On Sun, 12 Nov 2006 22:05:38 -0500, "C. J. Clegg"
wrote:


You guys sure are way beyond me in antenna and trap theory. :-)

I probably neglected to mention earlier, but this antenna is for portable
operation in the field on 75 and 40 (none of the other bands are needed),
so it needs to be easy to put up and take down and easily transportable,
also fairly rugged so things like airwound coils waving in the breexe with
ceramic insulators inside the coil probably won't work; instead the coils
will probably have to be tightly wound around small lengths of PVC pipe
and lacquered in to place.

I need to build several of these antennas (at least 10, I think, perhaps
more), and so $16+ each for doorknob capacitors from places like
rfparts.com isn't likely to be practical. That reason, along with ease of
construction (and now ease of design with programs like Hamcalc around) is
why I have been leaning towards coax traps.

The bandwidth (of the traps or of the antenna itself) is not important and
so high Q and narrow bandwidth is OK.

Power levels will be low, never more than 200 watts and probably never
more than 100 watts, and often around 10 watts.

If you guys say coax traps are a sub-optimal approach I'll take your word
for it.

But, the things you all are saying about traps coming with a lot of
baggage that no one ever thought of is making me nervous. :-)

It's not exactly a new concept, trap antennas have been used with great
success for what, 80+ years now?

If I forget the coax trap idea and go with a resonant coil-and-capacitor
approach on, say, a 1.5" coil form (which Hamcalc seems reasonably happy
with after warning me that 2" was too big and 1" was too small), what
should I look out for?


When you make your mind up about the MAXIMUM operating power level,
you will be in a better position to finalise the design. Capacitors
are likely to fail if you exceed their voltage at all, not for 90% of
the time, or 10% of the time, but at all, and in microseconds.

If you design a 75/40m trapped antenna, and place the trap resonance
away from 40m band (in fact any other operating frequencies), you will
reduce the voltage impressed on the trap. At low power levels, you may
well be able to use a capacitor or stack rated for around 1000V which
will come a lot cheaper and a lot lighter than a doorknob.

But, as Roy has told you, they don't fall into place. The question is
whether "hamcalc" will allow you to design with sufficient freedom and
rate the operating voltage on the traps. The technique of modelling
the proposed design will reveal the voltage expected across the traps,
so long as your estimates of the trap parameters are sufficiently
accurate.

It might be safer to copy a published design rather than looking for
design tools that might not be rock solid.

I cannot offer you a proven design, the thing that I use is pretty low
tech, it is an 80m dipole with insulators in the middle of each leg
and wander leads to switch between a 40m half wave and 80m half wave.

Owen
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