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

[Paul Christensen]

>  4) The major issue with SWR at the transmitter is giving the transmitter 
> a load that it is "happy" with. "Happy" means that it supplies full rated 
> power, AND is CLEAN.  Distortion rises when an amp is mismatched to its 
> load and if it's not carefully tuned. Distortion = SPLATTER. That's why 
> God made antenna tuners.

I think it's important to note that a tuner placed at the transmitter is 
performing several functions: (1) Z transformation from one complex Z value 
to another; and (2) creating a reflection mirror for the reflected wave that 
allows for re-combination with the forward wave back to the load.  At the 
load, reflected voltage is 180 degrees out of phase with the forward voltage 
and both the forward and reverse currents are in phase.   The tuner at the 
transmitter will again reverse the phase of the reflection voltage so that 
voltage and current of the reflected wave are in phase with the forward 
wave.

Before solid-state RF amp designs, our old VT amps cared little about line 
SWR, at least below ca. 3:1.  Early link-coupled tuners and later, the 
classic Pi network performed functions 1 & 2 noted above.  Once solid-state 
transceivers began incorporating built-in tuners, they were once again on an 
equal footing with the older VT/Pi designs.   Until L. McCoy and M.C. Jones 
authored articles on their respective Monimatch devices in the mid '50s, ops 
were making efficient use of their antennas and transmitters for several 
decades without SWR meters.  With low loss, moderate Z lines, the SWR meter 
was not needed.   Low-loss lines in conjunction with their link or Pi 
networks is all that was needed.

When I started as a novice in the early '70s, I was mentored by my OM, 
W9EAC.  Although he understood transmission line mechanics, we did not own 
an SWR.  One day, an 8th grade friend of mine brought his Midland Monimatch 
to the house.  We inserted the meter into the line.  The SWR on my 15m Yagi 
was 2:1, but fed with about 75 ft of low-loss coax.  My friend stated 
"You've got problems.  Your SWR is way too high."  From that day forward --  
well at least until I understood transmission line mechanics, I was obsessed 
with SWR, despite my OM's insistence that the 2:1 SWR was not nearly the 
problem my "know-it-all" friend made it out to be.  One day, I was happy 
making 15m contacts.  Then all of a sudden, I was spending more time looking 
at the SWR meter than the S-meter.

I echo K9YC's comments about SimSmith.  For those intimidated by the Smith 
Chart, consider starting with either TLD or TLW software.   Simulating line 
and load conditions is easy.  Then, move on up to Smith charting software.

Paul, W9AC