Technician: Cause of Erratic SWR Readings

From the Technician pool today we look at a question relating to a common issue in amateur radio, SWR, or Standing Wave Ratio. [T9B09]

What might cause erratic changes in SWR readings?

A. The transmitter is being modulated

B. A loose connection in an antenna or a feed line

C. The transmitter is being over-modulated

D. Interference from other stations is distorting your signal

Remember the definition of SWR, which is the ratio of transmitted power, to reflected power.  Now lets examine each of the possible answers.

A. The transmitter is being modulated?  Well, not to put to fine a point on it, but this happens every time you key up.  Of course its being modulated.  Whether its AM (Amplitude Modulation), FM (Frequency Modulation), CW (Continous Wave) or SSB (Single Sideband) your transmitter is modulating a signal of some kind….

C. The transmitter is being overmodulated? Now, this is definitely a problem you don’t want to have, but it shouldn’t result in erratic SWR readings.  Instead you will get reports of distorted audio or harmonic interference, meaning your signal is “wider” in bandwidth than it should be.

D. Interference from other stations is distorting your signal.  “Intermod” is another one of those problems you don’t want, where signals can intermix inside your radio and cause strange problems….  but again, it shouldn’t cause erratic SWR readings.  That leaves us with…

B. A loose connection in the antenna or feed line.  This absolutely, positively will cause SWR problems.  The answer, then, is B. A loose or intermittent connection can show acceptable SWR one minute, then spike the next and back again.  This can potentially damage your radio, and cause RF problems in your equipment.

This is a good reason why you should have a SWR meter in your setup somewhere, if possible, and that you monitor it.

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