Connecting your Rig to your SSTV Equiptment

Sounds simple enough, but there are a number of gotchas to avoid. Because there are a number of low audio levels involved, there is always the danger of hum loops.

Even more difficult problem is the danger of RF feedback in the audio. About 50% or more of the beginning SSTVers wind up with RF feedback on their pictures. It has a very distinctive raspey sound and results in QRNlike hash on the received picture. Stations that never had any problem with SSB phone find that all those extra audio lines involved in SSTV are real RF catchers!

Eliminating RF feedback starts with a good ground system and LOW SWR. A loose coax connector can cause RF feedback and computer birdies in the receiver. Transformer audio coupling for both transmit and receive helps. Ferrite clamp-on transformers and large beads over the audio leads and microphone lead will often cure the problem. Sometimes very small ceramic capacitors (100pfd or so) are placed across the audio leads and the microphone lead at the microphone element. In very stubborn cases a 1/4 wave stub of wire can be attached to the transmitter. On 20 meters, this is a 16 foot long wire, attached on one end to the transmitter, and floating on the other end. This electrically looks like a 1/4 wave antenna. The floating end will be high impedence and the transmitter end will be forced to a low RF voltage level, eliminating the RF feedback.

One of the PC COM ports, either COM1 or COM2, is used to key the PTT line on the transmitter. Either the RTS or the DTR line from the COM Port is fed to a 10K resistor, then to the Base of a 2N2222 transistor. The 2N2222's Emitter is tied to a common ground between the computer and the rig along with the ground pin from the COM Port. The 2N2222's Collector goes to the PTT line in the transmitter. The SSTV is then programmed to send a + signal to RTS or DTR on transmit, which causes the 2N2222 to saturate, pulling the PTT line to 0, turning on transmit. Some people prefer to completely isolate the computer and the rig, so they key a small relay whose N.O. contacts go to the PTT input, or they use an opto-isolator, the COM Port turning on the LED inside the opto-isolator which causes the phototransistor inside the opto-isolator to saturate.

When the SSTV computer is connected this way, the transmitter will automatically switch to transmit when the computer starts sending SSTV.

When using a Windows based program, the rig's received audio is sent to the Sound card's input. On transmit, the Sound card's output is sent to the rig's input. More about this in the Receive section that follows.

When using DOS based programs, the Sound card is not used. In this case the output from the receiver is sent to a limiter circuit that squares up the received waveshape. The ouput of the limiter is then sent to the receive pin on the RS-232 COM Port. On transmit, the transmit pin on the RS-232 COM Port is send to an RC network that rounds off the square wave output of the computer and sends it to the transmitter's input. There are several designs to do this, and you can see the circuitry that I used at http://www.kiva.net/~djones

The drawings below are from my Kenwood 950 manual that show the ACC2 connections that I use for SSTV, PSK31, etc. on HF. Most, but not all of the newer Kenwoods have the ACC2 socket on the back. Be sure to use pin 9 for the PTT input and not pin 13, or the microphone connector PTT line. Notice that pin 9 will turn off the microphone. Notice that the fixed audio ouput level is 300mv.

I also included the same things from my Yaesu 736R manual that I use on VHF & UHF. Notice that the fixed audio level ouput is 200mv., slight lower than the Kenwood. Unfortunately the newer FT-847 dropped this output level to 30mv, making it useless.