Automatic Half-Duplex WebSDR Audio Switch for Yaesu and Similar Radios

Band conditions on 75M lately have been terrible. So bad, that our long-time scheduled 75M SSB meetup of our four hams on the West Coast can’t work radio-to-radio due to high levels of noise. Also, propagation seems to favor distant stations over nearby ones, due to the inability of the ionosphere to reflect a near-vertical incidence wave at frequencies over 3 mHz.

We have been forced to use the Internet to make up for our inability to receive the distant station. One way to do this is to connect via Internet to a WebSDR page. A WebSDR is like a remote receiver that you can control yourself. Once you are connected to it, you specify the frequency and mode, and you can listen to the demodulated audio on your computer. By picking a WebSDR which is located in an area that your distant station can transmit to, you can listen in on your station even if the noise entirely drowns them at your own receiver. So this is what we do, when it’s necessary.

Some may say this is not how Ham Radio should work. That using someone else’s receiver, and the Internet, is cheating. Perhaps that’s true for contests, but networking with other radios in order to break thru propagation blocks is just another part of the hobby’s advance through the years. And it’s only fair — if Ham radio is a backup for the Internet, then the Internet should be the backup for Ham Radio. Right?

But manufacturers of modern transceivers haven’t kept up, of course. Our problem is that if we are listening to the WebSDR on our computer speakers, and we begin speaking into the microphone, we get feedback started and an embarrassing buzz or howl ensues. So to go from listen to speak, we have to shut off the computer audio at the same time we activate the push-to-talk button. It can be awkward and it invites mistakes, especially since it usually involves a mouse click. Then we have to reverse the process. Being forced to switch between sending and receiving is sometimes called “half duplex” operation. So the hobbyist’s answer is — design and build a device to switch between computer audio and radio audio. So here is such a box. Before we go too far I want to apologize for the shoddy construction methods and the reuse and abuse of old parts — I made this out of stuff that was within reach. If you make a version of this, yours can be pretty and professional if you like.

Rough interconnection diagram
Detailed Schematic (click for full size)

Operation is simple. There are two front panel switches, one marked “Radio” and the other “Computer”. As wired, turning on one or the other connects that source to the output. Turning on neither switch connects the computer UNTIL the radio goes into transmit mode, when it switches to the radio automatically. This will be the normal mode when listening to a conversation via the WebSDR. (Note: turning on both switches forces the computer to the output.) If you choose to use headphones, plugging them in will shut off the audio to the speakers. There is an output for an oscilloscope, to monitor the audio, if you like. Here are some photos.

View of the chassis rear.
View of the front. This was before the circuit board was screwed into place.
The LED is the little round indicator at the front top.

Finally here is a photo of the finished unit snuggled up alongside my Yaesu FTDX5000MP radio. It operates flawlessly so far. If you need to use an Internet device to receive sometimes, you need one of these too. Write me if you have any questions.

Advertisement