Well Roy (sorry I mis-spelled your name as Ray in a post above), you are not
the first to bring it to my attention, the fact that my manner and methods
are "unconventional", indeed, some have not been no kind and have referred
to me as eccentric! grin
As I type this, I am downloading a demo of your EZNEC program. I will spend
a little time with it and see if I can get the hang of it and, how to
describe the antenna problem to it. Quite possibly it will provide me with
answers, or even (and quite hopefully!) provide me with new questions.
Thank you for your time in responding and, if I have offended you, as some
others have pointed out may be the case, I apologize, I am sincere when I
state that was not my purpose...
Warmest regards!
"Roy Lewallen" wrote in message
...
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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