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[TowerTalk] V-beam Gain and R+/-jX

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Subject: [TowerTalk] V-beam Gain and R+/-jX
From: (Mark Shaum)
Date: Fri, 18 Jul 1997 03:05:16 -0500
> Don't know how he came up with 584 foot length back in
> middle 50's,  but it models well.  Per AO,  these are the
> results of Lloyd's array:
> Freq  3.510   3.795   7.005   14.005  14.195  18.110  21.015  28.050
> Gain,         6.4     7.14    10.4    8.9     8.9     10.5    9.9     9.9  dBi
> Zr    283     165     176     522     2985    414     405     865  ohms
> Zi    -j1144  j69     -j371   j752    -j1073  j534    -j872   j869  ohms
> SWR   12.4    2.8     4.46    4.19    7.51    3.23    6       4.14 :1
> @450 ref

In general, long wire arrays, Vee's, Rhombics, etc
(unterminated varities) are liable to show large swings in
reactance (and sometimes resistance) as you sweep over a
band, as your simulation indicates.  This is why tuned open
wire feeders are still the feed of choice, rather than to
come up with some 'magic' length that will permit some sort
of balun at the feedpoint with coax feed.

Terminated V-beams or rhombics are another animal.  These
can be designed to present a much more 'level' load over a
2 to 4:1 frequency range.
With today's automatic antenna tuners, the hassle of
finding non-reactive terminating resistors of sufficient
power level might be overcome by the convenience of 'no
manual tune' operation.  
If you don't need the 'auto tune' functionality, I'd
strongly suggest locating or building a tuner designed to
drive balanced lines, like my Johnson Matchbox tuners.  No
balun/core heating/arcing to be concerned with.  

Big Vee's and rhombics are neat.  Back in the early 70's,
we strung a (roughly) 600 ft. per leg rhombic between the
two engineering buildings and the physics buliding at W8UM,
pointed at Japan.  I don't recall which contest it was used
in, but it was nice to have a continuous pileup of JA's
that the majority of other folks couldn't even hear!  That
antenna came down quickly, as it was a 100% illegal
installation, but boy, did it work, with an average height
of 100 ft or so!

Mark Shaum  K9TR
Central Illinois Grid EN50

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