On Mon, 16 Nov 1998 12:49:43 +0000, Pete Smith <n4zr@contesting.com>
wrote:
>I know that antenna has a 24-foot boom, and the elements are also longer
>than on the EF-240S, but I was surprised at the difference. W9LT says that
>there has been some empirical work done that says lumped-constant loading
>like Cushcraft's is inherently superior to linear loading. I'd like to
>know more, and also wonder what the gurus of Towertalk-land think of this.
Just my humble opinion, having spent some considerable furtive time on
(attempts at) modelling linear-loaded antennas. (Sometimes my desk has
been covered by furtive.)
The lumped constant antennas *model* well, eg the existing programs
cope well with the concept. What is specified on paper is what shows
up on the tower. Huge advantage to the designer. Not necessary
indicative of final results (what will we be saying in 2003?). LL
design to date has the following problems (presumably soluble with
programming advances):
1) The modelling of LL is problematic at best. W4RNL has pointed to
the NEC4WINVM (brand new windows mininec implementation with
essentially unlimited pulses) as a possible improvement over known
limitations on other programs. This is still beta. I have a copy and
am playing with it as I have time.
2) Given variances in dimensions, front to back varies a lot quicker
than gain. The LL antennas have the problem of not having that
precision modelling to get the keen F/B. This will evaporate as soon
as someone nails it down. A lot of LL antenna design is still cut and
try, up and down the tower. Absent serendipity, the last nasty 5-10 db
of F/B awaits modelling that works.
3) Using a first try of NEC4WINVM, and a single LL element (1 foot
separations between the folds and 1" tubing throughout), variation in
height from 30' up produced a 70 khz variation in resonance at 40m,
with odd 1/4 waves more or less opposite even 1/4 wave heights. This
was over average ground. Although the driven element can be tuned at
height, a 70 khz variation in a reflector or director can make large
changes in F/B, and is not easily measured/adjusted once the antenna
is in the air. 65' up is very different from 95' up, and may account
for some of the installation vagaries reported here.
More to follow on this as soon as the kitchen upgrades are completed
(priorities, priorities...).
The above needs to be checked out for different styles of LL. It also
needs its priciples understood, formulas invented, and such
implemented in modelling programs. It is certain that stacking needs
to be examined, as well as proximity to other antennas.
It may be that directors/reflectors, and perhaps DE's as well, have to
be tuned separately for situation before being attached to the boom,
with a set of resonance settings for each element supplied by mfr with
variation of height and certain situations.
As always, above subject to peer review.
73, y'all.
Guy
Guy L. Olinger
k2av@qsl.net
Apex, NC, USA
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