The most fundamental cause of radio interference to other systems is the fact that the wiring for those systems, both inside and outside the box, are antennas. We may call them "patch cables" or "speaker cables" or "video cables" or "Ethernet cables," or printed circuit traces, but Mother Nature knows that they are antennas! And Mother Nature always wins the argument.

When we transmit, some of the RF from our transmitter is picked up by those unintentional antennas, and RF current flows on them.

What happens to that current determines whether there will be interference, and how severe it will be. We know that antennas work in both directions – that is, they follow the principle of reciprocity – so when RF trash from inside the box flows on those antennas, it is radiated as noise and we hear it on the ham bands.

An idea of  a simple antenna we’ve all used, probably with our first radio receiver. We connected a random wire to our receiver, and the antenna current flowed through the receiver to a "ground" that might have been a driven rod, but was more likely the safety ground of the AC power line (the third pin on the AC socket, known in North America as the "green wire").

Even if the radio was double insulated so that it didn't require the green wire connection, RF current still flowed through the stray capacitance of the power transformer to the power line and made the radio work. RF picked up on the antennas we call  loudspeaker wiring, video cables, the coax from the cable TV system or a rooftop TV antenna, flows through equipment to get to the AC power system safety ground. Hams understand that some antennas are more effective than others. An antenna that is close to resonance will work better than one that is not. Long antennas tend to pick up more RF than short ones. Think about these fundamental principles when trying to diagnose which cables are bringing your RF into a given system (or radiating their trash into your receiving antenna).

A path to "ground" or the power system is not always needed to produce antenna action. The whip antenna on our VHF and UHF handheld radios uses the radio, capacity-coupled to our hand that holds it, as a counterpoise (that is, to provide "the other half of the antenna"). All that is required for this to work is that the size of the counterpoise must be a significant fraction of a quarter wave (or larger) so that it can "sink" the antenna current.

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