[TowerTalk] Force12

[Tom Rauch]

> resistive with a purely reactive content.  The bottom line is it
> doesn't matter.  In a 2:1 environment, 88.9% of your power is still
> radiated by the antenna.

The worse thing anyone ever did to understanding how RF systems 
work was present SWR as "match efficiency" or "percent reflected 
power".

That is absolutely incorrect Jon, unless you happen to be feeding 
the antenna through a large attenuator pad that makes the rig look 
like a 50 ohm dissipative resistive source and you are comparing 
the additional mismatch loss in the situation where feedline loss is 
very high or you have an intentional large 50 ohm attenuator in the 
line.

In a real system, anywhere from zero percent to 100% of the power 
can be radiated with a 2:1 SWR. As a matter of fact, losses in the 
transmission line and the rest of the system can DECREASE with 
an increased SWR!

The tables in the Handbook and other places are only for 
somewhat long lines compared to the wavelength, where there is a 
place for waves "to stand". 

SWR does not tell you at all what the efficiency of a system is, 
because it does not indicate power dissipation nor does it tell you 
anything about the source. It does not tell you the transfer 
efficiency, unless the source appears as a conjugate with 
dissipative resistance comprising the bulk of the loss.

There are cases where system efficiency increases with a 
mismatch, cases where it remains the same, and cases where it 
decreases. It depends on much more than the SWR.


73, Tom W8JI
W8JI@contesting.com 

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