Wednesday, May 22, 2013

Lesson 10- Interference with Nearby Electronics

When I first started playing with my HF radio, I noticed something strange. When I talked on 20m, I could hear sound coming out of a set of near by computer speakers. I haven't extensively studied it out, but I believe that the speakers are poorly shielded, and actually acting as a poor radio receiver. Similarly, I have had computer speakers in the past that amplified AM Radio stations in the area. In this lesson, I will better familiarize you with the source of such interference, and what you need to do as a ham radio operator to mitigate it. I should state up front, don't take any of this as legal advice.

So, you've noticed that you are causing interference with one of your own devices, or a neighbor is complaining that they can hear you in one of theirs. Now what do you do? There are two possibilities as to what the source of the interference is, and you've got to figure out which one it is. The first one is that you are inadvertently broadcasting outside of amateur bands, and jamming their signal. The second case is that the device does not have any protection against strong radio signals. In the first case, you are required to stop the source of interference, in the latter case, the problem lies with the other electronics, and not yours.

So, how can you distinguish between these two types? The best way is to see if other devices have the same type of interference that the offending device reports. For example, if you have a TV that sees lines through it when you are transmitting, see if another brand of TV has the same issue on the same channel. Odds are pretty good that if you can't reproduce the exact same problem with a different device in a similar location, then the problem is in the electronics that is showing a problem.

If the problem is reproduced, then it is likely with your transmitter. First of all, you should stop transmitting on that radio until you can figure out what the source of the problem is! If you purchased an amateur radio, it should meet FCC requirements, so that means your problem is likely elsewhere. If you made your own radio, then you might borrow a commercial one and see if the problem is reproduced. Take a look carefully at the antenna, feedline, etc. Try putting in a balun if working with HF. If the interference is broad band, the most likely cause is arcing somewhere in your electronics. Take a look for bad solder joints. You can try doing a test where you take a receiver to find the exact source of the interference. Odds are pretty good that when you find the problem, your effective transmitter power will improve! Not to mention, the FCC won't come knocking on your door.

If the source is the other electronics, then you are technically not legally responsible to fix it. However, if you want to have a good relationship with your neighbor, I would suggest that you try something. There are two key things that can be done to mitigate this interference. The easiest is to install a ferrite bead on the cable. This is simply a device that snaps on to the cable, that will filter high frequencies, while leaving the lower ones alone. This is the best thing to do if you are hearing audio coming from a non-radio when transmitting. The other thing that could be done is to install a filter to prevent interference. There are 4 basic types of filters that might help, depending on the situation.
  • Low-Pass Filter- This type of filter will allow the low frequencies to pass, while filtering the higher frequencies. It can be made by putting a shunt capacitor, or a series inductor
  • High Pass Filter- This type of filter allows high frequencies to pass. It could be used to reject harmonics, for instance. It can be made by putting a shunt inductor, or a series capacitor.
  • Band Pass Filter- These allow a specific band of signals to pass without any problems, but reject anything outside of that band. Making these involves either both a series capacitor and inductor, or shunt capacitor/inductor.
  • Band Reject Filter- These will allow all signals to pass, except for a specific band. Placing one of these would allow one to filter out a specific band to electronics. You could, for example, filter our any 2 meter signals from a TV that is receiving interference.
Bottom line is, there are a large number of possibilities as to what could be wrong if interference is being caused, but interference is a solvable problem, in almost every case! For more information see the ARRL page on RF Interference.

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