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[TowerTalk] Force12

To: <>
Subject: [TowerTalk] Force12
From: (Jon Ogden)
Date: Thu, 4 Oct 2001 11:22:34 -0500

After thinking about it, you are correct.  Obviously, power reflected from
the antenna, will travel back down the line.  Unless the source is purely
dissipative as you say, some power will then be reflected from the source,
travel up the coax to be reradiated again.

So yeah, you are correct.  The percentage numbers are useful, but they don't
tell the overall efficiency of the system.  It is correct that in a 2:1
match that 11.1% of the incident power at the antenna is reflected back to
the source.  That's all it says.  Some of that reflected power will get back
to the antenna where it will then be delivered to it and be radiated.


Jon Ogden
Sales Engineer
Cain-Forlaw Company
847-202-9898 (Voice)
847-202-9896 (Fax) 

> -----Original Message-----
> From: Tom Rauch []
> Sent: Thursday, October 04, 2001 11:10 AM
> To: 'Ed Wolf'; Jon Ogden
> Cc: TowerTalk
> Subject: RE: [TowerTalk] Force12
> > 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

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