Hook Handheld To Intercom?

[itsjames2011]

I have an 1964 Cherokee 140 that is still having it's share of radio issues. I have since bought an IC-A14 handheld hooked to an external antenna and it is working incredibly well. I'm wondering if there is any way I can get this radio to go through the David Clark 4 place intercom in my airplane? I currently transmit via the handheld and listen on COMM1. I need to rip all of the radios out and have it rewired but I just don't have the budget at the moment.

James

 

[gismo]

I have an 1964 Cherokee 140 that is still having it's share of radio issues. I have since bought an IC-A14 handheld hooked to an external antenna and it is working incredibly well. I'm wondering if there is any way I can get this radio to go through the David Clark 4 place intercom in my airplane? I currently transmit via the handheld and listen on COMM1. I need to rip all of the radios out and have it rewired but I just don't have the budget at the moment.

James

The difficulty with using most handheld radios with an intercom comes from the way the handhelds implement the PTT function. Panel mounted radios activate the transmitter via a separate "Key" signal wire that's grounded when transmitting but the handhelds eliminate that extra wire and rely on detecting the current used by the external microphone. IOW the PTT switch for a handheld is in series with the microphone instead of being connected to a separate wire.

This causes several issues:

1) The transmit audio output of many intercoms doesn't draw any DC current so the handheld won't transmit unless you add something (a resistor works) in parallel with the transmit audio out.

2) Some intercoms do have a DC path from the audio out to ground but the bias current from the handheld Mic input can cause distortion in the transmit audio.

3) If your transmit audio out does have a DC path to ground or you add a resistor in parallel for that the handheld will be transmitting all the time unless you incorporate a way to interrupt that path except when you want to transmit.

So when you connect the output of your intercom to the handheld, you will need to insert a PTT switch in series with the handheld Mic connection and you may need to add a capacitor in series with and/or a resistor in parallel with the intercom's transmit audio out.

Most "portable" PTT switches include two switches, one in series with the mic and one that grounds the Key signal. If you connect one of these between the intercom and the handheld mic input it might work but it's likely that sound from both mic's will be transmitted. Some portable intercoms can "back feed" the Key signal from their transmit audio out plug and you could probably wire a panel mount intercom to accomplish the same thing but either way this only works if there's only one PTT (i.e. no copilot PTT).

For a PTT on a control wheel you'll either need to use a two pole PTT switch or use a single pole PTT to activate a remote 2 pole relay and those methods can allow two PTTs.

 

[flyingron]

One way to get around some of these problems is to get the matching "headset" adapter for the radio. Most audio panels and panel mount radios are compatible with the telephone transmitter carbon mike protocol enshrined in aviation history.

 

[coloradobluesky]

You can plug it into the music input if you have one. This will allow you to listen only through your headphones. If you want to talk, youll have to put the radio to your lips and talk on it. But yu can pick up the ATIS or AWOS this way. It helps.

 

[coloradobluesky]

Besides the music input, its possible to set up a "radio select" switch, but its just switches you back and forth for talking on the radios, while listening to both. For full flexibility, you need an audio panel.

 

[itsjames2011]

The difficulty with using most handheld radios with an intercom comes from the way the handhelds implement the PTT function. Panel mounted radios activate the transmitter via a separate "Key" signal wire that's grounded when transmitting but the handhelds eliminate that extra wire and rely on detecting the current used by the external microphone. IOW the PTT switch for a handheld is in series with the microphone instead of being connected to a separate wire.

This causes several issues:

1) The transmit audio output of many intercoms doesn't draw any DC current so the handheld won't transmit unless you add something (a resistor works) in parallel with the transmit audio out.

2) Some intercoms do have a DC path from the audio out to ground but the bias current from the handheld Mic input can cause distortion in the transmit audio.

3) If your transmit audio out does have a DC path to ground or you add a resistor in parallel for that the handheld will be transmitting all the time unless you incorporate a way to interrupt that path except when you want to transmit.

So when you connect the output of your intercom to the handheld, you will need to insert a PTT switch in series with the handheld Mic connection and you may need to add a capacitor in series with and/or a resistor in parallel with the intercom's transmit audio out.

Most "portable" PTT switches include two switches, one in series with the mic and one that grounds the Key signal. If you connect one of these between the intercom and the handheld mic input it might work but it's likely that sound from both mic's will be transmitted. Some portable intercoms can "back feed" the Key signal from their transmit audio out plug and you could probably wire a panel mount intercom to accomplish the same thing but either way this only works if there's only one PTT (i.e. no copilot PTT).

For a PTT on a control wheel you'll either need to use a two pole PTT switch or use a single pole PTT to activate a remote 2 pole relay and those methods can allow two PTTs.

Thank you so much for the insights. I'm going to take a hard look at the airplane this week and put together a schematic for when I'm planning to do. I will post it here when I'm done for any suggestions.