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[TowerTalk] Pardon the imposition

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Subject: [TowerTalk] Pardon the imposition
From: (L. B. Cebik)
Date: Fri, 6 Jun 1997 09:43:27 -0400 (EDT)
I hope this note will not impose too much on those not interested, but had
a couple of inquiries, with troubles getting replies to the senders.

First problem is my own darn fault.  I erased a message and hence lost
George Hiscox' address (and perhaps even the proper name spelling), but
wanted to note some limitations of NEC when modeling X-beams.  NEC 2 hates
corners where the diameter changes and NEC-4 is not much better.  Hence,
the X-beam is very hard to model.  I model them in MININEC, with tapered
segments toward the corners.  The 30-meter X-beam appears to have far
better F-b if the corners are 14.5, 14.5 rather than 13.641, 13.641, with
the same spacing between the tail ends.  Feed Z = 36 + j9 at 30' height
with gain = 9 dBi and direct F-B = 26 dB (with quartering rear side
lobes).  Not a bad small beam for 30 meters.  Can be adjusted
electronically with director loading, and if the feed Z goes down at max
F-B, a beta match works well on the driven element.  Max gain setting
give the lowest Z (down to 10 ohms) while settings that give 50 ohms give
very little F-B.  Hence, the max F-B setting is a good compromise.

John Moriarty asked a question about the latest Communications Quarterly
and the article on elevated ground planes.  Unfortunately, my mail to him
keeps bouncing, so I'm using this route.  The work appears to be very
solid.  My only question is why waste the current between the shortened
radials and the feedpoint by using a loading coil.  They waste current
available for radiation below electrical center as well as above.  For max
radiation, make this section linear, feed at electrical center or off
center (near radials) and use a top hat if there is a max length limit to
the vertical radiator.  That, of course, may introduce mechanical
problems, but it does provide the maximum efficiency.

These notes ought to be cryptic enough to worry everyone except the
targets.  Sorry to use the wide space, and thanks for your indulgence as I
catch up on my errors.



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