Expert: Marc Ebelini - 7/24/2009

Question
QUESTION: I'm working on a radio install on an ultralight using a Vertex VXA-220. Basicly running an extension with a PTT from radio to my headset. It worked on the bench, but in the plane I have had trouble with RF interferance. I think I have fixed that by use of ferrite chokes,as per Vertex, but my trouble now lies in the PTT. Using the PTT on the radio, it works fine, but when I add the external one, when i key up to transmit, I get a high pitch tone. This is a PTT that I wired up myself, so that is mostlikely the problem. On the mic side there are 3 parts. T S R. I am shorting the tip to the sleeve to transmit. Sometimes it works, but most of the time i Just get the tone. In reading  on line, it seems there should be 4 wires not just two. How does this work?

ANSWER: I'll try to analyze your problem but not knowing the details of your equipment I will make suggestions. First of all are the mic's carbon or Electret? Do both use the key line to transmit. Let me explain if I can. Some of the new mic's just connect the mic to the circuit. The current drain to the amplifier keys the transmitter. This prevents the transmitter to be key if the mic circuit is open. In the case of these amplified mic's RF feedback is a problem so that the mic audio line must be well shielded. The cable to the antenna must also had a good ground shield connection. Does the antenna have a good ground plane that is grounded to the electrical power source. Adding another mic in parallel should cause a problem unless they are of the different types I mentioned. Now is this feedback noise or generator(alternator)noise? You asked about four wires. The fourth line would be the audio shield. That wire must not carry any current. Grounding the mic tip is carrying the current for the relay or keying transistor in the radio. Many years ago when Plantronics came out with their soda straw mic's the ones used in light aircraft had to have additional shielding around the amplifier circuit board because of the strong local RF from the nearby antennas. Your problem can be a combination of things. Need to disconnect the audio mic line on the external mic to see if that line is the one picking up the RF and feeding to the radio. It could be acting like an antenna. Disconnect at the radio end not the jack. If that solves the problem then that's the line that needs shielding. Make only one change at a time to isolate the problem. If you still have a problem contact me with more details. Remember there is Carbon, amplified, grounding key and audio current keying types of mic's Have no idea of what type you are using.

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QUESTION: Sorry, i should have given more details. The radio is a handheld, so the antenna and mic are all internal. I'm just running an extended headset adapter with a PTT. Without the PTT in line, it seems to work ok. I just have to reach up to the radio to key it there.When I put the PTT in line is when I get the RF tone. Like I said, this is a PTT that I made myself just using a single pole momentary switch shorting between the tip and sleeve. It might be better to just buy a comercialy made PTT and fit it to my setup.
Thanks for your help.

Answer
Let's get some info on the Radio. It appears to be a straight forward aircraft type system where an intercepter jack is required to key the radio. A close look at the CT96 cable required to make this a work for headset use appears to have four connections on the plug into the radio. So I suspect it is a shielded line. That looks like a special plug so you would need to purchase it or the CT96 cable. The rest is simple, just insert the switch to key the radio. Just make sure that the mic is a compatible one for that radio. An electret mic is usually the standard. A carbon will not work. Marc