The use of "wire length" to describe the antenna impedance made it
diffiicult to understand what you were asking, and certainly deflected
my thinking. If you had only said that the coil had the inductance
necessary to make the antenna half-wave resonant (or antiresonant), it
would have been more clear. On re-reading it, I see that it's what you
meant, and it's what you did say in a slightly unconventional manner. So
that gives enough information for at least some general answers to your
questions:
to base of helical wound coil) is 1/4 wavelength, what would the
radiation
pattern of such an antenna be?
Would it favor the pattern of a 1/4 or 1/2 wave antenna--or, would the
pattern be a compromise between the two--or, would the pattern be
totally
unrelated to either?
It would have a pattern very similar to a quarter wave vertical, not a
half wave.
What could I expect the impedance of such an antenna be? Would the
reactance be capacitive or inductive?
The resistance would be high, on the order of a couple of thousand ohms
or more. If the coil resonantes the antenna, the feedpoint reactance, by
definition, is zero. However, a slight deviation of the frequency from
resonance will result in a fairly large reactance. It will be inductive
below resonance and capacitive above.
What would be the best way to
provide
a match to 50 ohm coax from such an antenna?
Connect one conductor of the feedline to the antenna base, and tap up
the helix with the other until a good match is achieved. This requires
that the antenna be resonated with the coil.
What software is available to model such an antenna?
EZNEC is able to do it. The demo program could be used to make an
approximate model with the inductor modeled as a number of lumped loads
along the wire. With the standard or plus programs you can model the
inductor directly as a wire helix. I develop and sell EZNEC, so I'll
leave it to others to recommend other programs -- I believe there are
several which can do the job.
Roy Lewallen, W7EL
John Smith wrote:
OK, if you notice in my post wire length is in quotes, i.e. "wire length."
In the example, it is a given that this length resonates at 1/2 wave along
with the combined length of the whip (hence, electrical 1/2 wavelength.)
So, your post, while presenting individual points and your opinions, in
general--seems to answer very little, if any, of the original questions
raised--and certainly cannot be accused of containing any specifics.
The diameter IS stated (1"), the wire length is the lenght of the wire minus
28 inches which is necessary to be a resonate 1/2 wave, AND this is divided
over (1/4 wave at 10 meters - 28 inches.) to arrive at the coil length--the
pitch is calculated from the coil length ((1/4 wave @ 10 meters) - 28
inches) and the wire lenght divided by pi (number of turns) spread out over
the coil ENTIRE length. And, of course, coil length plus whip length is a
1/4 wave.
If you refer back to my original post, you will see all of this IS there...
However, critical analysis is a wise beginning of any quest for answers.
Regards
"Roy Lewallen" wrote in message
...
If you wind, say, 100 inches of wire into a coil, it doesn't act the same
as 100 inches of straight wire. And no two different coils wound from 100
inches of wire will act the same. A coil's radiation characteristics are
very nearly the same as for a wire the outside diameter and length of the
coil, not the length of the wire it's made from. And the inductance of the
coil is dictated by the length, pitch, number of turns, and diameter of
the coil, not the length of the wire it's wound from. So your question
can't be answered unless you tell us the diameter of the coil and any two
of length, pitch, and number of turns; or the length and diameter of the
coil and its inductance at the frequency at which it's being used.
Roy Lewallen, W7EL
John Smith wrote:
My question(s):
For an example, on the 10 meter band:
If I take a 28" whip and mount it at the end of helical wound coil (wound
on
1" diameter form), where the "wire length" of the coil, PLUS, the length
of
the whip (28" + coil wire length) is equal to 1/2 wavelength (electrical
length)--BUT, the overall physical length of the antenna (top, tip of
whip
to base of helical wound coil) is 1/4 wavelength, what would the
radiation
pattern of such an antenna be?
Would it favor the pattern of a 1/4 or 1/2 wave antenna--or, would the
pattern be a compromise between the two--or, would the pattern be totally
unrelated to either?
What could I expect the impedance of such an antenna be? Would the
reactance be capacitive or inductive? What would be the best way to
provide
a match to 50 ohm coax from such an antenna?
What software is available to model such an antenna?
Thanks in advance,
warmest regards
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