There are two kinds of ham radio operators: Those who have RFI problems and those who are going to have them. Really? You bet! The electronics industry is hiring engineers at a high rate to design new radio receivers and transceivers called “Personal Communications Devices”. They’re going to appear in your house, your neighbor’s apartment, everywhere. And a lot of them are going to quit working when you start to transmit.
The FCC has made the manufacturers put warning labels on these devices stating “This device must accept any interference received. Including interference that may cause undesired operation.”
So when your neighbor calls to tell you that the screen goes blank on his new digital TV at the same time that your voice comes out of his microprocessor-controlled expresso machine what do you do? Tell him to read the warning labels on the rear of the machines? Forget it! If your transmitter causes RFI you have a problem. It’s partly a technical problem but mostly a people problem.
Now is the time, before you get that call, to get prepared with some technical knowledge about RFI, its causes and cures.
Let’s begin by looking at a problem that is common these days, telephone RFI. Anytime you talk on 20 meters the lady next door hears you on her telephone. She uses her telephone a lot.
How do you stop it? Low pass filter on your rig? A filter on you transmitter’s AC power line? Balun on your antenna? Better ground system for your station? All of these are considered “good engineering practice” and, if you are lucky, they may cure the problem but, more than likely, they won’t help at all. Why not?
Let’s look at the technical fundamentals of the problem. EMI engineers use what they call the “Source-Receptor-Path” model. To have an RFI problem there must be a SOURCE of RF, a RECEPTOR of interference and a PATH connecting them. In this case we think we know the SOURCE (our transmitter) and the RECEPTOR (the telephone), but we don’t know the PATH just yet.
To find the PATH let’s first look at the SOURCE. Our transmitter produces, let’s say, 100 watts. Because of our long history of TVI problems the manufacturer of the transmitter has provided a tight metal box to keep it from directly radiating. He also has put filters on the leads coming out of the box (power input, key and microphone leads, remote control wiring, etc.). Filters aren’t perfect but the RF leakage from these leads will be in the low millwatt or maybe the microwatt range.
There is one exit for the 100 watts and that’s the antenna connector. If your station is typical, a length of good quality coaxial cable pipes the 100 watts to an antenna where it is radiated for all the world to hear. So, for purposes of examining our telephone RFI problem, we can consider our antenna as the SOURCE instead of our transmitter.
If the antenna is the SOURCE then putting a filter on the transmitter’s power cord is not going to help at all. Improving the ground system won’t help either because the antenna will still radiate the full 100 watts. Now that we’ve identified the SOURCE these facts are obvious. But remember, before we identified the SOURCE they weren’t obvious. Thinking the problem through sure can help solve it!
Now we need to find the PATH. Since the RF is radiated from the antenna the path starts through the air. Is it then picked up directly by the telephone? Not likely – the ‘phone is too small to be much of a receiving antenna. But it is connected to a large “antenna”, the telephone wiring running through the building. If this cable runs through the attic it may not be far from your antenna.
Now that we identified the SOURCE, the RECEPTOR and the PATH we are ready to find the CURE. Clearly there is nothing to be done at the transmitter/antenna end (except possibly moving the antenna) because we want that radiation to continue so we can work DX with it. At the RECEPTOR end we almost certainly could get rid of the problem by shielding the telephone cable. That solution probably is not practical. Another, and easier, solution is to decouple the telephone from its cable at RF. This can be done without affecting the voice and data going through it.
The first choice for this CURE is the use of ferrite toroids. One inexpensive toroid commonly available has a half inch hole (Part No. F82-77, available separately or in the Palomar Engineers™ RFI-4 kit). The telephone line’s modular plug will go through the hole. Run it through three or four times then plug it back into the telephone. Using type 77 ferrite the toroid acts partly as an RF choke but mostly like a resistor that absorbs RF. It will prevent the RF from entering the telephone (or at least reduces the amount going through) thus decoupling the telephone from the line at RF. It will not absorb the voice or DC going through the line. This is because the RF is “common-mode” and the voice signal is “differential-mode”. We’ll go into this phenomenon in detail in a future installment.
This CURE works most of the time but, unfortunately, not always. It depends somewhat on the RF characteristics of the telephone and on the RF signal strength. In simple telephones the RF is detected by diodes that are used to keep the audio signal level constant. As long as the RF voltage is large enough to cause the diode to conduct it will be detected. Even though the RF voltage is reduced the sound does not change if the diode still conducts but once the signal drops below the conduction level the sound suddenly disappears. So, even if one toroid appears to not help at all, a second toroid may CURE completely. Experiment a bit.
If ferrites don’t do it the next step is to use a filter. These are available commercially for around $15 to $30. They are fitted with modular plugs and just plug into the line and the telephone. They contain inductor-capacitor filters that are quite effective.
One important feature of both of these CURES is that they do not modify the telephone in any way. You can’t be held responsible for any later problems with the telephone. Keep in mind that the lady next door probably hasn’t the faintest idea of how the telephone works. She just wants to use it in peace. Try to leave her that way as a happy neighbor.