Hello all,
Great job accolades to those who got the site back up.
Here is a description of an experiment that I did and I welcome comments.
I built an antenna from about 10 feet of 1/2 inch copper pipe. It is suspended from the rafters in my basement. At one end, the antenna connects to a coil wound with #12 wire on a 2" PVC form. In parallel is a 20 to 400 pf variable cap. The cold end of the coil is connected directly to a copper water pipe for ground. The AM transmitter is coupled through a 7 turn coil to the loading coil (79 turns).
Hello all,
Great job accolades to those who got the site back up.
Here is a description of an experiment that I did and I welcome comments.
I built an antenna from about 10 feet of 1/2 inch copper pipe. It is suspended from the rafters in my basement. At one end, the antenna connects to a coil wound with #12 wire on a 2" PVC form. In parallel is a 20 to 400 pf variable cap. The cold end of the coil is connected directly to a copper water pipe for ground. The AM transmitter is coupled through a 7 turn coil to the loading coil (79 turns).
I get a very sharp resonance by adjusting the tuning cap. The field strength looks good, and I am getting a range of several hundred feet. The audio is not good. I think the technical term for this is "it sounds like crap".
I have read that if the antenna Q is too high the sidebands do not radiate properly. The problem is that when I calculate the Q required to cause this it comes out to be 760. (1520 KHz. / 2 KHz.) The 2 KHz. bandwidth is just a guess I made based on how the audio sounds. It is not distorted, just sounds as if it is coming through cotton.
My questions are:
Has anyone run into this problem with high Q antennas?
A Q of 750 is hard to believe unless I am really good at winding coils.
Is there something else that may be causing this?
Neil
Neil & los otros,
Funny you should mention cotton. When I was very young & the world was magic, I thought there had to be some cotton in the radio to wipe the RF off the signal so I could hear my father's voice delivering the news.
Me, I figure a good AM signal has got to be close to 8 kHz wide. This would give you approximate 4 kHz sidebands. Which is why SSB sounds so cottony (and quacky too, but that's something else). Following your math for the Q, thinking in 4 kHz sidebands lowers your Q needs considerably.
The simple fact that we're talking about AM and AF & then having to face the reality of shortened antennas & all that math means that we're stuck trying to get an antenna that gives us enough leeway to have sidebands 4 kHz off the side of the perfect match you're shooting for. And that's where all the matching network stuff, the coils & taps and capacitors (and maybe even primary & secondary windings) puts you to work.
This is also another reason why QRP ham radio loonies (such as myself) find that it's waaaaay more easy to go 1km/watt CW (Morse) from a beach house rental in SC to some guy nuts enough to pull my signal out of the air down in Australia. CW ain't got that sideband problem.
Since you're stuck with a short antenna (which you have to see as a capacitor), any circuit below the antenna (element in the air part) is going to tune very sharply. And it will tune even more sharply as you move the tap feed point closer to ground. The only way out is to increase your ground system (radials) by numbers of radials, which, in effect (the way I see it) changes the capacitance of your radiating element.
At which point you're back to the coil settings. On and on like that until you're nuts with frustration or RF burns. And believe me, I've seen people with whole vast acreages of radials out bright and early every spring digging more trenches and trip lines through their yards. (Ah, the looks on the faces of the neighbors!) Not that I've done anything like that myself.
Probably the worst thing your facing, without even considering what I just gibbered, is that your antenna is, for all intents & porpoises, underground. In the basement, fer cryin' out loud. Let me guess: you live in a CC&R neighborhood where a flagpole has to be petitioned & only the right color of hardware to raise & lower la bandera, eh?
Get the antenna out of the basement first. After that, things will go easier. (He said, with a 6m four element antenna in the attic.)
73
Nils
W8IJN
Hi Nils and thanks for the response,
I have found cotton in radios. They used to wind cotton around the wires as insulation. Many times, mice found it too.
My antenna underground is just for development work. If my thoughts about the Q are correct, what surprises me is that the Q is high enough to be a problem. I usually use another antenna, also below ground, and the audio is just great with it. The only difference is that the problem antenna uses a 1/2 inch copper pipe and the working antenna uses #24 wire. Same coil and matching for both. Maybe the R of the thin wire lowers the Q?
Why underground? I only want to cover my modest 1/2 acre estate and the underground ants. do it nicely. I also don't worry about lightning, corrosion, heat, cold, powering the transmitter remotely, etc.
As for amateur radio, I have never run QRP but I did a lot of CW operating and you are correct. CW can't be beat for busting through noise. When I took the code test, the FCC had set up in a motel meeting room. Just as they started the code tape, a band began rehearsing behind the partition. Since I had learned CW by copying off the air, I was used to QRM so I and about 2 others passed. The rest complained but the examiners said they had to follow rules...no retest for thirty days.
I got my introduction to RF burns when I built my own transmatch. It worked great, but I never got around to putting it in a box. I touched the tuning cap while operating 280 WPEP on SSB. The louder I screamed into the microphone, the worse it got. RF burns take forever to heal don't they. Haven't had that problem with 100 mW. part 15.
73
Neil