Bavarian Contest Club

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Stefan v. Baltz 


#HF Radio Contesting & Personal Information

Basic SO2R Box

With several commercial products on the market Single-OP-Two-Radio (SO2R) gets more and more popular.  On the AF-end you donīt need a sophisticated setup to benefit from this mode, and you can easily homebrew a little box doing the basic headphone & mic switching and matching your specific personal needs.  On the HF-end, however, you must take care of protecting and/or interlocking both radios against each other, and reducing interference.  Fig. 1 shows the main circuit I have been using by myself for quite a while, occasionally adding (and dropping) new features.

Fig. 1
Basic SO2R Box

The left circuit does the headphone audio switching and offers 3 modes:

  • Both ears to Left Radio (mono)
  • Both ears to Right Radio (mono)
  • Left ear to Left Radio, Right ear to Right Radio (stereo)

Switch S2 selects mono/stereo, S1 the radio when in mono mode.  R1 has been found useful to enhance the stereo effect and its value will vary individually.  The reason for the enhancement is the ability of the human auditory cortex to process signals more efficiently when they are detectable on both ears.  R1 should be chosen large enough to still be able to assign a signal to the correct radio.

CW operators are done!

For Phone the simple relay switching to the right takes away the hassle with 2 microphones.  Assuming you are running on the left radio and you like to work a multiplier on the right radio, you hit the right footswitch and both Mic audio and PTT will be routed to the right radio.  It does not matter if you use VOX or footswitching on the left radio as the Mic is disconnected and routed to the right by relay K1.  If you prefer to run on the right you need to swap Mic- and PTT-cables.

Though this box can be improved in many ways it shows how easy it is to hook up a 2-radio environment on the AF-side.  When using the PTT to control additional relays in the headphones path, automatic switching can be easily implemented as available in commercial products. © by Stefan v. Baltz, DL1IAO