> I was wondering if anyone out there knows if the Bird 43 wattmeter and
> Coaxial Dynamics peak reading wattmeter work with 75 ohm coax or just 50
> ohm?
The line section has a surge impedance of 50 ohms, and the slugs
are nulled at 50 ohms.
You could re-null the slug at 75 ohms, assuming it has enough
range inside. I have no idea if Bird does this, but I doubt it.
A better choice would be to buy a meter using a normal directional
coupler like the AWM-30, and null it for 75 ohms. The most that
would require is adjusting a trimmer capacitor and a few resistors
in value.
The MFJ-259B and 269 both have a line impedance menu that lets
you normalize DIGITAL SWR readings to any resistance within the
reasonable accuracy limit of the bridge inside the unit. It works at
75 ohms and even with 450-ohm systems. My four-square uses an
unbalanced 600 ohm section that I can tune that way.
I use that function quite often since ALL of my receiving antennas
are 75-ohm systems.
> If they don't work properly with 75 ohm coax, is there anyway to "offset"
> the mismatch to get more accurate readings?
Matching transformers, if you can build them accurately.
73, Tom W8JI
W8JI@contesting.com
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