[GCARC] 2:1 SWR

[Kenneth Bozarth]

>From my experience designing solid state HF industrial RF systems... 2:1
VSWR with no fold-back can produce seriously high dissipation in the output
devices, depending on the phase of reflection (length of coax). The older
rigs using bipolar devices can tolerate it much better. New rigs are using
MOSFETs with smaller die area and higher thermal resistance, so they cannot
tolerate as much thermal stress. Yes, 2:1 is only about 11% loss of power
to the load, but I always opt for low SWR. It's good insurance. The mfrs
are smart in specifying 1.5 or less. It's always a gamble - do you use
speech processing, do you talk a long time, do you use max power, and the
unknown variable- what is your reflection phase? And, besides power
dissipation, there can be voltage stress too, a quarter-wave away from the
high dissipation phase. You are most likely OK with 2:1 but try to get it
lower.

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On Thu, Jun 6, 2024 at 9:46 AM Karl Frank W2KBF via GCARC <
gcarc at mailman.qth.net> wrote:

> To followup the topic of antenna matching/SWR from the meeting last night,
> I
> looked up the recommendations from ICOM and YAESU for two of my HF rigs
> that
> do NOT have an internal tuner.
>
> For the IC-7100, ICOM recommends an SWR less than 1.5:1 and says that power
> foldback will occur with an SWR greater than 2:1.
>
> For the FT-891, Yaesu recommends an SWR of less than 1.5:1 and does not
> mention power foldback.
>
> So Chris AD2CS was correct to question the statement that any SWR less than
> 2:1 is no problem.
>
>             Karl W2KBF
>
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