problem. Even on 144 MHz, a Yagi only a few feet long is good enough for hundreds of miles with QRP power.

But while you're trying to surmount your HF noise problems, you could try making a two-band dipole. An SO-239 connector with four pieces of solid copper wire (say 14-ga) set up on your tripod will way outperform the ducky. For receiving, you don't have to be compulsive about matching if you have only 15 feet or so of feedline. Use the formula 468/f-MHz to get the dimensions. Download the Japanese bandplan chart: http://www.jarl.or.jp/English/6_Band_Plan/JapaneseAmateurBandplans20090330.pdf I see that CW is done around 430 MHz there, while U.S. hams operate higher in the band. Our Two-Meter CW bandplans are similar, 144.02-144.10 over there.

Omnidirectional horizontal antennas called "halos," or even horizontal loops are another option, but you can whip up a simple two-band dipole in a few minutes.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

I want to thank you folks for starting this thread.

Built an embroidery hoop loop last night. 2 turns on a 23 inch hoop. 1 turn for receiver coupling.

I can peak 40 M and 31M with a 150 pF variable cap. I think my loop inductance is about 5 uH.

Doesn't work well without a ground on the receiver (my homebrew NE602/ZN414).

I want to demo the receiver to some friends tomorrow, so I am hoping that I can put the loop on my car roof and use the chassis as ground for an outdoor demo. If I get a strong local on 40Mtrs, it won't make any difference about the ground connection.

Rich

_________________
Homebrew Radio ex-Silicon Valley

Hi Rich,

What kind of difference are you seeing with and without the ground on your loop? I thought these small loop antennas, being balanced, were supposed to be ground-independent.

Initially, I thought I was seeing a large increase in both signal strength and background noise, when using a chassis ground.

After some playing, maybe I wasn't adjusting my variable cap accurately and the ground somehow compensated a little.

I'm surprised at how sharp the loop tunes... but given the coil size, I guess I should have expected that. Maybe a vernier should be my next step?

I'm taking my receiver and loop to my weekly "coffee with the retirees" group this morning. I just tried it out in the car with the hatchback open and could hear commercial AM from China, with the loop in the car. I hope propagation holds for another 2 hours.

Rich

PS: The demo went very well. Could hear a ham SSB net on 40, as well as WWV and commercial AM on 31 mtrs.

_________________
Homebrew Radio ex-Silicon Valley

The more I think about this antenna the better it looks. By going to one more turn, you quadruple the radiation resistance, and ohmic losses become almost insignificant. For receive-only use, this looks like the way to go. May be making another quilt-hoop shopping run!

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

Excellent. Very simple and it works great. The noise is gone! Thanks.

It was also pointed out to me that with my original schematic, I was tying one end of the loop directly to the 15m control cable. Seen that way it's obvious that connecting a 15m wire directly to the loop will introduce noise! I somehow overlooked that and just thought, "connect varactor across the loop... then connect varactor to DC." Now it's clear to me that both DC lines of the control cable must be isolated from the loop to avoid introducing noise. I guess this is a "floating varactor" configuration that I had asked about previously.

After some experimentation with my indoor loop antenna position for best signal, with the varactor control pot still conveniently at my operating position, I'm listening for the first time to booming 20m SSB QSOs. I heard three ops trying to use the same frequency and negotiating to QSY. Everyday stuff for you hams, but pretty interesting for me to hear, as I've still got a lot to learn about amateur operating practices.

After several months of fine service, this antenna today suddenly started giving me terrible 60 Hz hum modulation wherever I tuned. It turned out to be fixed by more securely taping the varactor wires to the antenna (they're not soldered to the loop element, just taped).

The only reason I can think of is that dirt or oxidation over time combined with a loose connection formed some kind of rectifying junction that then somehow conspired to hum-modulate all incoming signals. Does that sound plausible?

Your antenna turned itself into a mixer. With those power lines so close, I'm not surprised. Why not use nuts and screws?

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

Right, I should have thought of that. Hum modulation => multiplication of incoming signals by the hum waveform => mixing => mixer => rectifying joint. I haven't been working on homebrew receivers lately so I'm forgetting some basic principles.

I eventually hope to rebuild the antenna so it's less of an eyesore, at which point sturdier bolted or soldered connections will be used. Currently it tunes about 5 to 22 MHz, but if I rebuild it I'd like to extend that range upward and downward. I guess I'll have to make the loop smaller (for a higher maximum frequency) and add some switchable parallel capacitance (maybe with switching diodes?) to cover the lower frequencies. I think with 2 or 3 bands I can cover all of HF with one loop. For a casual listening antenna that's not too bad. It certainly beats hands down a random wire, as random wires pick up all sorts of noise at my location, whereas the loop is blissfully quiet (and can be rotated to null out intermittent nearby noise sources).

Goes to show, RadioBoarders will listen to anything. Static, hum, it's all good.

73,,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

rats...

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/