Interfacing the Aircraft Intercom With a Ham Radio

When operating VHF/UHF, HAAT (Height Above Average Terrain) is everything.  More important than power, antennas, and just about everything else.  The higher you are, the better your signal.  This is sometimes true for HF, as well, but sometimes not.  Depends on what you’re trying to do.  Since one of my many hobbies is ham radio, I figured I’d design and build some features into the plane that made ham radio from the air easier and more fun.  I’ve already put in a panel mount SO-239 and a coax run from that to an antenna mount under the plane back behind the main gear.  I also wired in power for a portable radio.  Last, but not least, I wired the PTT to a switch so I can flip back and forth between the aircraft comm radio and the ham radio when I push the PTT on the sticks (but still hear both of them at the same time).  Last bit of this was to wire the ham radio up to the “Radio 2” connections on the Dynon Intercom.

Aircraft headsets are old school.  They were designed back in the Olden Dayes of Yore to work with tube sets.  Technology then isn’t what it is today; any technology that’s related to aircraft is stuck, at best, in the 1940s, so impedances and levels aren’t what you’re used to with modern gear.  I did some poking around the ‘net and determined that the old radios, with their carbon mics, require a DC bias.  This bias, of course, is bad for modern radios.  Since we’re not tying a ham radio directly to an aviation headset, we don’t need that DC bias, but I wasn’t sure if the intercom tried to provide it to “Radio 2” or not, so I put a 22uf cap inline with the mic input, just to protect my radio.  Why 22uf?  I used the very scientific method of digging through my junk box until I found the first cap bigger than 10uf and smaller than 47uf.  Next, the level may not be quite right.  I did some more digging and found a 10k trim pot from another project.  Looks close enough for government work, so 10k became the Official Standard.  Last, but not least, my hope was that the headset might just simply work with no impedance or level issues.  Turns out I was lucky (for once).  It actually works just fine.

I decided to use my Yaesu FT-817 as my aircraft radio.  It’s 5W out on all bands, 160M through 70cm.  Given my HAAT, 5W (at least on VHF/UHF) should reach the horizon with ease, particularly on single sideband, though FM should work pretty well.  And the low current draw and handy size of the radio is a nice bonus.  The mic connector on an FT-817 is an RJ-45, so I went in my lab and grabbed a 6ft Cat5 ethernet patch cable and cut one end off.  That became my mic/ptt cable.  Standard shielded two-conductor audio cable was used for the speaker side of things.  The whole thing went together in less than an hour, and took about another 30 minutes to wire up to the plane.  I already had the PTT switch installed in the panel, so pin 12 (in the schematic below) actually goes to that switch, rather than straight to the intercom.  Otherwise, the schematic below is exactly as-built.

I did a quick test simplex to my VX-8R, and audio was pretty decent.  There’s some hum, which I need to work on, but the levels were pretty much spot-on right in the middle of the 10k pot.  Aviation mics don’t have the best frequency response, but then again, I’m not trying to sound like Tony Bennett, I’m trying to communicate.  I can live with that.  Received audio was 100% acceptable with no changes.

I need to figure out what kind of panel connector I’m going to use to make things easy to disconnect, and I need to build the very small amount of electronics into a box of some kind (they won’t last long just hanging by wires behind the panel), but otherwise, this is pretty much done.  Oh, and the hum.  But I’m not all that worried about the hum, I’ve probably got a ground loop or something.  Or maybe the cat5 cable is picking up the noise.  Usually not too hard to track down and fix.  It sounded like 60hz hum, anyway, and the plane was running on external power (ie, the battery charger) while I was testing, so it may go away the moment I disconnect the charger.

Anyway, I thought others might be able to make use of the info.  Cost me $0.00 out of pocket, since all the parts came from the junk box.  And it should work just as well with the FT-897, or any other Yaesu with the same mic pinout.  Won’t work with the Icoms, as they use a different RJ-45 pinout, and their mic levels are significantly different (at least on my IC-706MkII).

Now I just have to figure out how to string an HF antenna out behind the plane while in flight.  But until then, at least I’ll be able to get on VHF/UHF SSB and on the FM repeaters.  That’ll be fun.


3 thoughts on “Interfacing the Aircraft Intercom With a Ham Radio

  1. I’m working on this now. I’ve selected a Tarheel screwdriver type antenna and will mount it horizontally; hope it works. The 300 model has about 1.7 mhz-26mhz range which catches the oceanic ATC frequency ranges. I’m trying now to figure out how to integrate it with my Garmin GMA 340.

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