Posted by
Ron D'Eau Claire-2 on
Jan 06, 2006; 2:20am
URL: http://elecraft.85.s1.nabble.com/T1-Antenna-Considerations-tp385235p385247.html
Steve AA8AF wrote:
Alan Chester (G3CCB)(SK) proposed a solution for the high end impedance of a
1/2 wave antenna. Mr. Chester rationalized that there might be some
impedance-friendly length of wire usable for an end-fed antenna that didn't
present the tough-to-tune, high-impedance load on a select set of bands. ...
--------------------------
One way to look at it is that if the end-fed wire is longer than a 1/2 wave
antenna, the extra length is simply a single-wire feedline. Of course, the
'feeder' will also radiate UNLESS one manages to match the impedance of that
wire (roughly 600 ohms for a typical wire) to the feed point impedance. To
do that, you move the feed point for the single wire away from the high
impedance at the end and toward the lower impedance (73 ohms in free space)
that will be found near the center. At some point that wire will see a good
match and it will have no standing waves. No standing waves on even a signal
unshielded wire means it won't radiate (ya' gotta have standing waves to
make electromagnetic fields).
Of course, exactly that was done by the famous Loren G. Windom (8GZ/W8GZ)
and is known today as a Windom antenna. That's the REAL Windom, not the
modern variations using open wire feeders that are really just
off-center-fed doublets. Windom's antenna used a single wire feeder that did
not radiate significantly, but it only worked properly at the one frequency
at which the feed line was matched to the antenna.
Ron AC7AC
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