Expert: Marc Ebelini - 3/3/2010

Question
QUESTION: Hi. I have a gyrocopter and want to install a handheld uhf along with my existing handheld vhf radio. I want to wire in a changeover switch so that I can use the same PTT button. I cant find any diagrams on the net. UHF is a GME TX6200 and VHF is a Vertex VX 150 Pro. Any ideas?

ANSWER: Would like to help but need more info. If these are Handhelds how are they used? Do you use a remote PTT button for each set or are you keying then from the sets themselves? The method of keying can be different on each set. Some use a key line others use the mike current to key the transmitter and do not need a key line. Could not find any info on either set only that the GME uses a speaker mike according to the company web site. Did you try to contact each company by leaving a query? The phone number for help at Vertex was disconnected. If I am to understand you you have two remote PTT switches? In a normal PTT selector the double pole double throw switch is wired so one pole goes either to the #1 or #2 radio and the center goes to the PTT switch that grounds the line to key the transmitter. The respective poles on the other side connect to the mike input and the center pole goes to the mike. So that if number one is selected the key line goes to the key line of that radio and the mike also goes to the same radio. In this way when the ptt switch keys that radio the mike will be connected to it too. Now the problem is mike compatibility. Will the radio except that type of mike. GME claimed it uses a speaker mike. Does the Vertex use the same type? When most radios switched to electret mikes the mikes themselves did the keying. They used the current drawn to key the transmitter. Speaker mikes are usually high impedence speakers so you see the problem. Sorry that both companies were not helpful in giving me any technical help as to there keying method but you have the ability to get the info from your system and past in on to me so that maybe I can tell you if the about info I gave you will work for you. If the mikes are different a third pole can be used on the transfer switch so that each mike is connected to separate poles and the switch will only connect mikes to their our radios. Meaning the mike section will only connect to the radio used in that position. Hope I was able to make myself clear. If not let me know and I can arrange a diagram.

---------- FOLLOW-UP ----------

QUESTION: I use a remote PTT for the VHF. This arrangement grounds the mic.I haven't yet hooked up the UHF.It came with a remote collar mic/speaker. I pulled it apart and it has an electret mic and a 8 ohm speaker.The PTT on this collar unit switches the mic in series so seems to be mike current keyed as you were saying.
I measured the impedance of my headset and it is 12K ohms on the mic and 100-220 ohms on the speaker (depending on where the pot on the muff speaker is set#. My problem now is impedance matching. I want to use the one PTT#which is mounted in the joystick) and of course the one headset.
Hope this makes sense. I can buy a smart system to achieve all this and even patch in a cell phone, but they cost $1500.
This is a hairy problem, but I have heard of it being done.
Cheers and thanks a million,
Grant.

Answer
If I understand you correctly the mics are grounded as I suspected to key the transmitters then your only problem are the audio loads? Let look at it like this, isolate the speakers to assure that your are getting the correct impedance for each load. We will accept that the DC resistance is accurate enough for this. First give me the your headset Z then the Z's of each load for the radio audio. Because it is important to match as close as possible. The problem can be solved if there is adequate drive from each radio to absorb the loss of a transformer which should not be too much. You can use the selector switch to insert the matching transformer so the the headset will function properly. The radio volumes can be preset to so that switching will not effect the volume too much. So let me go over it again The headset Z is equal to X and the the UHF is Y and the VHF is W. Give me the numbers and I will look into it
Using an amplifier presents the same problem if the two radios how different output impedance's since the inputs to the amp would have to match closely too. Since the power to drive the headset is not very large the transformers will be small. You said the headset was 12K and one of the radios was 8 ohms. Find an old tube portable or parts for one. Get the output speaker transformer and  Connect the radio output to the speaker side (8 ohms.) The output side will be about 10K impedance that would be adequate to drive the 12K headset. Hope I have made my self clear. Don't like using an amp in the system because if it fails you lose sound from both radios unless you have a backup method. I can solve your problem but need to get a good picture of what i asked for above. The expedence's of X,Y %26 W. If I understood you correctly them the old radio transformer trick will work.