rfi

by: AA5AU

Getting Started on RTTY

Radio frequency interference (RFI) can be a problem when running RTTY. You may never have an RFI problem when running SSB or CW or not notice it anyway, but all of a sudden you find RFI creeping into your setup when running RTTY. Why? Because RTTY is full keyed down power out at 100% duty cycle. This does not happen during SSB or CW.
You will normally only get RFI with RTTY when your antenna is very close to your shack or you run a linear amplifier. Another cause for RFI is long cables. Try making your cables as short as possible.
RFI can be eliminated rather easily in most cases. Ferrite rods and torroids work wonders in keeping RF out of cables going into your radio and computer. I've found rods work better than torroids. And before installing rods and torroids, check all your antenna cable connectors to make sure you don't have a bad connection. Check your antenna cables to make sure they are in good condition and not leaking RF. Also, use shielded cable on all your cabling for audio or FSK. Make sure all equipment is properly grounded.

I use ferrite rods and toroids on almost all of my shacks cabling. I do this more as a precautionary measure more than anything. Earlier this year I started SO2R RTTY contesting using amplifiers on both radios for the first time. Instead of waiting for RFI to happen, I took a proactive approach to protecting against RFI.

If you decide to use ferrite rods or torroids, make sure you get ones made with the correct material that will attenuate the frequencies being used, mainly 3-30 mhz for HF.

Pictured above is the cable I use to go from my FSK interface to the FSK input of my radio. I use a ferrite rod on one end and a torroid on the other end. This is a little overkill. But at one time, I had RFI getting into my FSK cable and it caused the RFI circuit in my radio not to shift. So I decided not to take any more chances.

Probably the most common problem, and one that I have experienced, is RF getting into your keyboard. You can tell when this happens because if you are keyed up and typing away but the characters you type are not the characters going out on the air, then you know you have RF getting into the keyboard. Wrap the keyboard cable around a ferrite rod and it will eliminate the RFI. The same goes for your mouse. Although I can't recall ever having RFI problems with my mouse, I suppose it could happen. Try a short rod.

If you run AFSK, I highly suggest using a rod or torroid on the cable used for transmit audio from the sound card to the audio input of your radio. RF on the transmit audio cable will cause all kinds of problems. Even though I operate FSK on RTTY, I have to use an audio transmit cable when running PSK31. When I first started PSK31, I had bad RFI problems on this cable, even running low power (less than 50 watts). A ferrite rod took care of that problem and now I can run 100 watts on any band.

One symptom of RFI on a transmit audio cable can be seen by the RF power output jumping around erratically. When transmitting RTTY, your RF power output should be constant. If it fluctuates only slightly then it's probably OK and could be caused by the level of the Mark and Space tones being slightly different.

When transmitting AFSK, you should have the monitor turned on your radio so you can hear what the transmit audio sounds like. If the tones sound distorted, then RFI is probably the culprit.

You could have RFI on the transmit audio cable and not know it. If you have a second radio, it's always a good idea to listen to what your signal sounds like. Or meet someone on the air who can tell you if your signal is clean or not.

I have occasionally experienced RFI into a computer monitor, but only when transmitting high power on 80 meters. I don't think it's a common problem. Because the video cable is normally pretty thick, a clamp-around ferrite torroid could be tried.

It is possible to get RF into your computer. Many times, this is caused by RF getting onto the AC cable and into the power supply. An RF suppressor AC power strip can cure the problem or use a torroid on the AC power cord. RF might be getting straight into the computer or into the sound card. Not being an expert on RFI, I can only suggest you check other resources for information on RFI suppression.