Ian Jackson wrote:
In message , Ken Bessler
writes

Can anyone tell me why I get a worse SWR at
100w than at 5w? I know the reflected power
goes up but I'm using a cross needle meter here
so I'm referring to the actual ratio.

Which SWR should I trust?

Ken



It's all to do with the knee of the diodes. The rectification efficiency
of diodes drops off rapidly when the level of the applied RF signal is low.

Assuming that the SWR is fairly good, but not 1:1, with 100W the reverse
detector will get a reasonable level of RF, and will be reasonably
efficient. The forward detector will, of course, get lots of RF, and
will be very efficient. The result of this is that you get a pretty
accurate (if slightly optimistic) indication of the SWR.

At 5W, the reverse detector will get less than a quarter of the voltage
than at 100W, and this will make it fairly inefficient - so inefficient
that the diode hardly works at all. However, even though the forward
detector will also get less than a quarter of the 100W voltage, this
will still be sufficient for it to be reasonably efficient. As a result,
you still get a good reading for the forward power, but essentially
nothing for the reverse. As you are not really seeing the reflected
power, the SWR looks decidedly optimistic.

Ian.
One other possibility, could be mismatched diodes simple test is to reverse
your swr meter (The Antenna to the Transmit connector, the Transmiter to
the Antenna connector, and see if you get the same results. this should at
least test the knee of the "square law response" of low level voltage from
the diodes . for a dual , crossed meter, this jist is to make sure that
the pointers (opposite) are at the same relative positions as measured
in the correct direction, as obviously, the swr curve on the meter will
not be valid-- Do this on both low power, and high power. Tests not
only the diodes, but the coupling links in your swr bridge. Jim NN7K

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