> Antenna is an 80/40 dipole with no balun, fed with some
vintage 75
> ohm KW twin lead, with both ends of the twin lead tied
together and
> fed as a long wire on 160m. The tuner has a good
counterpoise on it
> (roughly 140 feet of 6 ft high wrought iron fence that
runs around
> the front of my house).
You are using the term "good counterpoise" loosely aren't
you Jim?? ;-)
Every single milliampere of current flowing into the antenna
must be matched by equal current flowing into a counterpoise
of some type. That means if the feedline connection at the
tuner had three amperes of RF current, three amperes must
flow to some type of counterpoise.
If the common mode terminal impedance of the fence was 30
ohms J0 and it was the only ground, and if you had three
amperes flowing into the antenna, the chassis to "ground"
voltage on the tuner would be 90 volts.
It's only when counterpoise common mode impedance is zero
ohms that current does not try to flow though your shack
equipment to power lines or whatever other "ground paths"
the RF might find.
This thing tunes nicely, and indicates a nice
> low VSWR at the beginning of transmission, but the SWR
rises, and the
> lights start dimming when I key it (indicating more power
supply
> current), about 15 seconds into the transmission. I'm
seeing this now
> with a Ten Tec Herc II at only 500 w, but I've also seen
it with my
> Titan.
It could be impedance of the fence is changing as RF heats
poor dielectics in the fence mounting. It could be
something in the tuner heating. It might be something in the
shack heating from the RF trying to flow back through wiring
and gear to the power lines and everything else connected at
the shack to your gear.
> Any ideas what's going on? The fixed capacitors in the
tuner high
> voltage types, but with significant temperature
coefficient. Are they
> heating and changing value?
They can. It is almost impossible to get high voltage high
current caps with NP0 (negative positive 0, not the letter
O) temperature characteristics. The higher the capacitance
the higher the negative temperature coefficient. This means
high value doorknobs are very susceptable to drift. Most of
the ones I've seen actually have a significantly higher
temperature drift than the manufacturer claims!
> The other possibility is that the loading coils are arcing
or
> drifting as they heat. They are pretty near each end of
the antenna.
That could be, but generally air wound coils don't change
value nearly as much as capacitors do with temperature
change. How long does it take to settle back to normal? Are
the caps in the tuner getting warm? Any temperature change
will affect capacitance in high value ceramics unless they
are NPzeros.
73, Tom
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