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Signal Bandwith and "Wide" Reports

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Author Topic: Signal Bandwith and "Wide" Reports  (Read 7362 times)
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« on: February 08, 2011, 12:58:26 PM »

It is not unusual to be engaged in an AM QSO and have a SSB station attempt to break in and report that the signals are "wide" or "splattering all over the band".  Most of the time, I take these reports with a grain of salt.

Many operators seem to be unaware that the apparent bandwidth of a signal as it is tuned in on a receiver is the sum of the transmitted bandwidth and the width of the  passband selectivity of the receiver.  For example, when tuning in a CW signal or unmodulated carrier using a receiver set to 3 kHz in SSB mode, the signal will appear across 3 kHz of dial space on the receiver, even though a clean unmodulated carrier has zero bandwidth. A 3 kHz wide SSB signal tuned in on the same receiver will appear across 6 kHz of dial space. A 7 kHz wide AM signal  will appear across 10 kHz of dial space.

If the signal is high in strength and well above the background noise, it will likely appear even wider, since the selectivity passband characteristic of any receiver is less than a perfect rectangle; even the best bandpass filters have some slope at the skirts.  In addition, no transmitter is 100% free of spurious distortion products and no receiver is 100% free of spurious responses.  If a signal is coming in at 40 dB over S9, even though the spurious products outside the passband are within FCC specifications at -40 dB, these distortion products will still appear as S9 at the receiver!

The total bandwidth of a signal is a product of two distinct characteristics: the bandwidth (frequency response) of the signal used to modulate, and spurious distortion products.  Both characteristics may be excessive, but the latter is far more likely than the former to cause harmful interference to adjacent channel communications, particularly when the source of modulation is the human voice.

Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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