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A quick follow-up to this answer

... If the impedance mismatch is large, you risk damaging your radio, particularly on the lower frequencies ...

I'm sorry to be so dense, but uh why is the risk of damage to the Tx due to an antenna mismatch greater at lower frequencies? Is it merely that the wavelength is longer?

Kevin Reid AG6YO
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VU2NHW
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2 Answers2

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Another phenomenon that might play into the impression that HF transmitters are more susceptible to damage from high VSWR is feedline loss. Feedline loss increases as frequency increases. So at HF frequencies the power reaching the antenna feedpoint is higher due to lower feedline losses, and for the same reason the signal reflected back from the antenna feedpoint toward the transmitter is also higher when it reaches the transmitter. But at VHF frequencies the same feedline will absorb more of the signal going in both directions: making the feedline more effective as a dummy load. So for two antennas, one HF and the other VHF with identical VSWR measured at the antenna feedpoints, using identical feedlines, the VHF transmitter will see a lower VSWR at its antenna connector. This effect will be more apparent for higher-loss feedlines (e.g., coax cable) and less apparent for low-loss feedlines (ladder line, air-dielectric cables, etc.)

user2338215
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I'm aware of no such risk. A SWR of 3:1 on 7MHz is just as bad as a VSWR of 3:1 on 144MHz. There are other things that change with frequency that might contribute to this misconception, however:

VHF radios typically do not operate at such high power as HF radios. Ultimately, high SWR damages the radio when the final stage is made to operate outside its voltage or current limits. A bigger transmitter means more power to make higher voltages and currents when SWR is high.

Also, low frequency antennas have a narrower bandwidth. If we take a simple antenna such as a dipole and scale it up in every dimension to decrease the frequency, the fractional bandwidth remains the same, but since the center frequency is lower, the bandwidth, in terms of the difference between upper and lower frequencies with an acceptable SWR, is less. Thus on low bands it's harder to keep a low SWR across the band.

Phil Frost - W8II
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