"How does one accurately measure RF power into an unknown impedance
using the typical wattmeter either internal or external to the rig?"
One most likely does not, nor does one need to. All that is necessary is
that the power is low enough to reduce relay contact damage as they are
hot switched during the tuning process, and the power is high enough for
the SWR bridge (if it it really is an SWR bridge) to detect the forward
and reflected power. I understand that some auto tuners are not tuning
only for minimum reflected. They also look for RF current and RF voltage
phase relation. When voltage and current are in phase the load is
non-reactive. Either way the circuitry does not need a specific power
level, it just needs enough, and to save the relays you want the power
to be not too much.
"Then how could one tune even a manual tuner with an automatic SWR bridge?"
Really we don't need to know the absolute SWR we are just trying to
adjust for minimum reflected. True, if you don't have enough forward
power, there also won't be enough reflected power to tell the tuner it
needs to keep trying for a better match. Once you have adjusted for
minimum reflected, you can check the SWR if you really want to know.
I have a SG-235, which is also a tuner that uses relays to select from a
finite number of fixed inductances and capacitances. It is rated at 500
watts, and can be used with SGC's 500 watt solid state amplifier, the
SG-500. The manual says to use low power (it think they say 50 watts,
which also happens to be the nominal drive power for the SG-500) for
tuning, and to only run higher power levels AFTER a successful match has
been found. It even has a control line coming from it that can be used
to inhibit the SG-500 until the tuner has found a match.
I was using my SG-235 with a Kenwood TS-440. I had a problem with an
interaction between the tuner and the transmitter. The reverse power
protection circuitry on the TS-440 would vary the TX power output as the
reflected power varied during the tuning process. This confused the
SG-235, and it usually failed to find a proper match. Turning the RF
level (I think it is called "CAR" for carrier on the TS-440) down
sometimes worked, but sometimes not. Eventually I solved this problem by
building a 2.5 dB "cantennuator" T network out of a bunch or 2 watt
carbon comp resistors in a quart paint can full of mineral oil. This
also solved another problem: the TS-440 when turned down to the 50 to 60
watts that the SG-500 wanted to be driven by, had a funny soft CW keying
envelope. Adding 2.5 dB of loss makes no important difference receiving
on the lower bands, where we have receiver gain to burn due to high
terrestrial noise levels.
Interaction between the transmitter's reverse power protection and the
auto tuning process is probably less of a problem with most Ten-Tec
transceivers, which have PAs built to handle a lot of reflected power,
and don't have the automatic power reduction with high reflected power
like the Kenwood TS-440. Also Ten-Tec radios tend to maintain a proper
CW keying envelope at whatever TX power they are set to.
To sum it up:
While tuning an antenna tuner, either automatic or manual, it is fairly
common practice to use reduced TX power.
We don't need to know the exact power level to get the tuner adjusted.
We don't need to know the exact SWR while we are adjusting the tuner.
DE N6KB
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