Hi Tom.
A good example of what a "good counterpoise" is would be to take a looking any
professional level antenna installation handbook. Granted, most of us cannot
install our antenna systems to the same level of expertise taht are demanded
(and
required) by broadcasters, but a strong aim in that direction does wonders for
improving signal. Fer instance, I have heard from some who have a good antenna
and
not much of a counterpoise, who after making only changes to the counterpoise
itself
have experienced a ten fold increase in both xmt and rcv capabilities.
Remember: when a antenna manufacturer suggests a counterpoise, they are usually
speaking in minimums , not the otherwise.
Tom Rauch wrote:
>>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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