Very lightweight SOTA microphone

I have been looking for an alternative to the rather bulky and heavy Elecraft MH3 handheld microphone for quite some time. Since I mainly operate CW, the solution could be somewhat improvised.
Inspired by an article about a very light microphone in the SOTAreflector, I started the following mini-project:

A headset from my Baofeng UV-5R was available, lying around unused. The aim was to replace its weird dual plug with a simple 3.5 mm stereo plug and change the wiring so that it could be used with the KX3. The tiny speaker I would simply trim as I’m already using earbuds for my activations.

UV-5R-HS

This is what the Elecraft MH3 wiring looks like (Note: the function of up/ dwn keys isn’t used):

KX3-Mic-Wiring-1This is the schematic of the UV-5R headset:

uv5r

After cutting off the dual plug, I found four wires, each in a different colour:

2021-03-15 12.43.46

The meanig of the colours is:

  • clear:   MIC – Rx Data PTT      -> initially connected to ring (later changed to sleeve)
  • red:      MIC +                              -> connect to tip of stereo plug!
  • blue:    SPK – PTT Gnd Data   -> initially connected to sleeve (later changed to ring)
  • green: SPK +                              -> not used!

The wiring was done straight forward. But when pushing the PTT, a strong 50Hz hum came from the KX3’s speaker.  After some pondering, I swapped the blue and clear wire just in case. Now the hum was gone.

Some settings in the KX3 had to be modified:

  • MIC BTN -> change to „PTT“
  • MIC BIAS -> change to „ON“
  • MIC Gain -> change according to your preferences
  • TX CMP -> change according to your preferences

Comparison:

2021-03-15 13.05.53
The Elecraft MH3 weighs 147 grams
2021-03-15 13.06.22
The homebrew mic weighs 10 grams

Conclusion:

Weight saving is 137 grams with the homebrew mic. Additionally, there is a significant saving on space in the rucksack 🙂

Edit 2021-06-14:

After a couple of activations I noticed a strong susceptibility to RF from my own signal. Especially on the low bands, where most of the SOTA-QSOs happen, a strong noise appears as soon as I push the tiny PTT.
The mic works well with the KX3 in my shack and the antenna at the roof. This seems to confirm the idea of RF feedback into the mic. However, several ferrites on the mic cord didn’t solve the problem. More investigations are necessary but priority is low since I’m operating cw most of the time.

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