[Amps] SWR and amplifiers

[Jim Thomson]

Date: Wed, 24 May 2017 23:12:54 +0000
From: Bill Turner <dezrat at outlook.com>
To: Amps group <amps at contesting.com>
Subject: [Amps] SWR and amplifiers

<There is something I don't completely understand about the effect of
<SWR on an amplifier. Perhaps someone here can clear it up?

Let's say my SWR meter indicates 1000 watts forward and 100 watts
reflected. 

1. Does this mean the actual radiated power from the antenna is 900
watts?

2. How much power is the amp actually putting out - 1000 watts or just
900? 

2. If the answer to #2 is only 900, why does the SWR meter indicate
1000? Is that a false reading caused by the actual 900 added to the
100 reflected? If so, why does reflected and actual power add to
produce a false reading?

After 60 years in ham radio I suppose I should know this stuff, buy I
must have been absent that day.  :-)

All comments appreciated. 

73, Bill W6WRT

###  Forward power minus  reverse power = net power.  The amp is only putting out
900 watts.   It would also being putting out  900 watts  into a dummy load. 

##  Into real high swrs, the forward  reading is sky high..and bogus.   The reverse reading 
with high swr will also be real high.  Again,  forward power minus reverse power = net power.

##  Even Bird, CD etc, will tell you to subtract the reverse power from the forward power to
obtain the real power output.  Commercial setups will do the same thing to calculate true power.

##  at least this is the results you see when using a tube PA..with a PI net that will  tune into a high swr. 
A SS pa  will no doubt throttle back with higher swrs. 

##  My old  drake  T4XC  tube tx, put out  100w.   Into a high swr, would show 190 watts forward....and  90 watts reverse.
Net power is just 100 watts....the same as what I get into a dummy load.   Blatantly obvious that the T4XC is not capable of 
putting out 190 watts.   Esp when its only running 200 watts  dc input.  

##  back in the old heath HM-102 wattmeter days, the heath hm-102 would indicate forward power and swr...but no reverse
power.  Folks operating into higher swrs would all see abnormally high forward power readings.

##  The array solutions power master series of digital wattmeters has a provision to display either ..forward power...
or... net power.   Set to net power, it always reads true power output....  regardless of swr. 

Jim   VE7RF