> What's the limiting mechanism and/or failure mode from
running too much
> power on a screwdriver antenna?
I tested several antennas to destruction. The weakest
frequency is in upper HF near 15-6 meters (depends on
manufacturer and style) where the coils in some antennas
have series-resonance. At those frequencies the coil is
fairly easy to overheat in a short time with only a hundred
watts or so. You can find the self-resonant point by
sweeping the unit with the coil almost totally collapsed and
looking for a base impedance that is abnormally high.
At other frequencies, even a few hundred kilohertz off
series resonance, the standard sized Tarheel and High Serria
antennas took 400 watts or so for long periods of carrier.
An MFJ cheapened antenna took less time to fail.
I wouldn't be afraid to run a kilowatt PEP SSB or more,
watching the SWR on a peak reading meter for a sudden spike
as a warning something was arcing. For CW, 500 watts would
probably be OK.
> The 4:1 transformer or matching network at the base?
I never use a 4:1, I always use a shunt coil. I never tested
matching systems, except to say the tiny enameled wire used
to match handles a kilowatt OK.
I expect most antennas would fail by arcing limits for low
duty cycle modes and by heating on high-duty modes like CW.
The failure points I've found were always heat related in
the coil. Away from coil self-resonance near the finger
stock, otherwise in the area of maximum voltage where the
coil is self-resonant.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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