|[AMRadio] Windom Antenna|
W7QHO at aol.com
W7QHO at aol.com
Mon Dec 27 20:51:02 EST 2004
In a message dated 12/27/04 2:38:01 PM, k4xm at arczip.com writes:
> Okay, let us try again. I thought the question was.." how can it be
> RESONANT" at more than one place........
It's important here to draw a distinction between the antenna itself, in this
case a single wire cut to a half wavelength at the lowest operating
frequency, and the feed system connected to it. Neglecting details such as end
effects for the moment, the antenna will exhibit resonance at the frequency for
which it was cut and at multiples thereof.
However, we propose to feed this antenna in a novel way, i.e., with a single
wire line. Now, a single wire transmission line will exhibit, under some set
of conditions, a characteristic impedance around 600 ohms. (I've never seen
the derivation of this figure but all the books say so.) Mr. Windom, way
back in the 1920s got the idea that by connecting such a feedline to just the
right point on a halfwave piece of wire would make for a simple, easy to erect
antenna AND FEED system. One would suspect the "right point" would be
somewhere off the center of the antenna near where the radiation resistance also
measured 600 ohms. Haven't read the 1929 article, but would suspect more than a
little "cut-and-try" was involved here. My 1936 RAHB cautions that "Correct
antenna length and placement of the feeder should be checked experimentally
for best results..." In any case, Windom did apparently find a sweet spot and
achieved something close to a flat, non-radiating feedline. And, the design
was used successfully by a lot of guys for a long time.
This is OK for a half wave (fundamental frequency) antenna but what about
operation at 2, 3, 4, etc., times the original half wave frequency? Is it
possible to find a single feedpoint that will be correct (i.e., match the 600 ohm
feedline) for the fundamental AND all harmonics? Haven't read the original
1929 QST article but would be inclined to say no way. My 1936 HB (see above)
agrees and goes on to say that the antenna SYSTEM can be used for harmonic
operation but the feedline will always be radiating along with the antenna.
Fine old classic design, though. Hope to get another one up again someday.
Dennis D. W7QHO
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