> How does one accurately measure RF power into an unknown
> impedance using the typical wattmeter either internal or
> external to the rig?
>
> All RF wattmeters that I know of are calibrated using 50 ohms
> as the standard impedance. As you deviate from that 50 ohms,
> the calibration becomes meaningless.
Then how could one tune even a manual tuner with an automatic SWR bridge?
The absolute VSWR readings may not be accurate in a grossly untuned
condition, but clearly they're good enough to clue the operator (or
computer) through trial-by-error what to try next to lower the SWR reading.
As I read the LDG docs (although they're a bit sparse on theory), that's
effectively what they do.
So perhaps the calibration is meaningless in an absolute sense, but as long
as you can see which way to move L/C values you'll get there. I can't tell
form the documents whether LDG uses a system similar to SGC, for example,
but they must at least have an impedance detector because there are relays
which switch in special L's for hi or low Z. You might take a look at one
of the SGC manuals (available on-line, at
http://www.sgcworld.com/PubInfoPage.html, which contain an excellent
description of how it works, even with internal sensors calibrated to 50
ohms.
The Elecraft T1 ATU will tune on 1/2 watt. It's a 20 watt rated tuner, and
Elecraft recommends power reduction to 2-5 watts for initial tune-up. SGC
recommends their relay-switched tuners be tuned at full power.
Maybe I've just misunderstood the question :-)
Grant/NQ5T
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