At 6:37 PM -0500 2001/2/21, Bob Alexander wrote:
>If the amp input SWR is 1.2:1 with the FT847 in the CW mode and has a
>problem in the SSB
>mode, I would think there is a problem with the Yaesu ALC and/or SWR
>monitoring circuit.
>Input SWR to the amp should not change from one mode to the other.
1. The "SWR monitoring" circuit in the Yaesu transceiver, like the one in
my Icom transceiver, almost certainly does not function as intended when
the load is nonlinear -- as the input to a G-G amp is.
2. I don't know why the transceiver's behavior when faced with this load is
different depending on whether the transceiver's mode is CW or SSB, but I
think a plausible explanation is that the mode of operation of the
transceiver's output stage is different, e.g., Class C for CW and AB2 for
linear operation in SSB. Thus, the output I-V characteristic is different.
(I won't say that the output "impedance" is different, because impedance is
really defined only for linear circuits, but it's something like that.)
Therefore the funny nonsinusoidal wave reflected back to the transceiver
from the amp impacts the transceiver differently, and does or does not
raise the monitored "SWR" signal to the critical point in one mode or the
other.
3. Just a thought: Try connecting the transceiver to the amp through
different lengths of cable. Other tricks that some have used successfully
are putting an "antenna tuner" or "matchbox" between the transceiver and
the amp, and putting a high-power pad (attenuator) between them. However,
the right place to put a pad is inside the amp, past the input T-R relay,
so the pad is not in the line for receiving.
-Chuck W1HIS
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