When a transmitter has spurious output and fed into a dummy load, since
the load is resistive and 50 ohms at all frequencies, the SWR will
always be 1:1 regardless of frequency or frequencies applied. When
the same transmitter is fed into a resonant load {antenna} the spurious
energy will be reflected and shown as SWR. If it is related to power,
this likely indicates the transmitter PA or driver is breaking into
oscillation and is generating spurious energy.
What you observe is nothing unusual. Except one needs to find the
reason for the spurious energy related to power. I trust you are
measuring SWR with an external meter. If you are using the internal
meter it has been to known that one or more of the diodes in the SWR FWD
REF circuit has failed. Since power control is derived by the voltage
from the FWD diode and circuit, the PA and driver may be breaking into
oscillation do to "no control voltage". The diodes are D10 and D11 on
the SWR board. My documentation does not identify the type.
Photos are not being processed by the server.
73
Bob, K4TAX
On 10/10/2019 5:07 PM, Gary Follett wrote:
Has anyone else seen this before?
He Corsair II operates perfectly into a dummy load, all bands.
The owner’s complaint was that the radio shows high SWR at high power, a
condition I could not replicate into a dummy load on the bench.
However, when I tried a real antenna, things changed a LOT. I observed exactly
what the owner saw, but only when the SWR was a LITTLE high, maybe 2:1.
These are photos from my spectrum analyzer, full power and about half power.
The radio is pretty clean with 14.6 MHz harmonic about 40 dB down.
It is clear that things get really bad as power goes up and the “SWR” readings
are the result of all of this out-of-resonance signal, which is why the problem only shows up
on a resonant load, not on a resistive load.
I did perform driver bias adjustment and final amplifier adjustment with no
change in performance. Prior to adjustment, the bias for the final was quite
high at 2+ amps, well over the 500 mA specified in the manual. I set it to the
proper 500 mA. Still no improvement.
Gary Follett
W0DVN
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