Recently I aquired a Kenwood TH-D7 handheld radio to work FM satellites with a QRP setup. As this would work full duplex at least headphones are required. But as a Heil BM-17 IC was available I made an adapter to work with the headset also using its microphone and added a separate input to connect the built-in PTT switches of my arrow antennas.
Studying the manual of the TH-D7 it turned out to be a rather simple circuit:
So it is basically just connecting pins. The RX audio is connected to L and R pins for the headphones. MIC can be connected directly to the tip of the mic plug of the Heil BM-17 IC (which has an electret capsule). The bias power is provided by the radio. PTT is switched by connecting the two sleeve pins.
As Kenwood uses 2.5mm jack for phones and 3.5mm jack for mic I scraped an old external hand mic originally designed for a Tytera MD-380 which only costs a few bucks and added the 3.5mm jacks to the other end.
This setup was tested on a pass of TEVEL-6 satellite 2023-07-05 2107z. Successfully made two QSOs with I/4O4A and M5JFS. The handheld radio was powered with an external 3.4Ah battery and the 10⁄4 el Alaskan arrow antenna was used .
# Update re Low Audio Level
As I used the adapter for some portable activation from JN49 people reported a low audio level on the transmissions. I used the BM-17 with the adapter connected to a Kenwood TH-D72. PTT was switched on the radio itself. Disconnecting the external headset immidiately increased audio level while using the built-in mic of the TH-D72.
So next up was a study of schematics of external mic/headset hardware for the TH-D7 / TH-D72 as well as doing some measurements on the radio(s). First measurements at the mic pins (red and dark blue) showed about 1.6V bias power during RX and TX. This is obviously the reason for low audio level during transmit (while using internal PTT button). The headset is rated for 5V bias power so 1.6V is way too low.
But further views and thoughts at the schematics of the external handmic SMC-34 showed that PTT also connected GND pin of the speaker line (light blue) to mic GND during transmit. So while pressing the PTT button on the SMC-34 it shows about 3.3V at the mic pins now. So 3.3V bias is only applied to the external mic if the external PTT button is used.
Back to the adapter I built: Luckily I routed the PTT pins to a separate jack so I could connect a (third) cable to the Arrow antenna which I equipped with two PTT buttons (see ). I was just not using it during my latest tests and that is why the audio level was low.
So in order to be able to use the BM-17 without PTT buttons on the Arrow II antenna I made a little microPTT button built from a 3.5mm headphone plug and a micro switch.
Conclusion: If using an external headset also the external PTT button has to be used. And for the case that the PTT cable to the Arrow II has been forgotten have a nice little microPTT button at hand :-).