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Re: [TenTec] SWR VS Power Loss

To: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: Re: [TenTec] SWR VS Power Loss
From: Michael Tope <W4EF@dellroy.com>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Fri, 04 Jul 2008 11:01:34 -0700
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>

Steve Hunt wrote:

>That table could be very misleading without a lot more info.
>For example, is it showing the total loss, or the "excess SWR loss"?  
>What type of cable is it for? And what length? At what frequency?
>If you believe the text, it's how the SWR "affects the power output of 
>your radio" ! Pretty good radio that's still putting out 33% of its 
>power into a 10:1 SWR load :)
>Steve G3TXQ
>Jim WA9YSD wrote:
>>Found this nice table.  Thought I would pass it along.  
>> Keep The Faith, Jim K9TF/WA9YSD


This table just shows the percentage of incident power reflected and 
transmitted at a mis-matched RF transmission interface. You'll notice 
that the "% of Loss" versus transmitted power (what they call "ERP") 
always add to 100%. This has nothing to do with "excess SWR loss" on a 
mis-matched lossy cable. As you point out, it also doesn't include the 
impact of power-reduction due to final amplifier bias foldback in the 
presence of a high VSWR.  The best way to think of this table is a 
measure of how much power in a short RF pulse would be reflected versus 
how much would be transmitted when that short RF pulse encounters a 
mis-match junction along a very long losseless transmission line. For 
instance, at 6.0:1 VSWR and a short RF pulse say 1KW in amplitude (old 
NTSC TV horizontal sync pulse for instance) hits a damaged antenna on 
transmission line that exhibits a 6.0:1 VSWR at the transmission 
line/antenna interface. According to the chart, 50% of the incident 
pulse power (i.e. 500 watts) would get reflected back toward the TV 
transmitter and 50% would be transmitted into the damaged antenna (i.e. 
the remaining 500 watts).  Not terribly useful for most ham applications.

73, Mike W4EF..............................................

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