ANTENNA CAUSES RFI
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 need to do to fix the problem? Low pass filters on your rig? A filter on your rig’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 first look at your transmitter. It is generating the 250-watts that is causing the problem. Because of our long history of TVI problems the manufacturer 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 milliwatt or even the microwatt range.
There is one exit for the 250 watts and that’s the antenna connector. If your station is typical, a length of good quality coaxial cable pipes the 250 watts to an antenna where it is radiated for all the world to hear. On the way the power passes through some well shielded metal boxes (SWR meter, tuner). These are not going to radiate if properly connected to your ground system. So for the purpose of examining our RFI problem we can consider the antenna as the source of the radiation instead of our transmitter.
If the antenna is the source then putting a filter on the transmitter’s power cord is not going the help at all. Improving the ground system won’t help either because the antenna still will radiate the full 250 watts.
Now we need to find the path the radiation takes from your antenna to the electronic coffee maker. Is it picked up directly by the coffee maker? Not likely – it is too small be much of a receiving antenna. But it is connected to a large “antenna”, the power wiring running through the building. If this cable runs through the attic it may not be far from your antenna. It takes the radiation it receives and conducts it right down to the coffee maker.
Now that we’ve found the radiation source and path it took to the coffee maker we need to find the cure to our problem. We could get rid of he problem by shielding the power wiring. Not practical. Another and easier solution is to decouple the coffee maker from its power cord. This can be done without affecting the power going through it.
The first choice for this cure is the use of ferrite split beads. You just clamp them over the cable and they stop the RF from getting into the coffee maker. It can take from one to five of them depending on the strength of the RF picked up. This cure works most of the time but not always. If it doesn’t, we move to a ferrite toroid. This needs to be large enough to take the power plug and to allow us to wind about five turns of the cord through the hole. This is strong medicine.
If even this doesn’t work there are filters available commercially. They are fitted with a plug and socket and connect in-line with the power cable. Sometimes they work better than the toroid and sometimes not. As with almost all RFI situations you have to try to see what works and what doesn’t.
One important feature of all these cures is that they do not modify the expresso machine in any way. You can’t be held responsible for any later problems with the machine. Keep in mind that your neighbor hasn’t the faintest idea how the machine works. He just wants to use it in peace. Try to leave him that way as a happy neighbor.