Signals From The Real World

Understanding Spurious Signals

By Jonathan Imberi

An ideal transmitter emits its signal only on the operating frequency in use and no where else. In the real world, transmitters radiate undesired signals, or spurious emissions. It doesn't matter if you are using a 100 watt HF transceiver or a 2 watt handheld VHF or UHF radio, any transmitter can produce spurious emissions. A spurious emission can be defined as any signal produced by the radio that falls outside the band on which you are operating. Using good design and construction practices, manufacturers and home builders are able to reduce spurious emissions so they don't cause problems.

Harmonics are whole number multiples of a given frequency. The the third harmonic of 100 Hz is 300 Hz. The fifth harmonic of 100 Hz is 500 Hz. Every oscillator generates harmonics in addition to signals at their fundamental frequencies. Simply multiply by the whole number of the particular harmonic to find its fundamental frequency.

Harmonics can interfere with other amateurs and other users of the radio spectrum. The second through the fourth harmonics of a 40 meter transmitter fall in the 20, 15, and 10 meter amateur bands, so imagine the interference (QRM) that would result if everyone transmitted two, four, six, or more harmonics in with the desired signal. If you receive a report from another amateur that your signals were heard on 28,640 kHz when you are operating your station on 7160 kHz, you should suspect that your transmitter is radiating excessive harmonic radiation.

FCC regulations specify limits for harmonic and other spurious radiation to prevent the chaos that would occur if everyone transmitted harmonics. If operating a 100 W output transmitter, harmonic signals can total no more than 10 milliwatts. A transmitter that complies with the rules still generates some harmonic energy, but that energy is so small that it is not likely to cause problems.

Good engineering calls for tuned circuits, which reduce or eliminate spurious signals, between stages in transmitters. The tuned circuits allow signals at the desired frequency to pass, but they attenuate (reduce) harmonics.

It is easy to be sure that your transmitter does not generate excessive harmonics, because the FCC requires all transmitter manufacturers to prove that their equipment complies with its regulations. This means that commercially manufactured equipment usually produces clean signals. If you choose to build a transmitter from a magazine or book article, check for information on harmonic radiation.

Controls adjusted improperly on your equipment can also cause spurious emissions. If you operate an SSB transmitter with the microphone gain set too high you can cause splatter, or interference to frequencies near the one on which you are operating. Talking too loud into the microphone or having the microphone gain set too high can also cause the transmitter to over-modulate its signal. Your transmitter may be putting out spurious emissions or splatter that could interfere with other stations if you operate it this way.

Many SSB transmitters include a speech processor to add "extra punch" to your voice, which will help another operator hear you under poor band conditions or interference. But, too much speech processing can distort your audio and cause splatter interference on frequencies close to the one on which you are operating.

If the microphone gain or deviation control is set too high, even a handheld FM transceiver can cause interference on nearby frequencies . On an FM transmitter, the microphone gain or deviation control is usually inside the radio, and you won't normally have to adjust this control. If, however, you consistently get reports that your audio is distorted or that you are causing splatter interference to nearby frequencies you may have to make an adjustment. Other operators may also say that you are over-deviating. It all means one thing: the problem needs to be corrected.

Your voice characteristics and the microphone that you are using may require a small adjustment to the deviation control. This would be especially true if you change microphones from a mobile microphone to a base station type microphone. Any time microphones are changed an on-the-air check with another station is necessary to ensure the quality of your signal.

You should hold a microphone close to your mouth and speak in a normal voice, but if you get reports that your FM transceiver is over-deviating, you should try holding the microphone a bit farther from your mouth when you are talking. You may not need to make adjustments to the deviation control if this works.

Be sure to reinstall and tighten all the screws before operating the radio again, if you have to remove the covers of your radio for any reason. In addition to protecting the electronic components from physical damage, the covers also provide shielding to the circuit. This shielding stops any spurious emissions or unwanted RF signals from being radiated.