Hello,
I just had a chance to read the posts from the weekend and saw some
builders having low SWR on dummy load and high SWR on antenna.
I have had this trouble also. I finally found a way to tune up my
transmitters with out a spectrum analyzer.
It seems that some TX output stages can be tuned for what appears to be
full output into a dummy load, but when you hook a know good and matched
antenna to the TX the SWR is high. This can be checked by lightly
coupling a frequency counter to your dummy load as you are tuning the
transmitter and check that the "bulk" of the RF output is being read on
the counter at the frequency you are tuning for. Sort of a poormans
spectrum analyzer.
I had to do this a long time ago when I built "A PAIR of SLIPPERS FOR
THE HW-8". I was getting good output on the dummy load, but high SWR on
the 40M dipole. After hooking up the counter to my dummy load I saw that
I was tuning the output circuit to 5 MHZ! Some more tuning and tweeking
got the counter to show 7MHZ and some more tuning resulted in full
output on 7MHZ. Then going to the 40M dipole again indicated a low SWR!
Now, whenever I tune up a new TX I couple my counter to the dummy load.
The coupling must be light or the counters input can be overloaded and
give incorrect readings.
I wrote this up in QRP QUARTERLY about 6 years ago. In the IDEA EXCHANGE
column.
Hope this is of some help.
73 es GL, Mike W3TS
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