{"id":688,"date":"2026-09-27T08:17:51","date_gmt":"2026-09-27T16:17:51","guid":{"rendered":"https:\/\/w6dxw.com\/?p=688"},"modified":"2026-09-27T11:59:12","modified_gmt":"2026-09-27T19:59:12","slug":"simulcast-theory","status":"publish","type":"post","link":"https:\/\/w6dxw.com\/?p=688","title":{"rendered":"Simulcast Theory"},"content":{"rendered":"\n

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Putting several transmitters on one frequency on purpose means managing interference instead of avoiding it. This post touches on the science behind simulcast, and how coverage and timing are tuned in a real system.<\/p>\n\n\n\n

Simulcast is controlled interference<\/h3>\n\n\n\n

Every mobile radio already deals with interference from its own signal. At a single site, the signal reaches the radio by a direct path and also by reflections off buildings and terrain. The reflected copies arrive a little later and at a different strength. This is ordinary multipath<\/strong>, and receivers handle it every day.<\/p>\n\n\n\n

Simulcast adds more of the same. A second site on the same frequency is, from the radio’s point of view, one more source of “reflections”: its direct path and its own reflections all land on top of the first site’s. This is co-channel multipath interference<\/strong>. The job of simulcast design is to keep that interference under control, so that where two sites overlap the radio still recovers clean audio or data.<\/p>\n\n\n\n

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What the receiver does with it<\/h3>\n\n\n\n

A receiver amplifies what arrives at its antenna, mixes it down to an intermediate frequency, and demodulates it. When two or more signals are on, or very nearly on, the same frequency, they go through the mixer together and come out combined. Distortion is added along the way. What the radio finally produces depends on how its demodulator handles that combined signal. Everything else in simulcast design exists to make sure that combination still demodulates cleanly.<\/p>\n\n\n\n

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Carrier frequency and phase<\/h3>\n\n\n\n

A mixer produces the sum and the difference of the signals fed into it. Put two transmitters at 147.000000 MHz and 147.000001 MHz, only 1 Hz apart, and the receiver sees a 1 Hz difference product. That is heard as a rolling fade, sometimes called “mobile flutter”.<\/p>\n\n\n\n

Being frequency<\/em>-locked is not the same as being phase<\/em>-locked. To a receiver, a small change in phase looks just like a change in frequency, so a slow phase drift between sites also shows up as a rolling fade, and there is little that can be done about it at the receiver. <\/p>\n\n\n\n

Hear this your self<\/h3>\n\n\n\n

You can reproduce the effect on a bench with two audio signal generators. Set one to 440 Hz and the other to 441 Hz at the same level, and mix them into a single speaker. The combined tone swells and fades once per second, because the two waves drift in and out of step, adding together and then cancelling. The beat rate is simply the difference between the two frequencies. Two carriers at 147.000000 and 147.000001 MHz beat at the same 1 Hz as the two tones.

This analogy has a limit. In the radio the beat happens to the RF carrier, before the audio is recovered. An FM receiver ignores changes in signal level, so the listener doesn’t hear a smooth swell. They hear the audio flutter and break into noise each time the two carriers cancel. Try turning one generator down: the fade becomes shallower. That is the capture effect at work, as described below. When one signal is much stronger than the other, the weaker one can barely pull it down.<\/p>\n\n\n\n

Note: the demo needs both tones summed into one speaker or one channel, not one tone in each ear.<\/p>\n\n\n\n

\"1Hz<\/figure>\n\n\n\n

Audio timing: the 30-degree rule<\/h3>\n\n\n\n

The decoded audio from each transmitter has to reach the radio within 30 degrees<\/strong> of phase of each other. Turning that into a time depends on the audio frequency. Using 1,000 Hz as the reference:<\/p>\n\n\n\n