Technician: SWR and Output Power

The Technician question of the week comes from sub-element 7 (antenna measurements) section C (SWR measurement) [T7C05]

What is the approximate SWR value above which the protection circuits in most solid-state transmitters begin to reduce transmitter power?

A. 2 to 1
B. 1 to 2
C. 6 to 1
D. 10 to 1

Once you understand what SWR (Standing Wave Ratio) is, and how it affects your transmitter, this question becomes common sense, really.

SWR is a measure of how much power is being output by your transmitter, compared to how much power is actually being transmitted by your antenna.  In an ideal situation, you have a 1:1 SWR, where all the power from your transmitter is being emitted as RF by your antenna.

Practically speaking, though, its almost never that good.  The energy that doesn’t actually get transmitted reflects back to the transmitter, and it turned into heat.

We can automatically eliminate answer B) 1 to 2, since that isn’t a valid SWR measurement.  It’s not possible.  Your SWR will always be equal to or greater than 1:1.

C) 6 to 1, and D) 10 to 1 are way, way, way too high.  If you transmit at any appreciable power with these SWR ratios, you will damage your radio.  It will be too late.

The only answer left is A) 2 to 1.  This means that a full 50% of the power leaving the transmitter is getting reflected back into it.  Most modern transmitters will start to dial back the power to protect the internals above this point.  While C and D might also be correct, approximately 2:1 is where the dial-down starts.

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