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

[Jim Brown]

On 12/13/2013 3:32 PM, kr2q at optimum.net wrote:
> Interesting comments.
>
> Contributor A says the SWR will be 2:1 while Dan says 1.5:1.
>
> I agree with Dan.
>
> Contributor B says, "What if the impedance at the antenna is only 33 ohms....
> and you have a quarter wavelength of 75 coax."  Then...Oh horrors.
>
> Well, don't look now, but if the feedpoint is 33 and the coax is 50, I calculate that to be the same VSWR as if the feedpoint were 50 and the coax were 75.  No?  Nobody worries about the former, do they?

N6RK and others have described worst case scenarios.  To paraphrase a 
hero of mine, the late Richard Heyser, trying to describe an antenna 
system using only SWR is like trying to write poetry with only one word 
in your vocabulary.  I have three suggestions. First, pull out your copy 
of the ARRL Handbook and STUDY the section on transmission lines to 
learn how transmission lines work. Second, STUDY my tutorial on 
transmission lines, on my website. k9yc.com/publish.htm  Also STUDY the 
pdf about using SimSmith to design antenna matching networks. It talks 
about these specific issues. Third, download and learn to use Sim Smith, 
then use it to help you learn this stuff. "Study" is in caps to 
emphasize that this is not a casual read -- you've got to work at it.

Some fundamentals.  1) SWR causes increased losses in the line, but with 
the small mismatches discussed here. the increased loss is VERY small. 
2) ANY transmission line will transform any load that is not a perfect 
match to some different load Z, depending on the load Z, the line Z, and 
the line length -- UNLESS the line is some multiple of half wavelengths 
at the operating frequency, in which case the line only adds loss. 3) 
The Smith Chart is an excellent tool for understanding and predicting 
the contribitution of the transmission line to any specific situation IF 
you have the data to plug into it. 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'm feeding my 3-el SteppIR with 350 ft of 7/8 50 ohm hard line, but I'm 
using 1/2-in  75 ohm CATV hard line to feed monoband antennas for 20, 
15, and 10M. All of the 75 ohm sections are tweaked (using an MFJ 259) 
to some multiple of half wavelengths, and I've added short lengths of 50 
ohm cable on both ends to finish the runs. All result in a nice match 
and pretty low loss (SimSmith computes the loss).

73, Jim K9YC