Russ Hines wrote:
Some related comments, if you don't mind.
Temperature changes seem to be the biggest "detuner" of largely
mechanical devices like cavity duplexers. We often send our repeaters
off to live in less-than-ideal environments, then expect cavity
input/output impedances to remain as we measured them in the shop?
Don't think so.
I largely disagree. Most modern duplexer designs (within the last 25
years or so) use compensating elements to make the duplexer or cavity
temperature stable. Invar is a nickel-steel alloy that exhibits about
1/10 the thermal expansion as a common carbon steel counterpart. Invar
is used to make the tuning rod - many times it's threaded. The rest of
the duplexer or cavity is usually made of similar metals and generally
thermal expansion occurs across these components equally, resulting in
extremely low frequency drift over its rated operating temperature.
Our in-line power meters, like our trusted Bird 43, do not directly
measure power. They're really voltage meters calibrated in watts at a
specific impedance. That's why they can be fooled into displaying an
erroneous reflected power reading, perhaps lulling us into a sense of
security that the VSWR on the line is acceptable when it may not be.
What? Maybe you would like to have another chance at that one....
Kevin Custer
