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Thursday, May 5, 2011

T/R Success with WSPR

Thanks to everyone who sent in advice on my WSPR t/r troubles. I now have it all sorted out. In the image above you can see the cable from the serial port that carries the RTS T/R signal from the computer. It goes to a switching transistor that controls a relay that in turn switches the three relays that actually do T/R for the rig. One question: On the Linux computer the RTS signal seems to switch between +5 and -5 volts, but on the Windows machine it was +10 and -10. Why?
I'm really pleased to have the computer interface working. It is kind of neat to bring together the complex technology of the computer and the simple technology of a DSB/Direct Conversion rig.


  1. Both are valid.

    A range of voltages is allowed. The range allowed by the sender is supposed to be from +/-5 to +/- 15. The rage allowed by the receiver is supposed to be from +/-3 to +/-25.

    Paul - K0EET

  2. I am thinking this would be determined by the hardware, not the OS running on it. Am I right?

    I've read that these sort of switching arrangements don't always work with USB to RS232 adapters because many of them are underpowered. Mine seems to work just fine though.

  3. The RS-232 standard is +-12V
    but since most serial communication is terminated with a TTL chip the defacto
    standard is now +-5V. i.e.,
    10V on old computer, 5V on new.
    Frank, K09K

  4. FB on the pretty circuit board! I have a question for you. I noticed the briding coax at the front of the picture. What is its purpose? Put more specifically, could the same thing have been accomplished with twisted pair, or a single wire?

  5. I like to carry RF via small coax. RG-174 I think it is. It helps prevent unwanted feedback and keeps amplifiers from turning into oscillators. Also, shielding of the VFO is important in DC receivers. You can see that the oscillator on this rig is still unshielded, but I have left room for half of an Altoids box -- that will be the shield.

  6. Thanks for the answer! I was curious because I'm forgoing the coax on a small QRP radio and wiring the antenna straight into the board to loft the entire radio as part of the antenna. I recently let the smoke out of one of the parts when I inadvertently reversed the battery pack, so no results yet.


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