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DIY base station COM radio question

rv6n6r

Well Known Member
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I have an old COM radio I'm trying to use as a base station in my hangar (to listen to RVs flying in = RV related :D) which I wired to use with an aviation headset or speaker / microphone. I made up a quarter wave antenna out of copper sheet and heavy copper wire based on this. It receives okay though somewhat staticky. But on transmit it just broadcasts a very loud HMMMM along with maybe a tiny bit of voice signal.

I've gone through the wiring many times and even wired up a couple of different radios independently, and tried a couple of different antenna setups including a handheld rubber-duck, and I get the same thing always.

I'm quite comfortable with avionics wiring having done my RV-6 and work on other planes and non-aviation electronics, and I'm pretty careful with grounding only one end and common ground etc. And I've never had a problem like this in any A/C. It seems likely that it's antenna related though I have tried more than one. The only difference I can think of from what I've done before is using an antenna without an airframe for a ground plane, but the quarter wave ground plane antenna is supposed to be in lieu of that (right?)

Any radio gurus with an "oh that's always X" or diagnostics to try? Something specific to the antenna coax or connectors maybe?
 
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and I'm pretty careful with grounding only one end

What does the above mean? This is a transmission line, the shield is part of that and must be connected to the downward pointing radials at the antenna (all of them) at one end, and to the transmitter ground at the other end.
The antenna is not inside a metal hangar, is it?
I’m not positive, but I suspect it’s a violation of FCC rules to operate the transmitter outside of an airplane.
You said you used heavy wire. Is it self-supporting over 60 cm (2 ft)?
 
How are you powering this rig?
Just some 12v power supply I had laying around.
What does the above mean? This is a transmission line, the shield is part of that and must be connected to the downward pointing radials at the antenna (all of them) at one end, and to the transmitter ground at the other end.
Sorry I mean the shielded wiring with the exception of the antenna.
The antenna is not inside a metal hangar, is it?
For testing yes, as is the handheld I'm testing it with. Problem? I did try an earlier iteration at my house at one point with similar results.
I’m not positive, but I suspect it’s a violation of FCC rules to operate the transmitter outside of an airplane.
That's as may be, I'm mainly using to listen just would like to know I could txmit if need be.
You said you used heavy wire. Is it self-supporting over 60 cm (2 ft)?
Yes
 
As this antenna is about as dirt simple as it gets, the list of problems is short:
- You shorted the radiating element (the vertical wire) to the base. Disconnect the feed line and verify that these are not shorted together.
- You have a bad feed line. If you made it either you shorted the center conductor to the shield, or you have broken the center conductor. Check wiht a multi-meter.
- You did the math wrong. Here the radiation element (the vertical wire) should be around 23” log and the counterpoise elements (the legs pointing down) should be around 26” long (each).

Carl
 
As this antenna is about as dirt simple as it gets, the list of problems is short:
- You shorted the radiating element (the vertical wire) to the base. Disconnect the feed line and verify that these are not shorted together
Definitely tested that (with ohm-meter)
- You have a bad feed line. If you made it either you shorted the center conductor to the shield, or you have broken the center conductor. Check wiht a multi-meter.
Ditto
- You did the math wrong. Here the radiation element (the vertical wire) should be around 23” log and the counterpoise elements (the legs pointing down) should be around 26” long (each).
Vertical is 23", counterpoise are 25" each. Should be close enough? I calculated it for 122.9 MHz.

One odd thing is that if I put an ohmmeter between the feed line and shield on the coax connector coming out of the radio (not powered on), it shows almost no resistance. Same result on another old radio I have sitting around but not a third. Huh? I haven't checked that on my actual working radio in the plane. What should that normally be showing?

I really would like to have a base station at my hangar and thought DIY would be cool but I suppose there might be an $off-the-shelf base station in my future :rolleyes:
 
It’s not uncommon to have an RF choke across the output; high impedance at 100 MHz, near zero at DC.
You aren’t sitting this whole thing on a metal floor?
 
One last thought. Are you testing this by listening with another receiver, inside the same hangar? You may just be grossly overloading that receiver. Repeat your test, with the hangar door open and an assistant monitoring your signal from 1/4 mile away.
 
Also make sure the TX current draw isn't overloading the random old power supply you had lying around. Look at the voltage to the radio key-down and with audio signal applied to the mic. This loads the transmitter maximally and your power supply must be able to keep up.
 
Also make sure the TX current draw isn't overloading the random old power supply you had lying around. Look at the voltage to the radio key-down and with audio signal applied to the mic. This loads the transmitter maximally and your power supply must be able to keep up.
^^^^^ THIS ^^^^^

Is a common situation with "home made" base stations. :cool:
 
Also make sure the TX current draw isn't overloading the random old power supply you had lying around. Look at the voltage to the radio key-down and with audio signal applied to the mic. This loads the transmitter maximally and your power supply must be able to keep up.

Good one I'll check it. I checked the power supply and it's rated at 4A. I don't know what the radio draws during txmit but it specifies a 7.5A fuse so hmm.

Whatever the result, thanks everyone for your suggestions!

EDIT and yes the handheld is close by so I'll try distancing too!
 
Test the radio connected to a 12 volt car battery, if the radio performs well, the power supply is undersized.
 
Works!

Also make sure the TX current draw isn't overloading the random old power supply you had lying around.
In case anyone's keeping track, this appears to have been the problem! I just pulled the leads off the (old 4A Radio Shack) power supply and put them on a new fully charged aircraft battery and the thing works fine.

Definitely one for the aviation version of Stump the Chumps. Thanks VAF posters!
 
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