Most amps have an output "range" they will match and are quite happy
within this range. Probably less happy near the edges.
But they could care less whether it's 50 or 75 ohm in my experience. I
have a "T-network" tuner and use the tune function of a VNA calibrated
to 50 ohms.
If your amp can match what it sees, there is very little extra loss.
According to The Handbook a line with 1 db of loss when matched, only
has an additional loss of less than 0.2 db with a 2:1 SWR. With 3:1
it's about 0.4 db additional loss. (fig 20.4 in the transmission lines
section 2013 HB)
Trying to get an SWR of 1:1 (which you don't really need) on that today
with the wind and snowstorm would drive you nuts as I can measure to 4
decimal places. That changes with antenna movement, trees in the woods,
and of course precipitation. (Raging snowstorm)
The point is, I tune for minimum SWR on a standard "T" match. it is only
my method of measurement that is precision and any time the "T" gives me
~1:1 the Rp = ~50 ohms. This precision is not needed and 1.1 to 1.2:1
should be good enough in most cases, but it lets me know if any changes
are taking place.
The auto tuner, or any tuner is only as good as its measuring
capability. If its SWR bridge is reasonably close to 50 ohms, I'd
expect to see the same results. You would need to run these scans with a
VNA using the auto tuner to see just how close to 50 ohms the reference
is set.
Depending on how close to 1:1 you set the trigger level on the autotuner
whether on 50 or 70 ohm line I would think you should get > 1.5:1 R ~ 50
ohms.
73
Roger (K8RI)
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