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Radio transmission equipments emit unwanted common-mode radiations..…how much is just a matter of design and installation. Radio transceivers should have adequate common-mode filtering built in. But is it really adequate even if any is built in? Time will tell you one way or the other but do you want to wait till then?!

In amateur radio applications and any other radio services, common-mode filtering is crucial. As per FCC requirement, Radio transceivers and RF power amplifiers have low-pass and/or band-pass filters built in to attenuate unwanted differential-mode currents, harmonic transmissions. However, in order to prevent RFI to nearby equipments, common-mode currents must also be suppressed. Don’t take chances. Do what you can to prevent it before it happens. Prevent your transmitted common-mode currents from flowing to neighboring homes by transmission line radiation or ground line radiation or, through AC power lines.

It is advisable to filter out common-mode currents on your radios in the first place. Treat your transmission lines (coax or twin-lead) and power cables (AC or DC) which are the most suspected cause of interfering common-mode transmission sources. If you experience RFI on your home/office electronics, you should then treat them as outlined herein.

You can stop noises you receive on your rigs! I once experienced nasty noises (S5 to S7) received on HF bands when my wife was cooking in the kitchen. After a few observations, I found that a small AC/DC adapter used for beam lights under the cabinet was emitting the noise. I clamped a couple of split ferrite cores on the AC/DC adapter and the noise was gone. A poorly designed switching power supply!

Another example: One night my daughter turned a portable fluorescent lamp on in her bedroom. (This is the one you use at campsites with batteries. She uses it for reading in bed.) I then got a nasty noise (S7+) on the HF band. I had set it up for her, using my old "regulated" AC/DC power supply. So I knew that power supply was not the problem, but the fluorescent lamp was. I wound the DC cable several times over a split ferrite core and clamped it on the lamp. And the noise was gone. The power inverter was emitting the noise. There are some home electronics that spread noises around. Watch out! These are examples how you can stop annoying noises that you receive on your rigs right at their sources.

Installation for Ham Radios

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