[TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?

[Roger (K8RI) on TT]

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)