[Elecraft] Measuring SWR (Long)

[Fred Jensen]

Well, the reflected power is created by a "virtual transmitter" at the 
feedpoint of the antenna and heads down the coax which has a surge 
impedance of Z0 ohms.  It meets the SO-239 at the TX and sees an 
impedance of Z1, the impedance presented by the PA and output filters.  
If Z1=Z0, the power is dissipated as heat in the PA and associated RF 
circuitry.  If Z1<>Z0, some is dissipated and some is reflected, where 
some is radiated, and some is reflected [virtual transmitter again].  Ad 
infinitum, and when forever is over, it is all gone and everyone lives 
happily ever after.

SWR and all the associated measuring equipment and concern with it only 
became an issue when: 1) Coax replaced parallel lines and; 2) Resonant 
output circuits were replaced by solid state amplifiers with 
non-resonant filters.  When I sat for the Extra in early 1956, the only 
question that involved standing waves was one about how Lecher Lines 
could be used to measure transmitter frequency.

It's important today but calculating it hasn't changed.  At 10 W, a 2:1 
SWR will probably work ok.  At 1500 W, a solid state amplifier may not 
be happy with the voltages developed at its output connector.

73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County

On 4/9/2019 5:56 PM, Don Sanders wrote:
> Since no one has answered your question. I will venture a
> supposition that many know but few will speak out.
> And some still remember their CB days and the erroneous
> advertising saying the power is lost.
> Reflected Power flows back down the feed line and is subjected
> to the normal feed line loss. It then is "reflected" back toward the
> antenna, again subject to the feed line loss, where some of it is
> radiated and some reflected again.
> This continues until the power is dissipated. The book "Reflections"
>   has a very good but somewhat technical explanation.
> Therein also is the reference to forgetting about striving for the
> "Holy Grail" of 1:1 SWR.
> And concentrating on getting it low enough that the transmitter will
> supply full power output. Also using the best low loss feed line and
> proper matching when possible of the feed line to the antenna.
>
> Dr. Don W4BWS
>