[AMRadio] watt meter

Donald Chester k4kyv at hotmail.com
Thu Mar 2 16:59:41 EST 2006

>The Best measurement I ever got was using a dummy load of know resistive
>accuracy.  I used a new "Cantenna" from heath kit.  I said "new" because if
>over heated once they can change resistance.  The older one seemed to be

I use a dummy load made of a dozen 600-ohm Glo-Bar resistors in parallel.  
The composition material looks like it is tinned at each end, and plugs into 
clips just like a cartridge fuse.   Not exactly sure the power rating of 
each, but they are hollow composition tubes about 1" in diameter and 18" 
long.  They were new in the box, dated 1945, apparently WW2 surplus intended 
for use as rhombic antenna terminating resistors.  I can load the 
transmitter up to 1 kw output and run that into the load for 30 minutes or 
more, and although the resistors get very warm, nothing looks like it is 
anywhere near the self-destruct point.  I have the resistors mounted 
vertically to produce a chimney effect for convection cooling without a fan.

When cold, the parallelled resistors measure exactly 50 ohms with my Fluke 
DVM.  But if I run the load hot for a while, the DC resistance changes a few 
ohms (don't remember if it increases or decreases), but the SWR meter still 
reads exactly 1:1.

If I want to measure the power output of a transmitter, I load it into that 
dummy load, measure the rf current with a thermocouple meter, and calculate 
using ohms law.

On the air, none of my feedlines look anything near like 50-ohms 

On 160m, I use an outboard L-network to make the transmitter see a 50-ohm 
load, since the el-cheapo Gates is designed to work into a very narrow range 
of 50-70 ohms (much like a ricebox), whereas other BC transmitters of the 
same era were rated to work into 30-600 ohms or so.

If you use an outboard L-network, beware of transmitting into it with the 
feedline disconnected.  I did that twice.  Once when the flexible stranded 
copper lead on my T/R relay failed, and once when I forgot to re-engage the 
antenna switch following a thunderstorm.  Each time, I blew up the rf 
ammeter mounted in the transmitter, wired in series with the rf output line. 
  Apparently, just working into the L-network without a proper load on it 
generates ENORMOUS circulating rf current and blows the thermocouple in the 

I just strapped across the output rf ammeter in the transmitter.  That meter 
needs to go between the last element of the matching network and the 
feedline itself.  It would be nice to be able to read rf power output 
directly using the rf ammeter, but thermcouple meters are too rare and 
expensive to blow up every time the transmitter is accidentally keyed up 
without a load.

The broadcast station where I once worked had a matching network between 
transmitter and tower, and I never remember blowing the rf ammeter.  They 
had a disconnect switch to remove the meter from service when readings were 
not being taken, to avoid lightning jolts wiping out the thermocouple.

Of course, since it was a broadcast station designed to run 24/7,  there was 
no disconnect switch to remove the tower from the output network, nor any 
T/R switch in the line, so the L-network never worked without a proper load.




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