In , "Leland C. Scott"
wrote:
"Frank Gilliland" wrote in message
.. .
BTW, the easiest way to do a pattern test is to park the meter with a
spotter a couple hundred feet away, drive the test vehicle in a tight
circle, stop every ten degrees, key up and transmit the heading. It
takes all of ten minutes, give or take, depending on how fast your
spotter can read the meter and write down the data.
My reason for asking is the pattern is of course 3d. Doing an elevation
slice through the pattern I have seen extensive variation in the pattern as
the elevation slice is moved up and down in the z-axis when multiple
elements are involved. Normally the main lobe in the field pattern is not at
90 degrees to the vertical antenna, but tilted up by some number of degrees.
Standing at ground level a few hundred feet away doesn't really give you a
good idea of the of what the field pattern looks like.
It gives a good idea of the field pattern where it matters. How many
aeronautical CBers do you talk to in any given year?
And I still don't understand the desire for front/back gain on a
vehicle. Unless you drive most of the time on the long, straight
highways of the desert and plains,
Long haul truckers spend a lot of their time driving over long sections of
straight highways.
I think that's what I just said.....
a directional pattern isn't going
to do much good at all, and what little bit gain you can get from a
directional pattern won't amount to anything you can hear from the
speaker.
Tell a hard core Dxer that a db or so difference don't matter and see what
happens. They will swear up and down it does.
And some people still claim to see Elvis at shopping malls.
Oh well, to each his own. As for me, I'm going to try dual 9'
whips on the rear bumper of the GMC.
A buddy of mine used a 4-point mag-mount on the roof of his car to hold a 9
foot whip when he was on 11m years ago. He claimed around 100+ miles with a
standard radio.
I have pulled in signals from just about every distance between here
and Mexico with my homebrew base vertical, but propogation just isn't
very reliable beyond 5 or 10 miles. Regardless, I have no intention of
mounting an 18' stick on the truck. I think I'll stay with the idea of
dual niners on the rear bumper. It should make the most out of the
range that's available, and I'll also be able to test some of these
dual-antenna theories.
Antenna modeling software is a great tool for learning theoretical
antenna design. But unless the software was written by a team of grad
students at Cal-Tech and runs on Big Blue, it cannot possibly account
for all the variables involved. It is not, and should not be used as,
a substitute for actual field measurements.
I think if you contact Mr. Cebik, W4RNL, you will find he does both, model
and test. Seems his models predict real antenna performace rather well. If
he didn't then there would be no way to advise others about areas that
require care when modeling atennas. Also Roy, W7EL, has a lot of experience
too.
I am familiar with W4RNL, and he is very good with the software. But
if you haven't noticed, he tends to play with software much more than
hardware. But if modeling software had the capability to accurately
predict the performance of any given antenna then there would be no
reason to test them in the field. Yet he -does- test his models, which
is indicitive of the fact that the software has not evolved far enough
to stand on it's own, and he evidently recognizes this fact.
I worked with Mr. Cebik on a Turnstile matching idea. As he pointed out in
an article in QEX the schema has the draw back of producing a gain variation
of around 2 db from perfect omnidirectional. While not a big deal it can be
improved by a small adjustment to the length of the phasing lines that do
the impedance match at the expense of a small increase in SWR. The
analytical solution was confirmed by the antenna simulation software, which
showed an improvement in pattern circularity and equal currents in the
antenna elements.
Now this is what I'm talking about.... some people just can't sleep at
night knowing that their radiation pattern is not a perfect circle (or
whatever desired pattern they have drawn in their mind). Smoothing the
dimples of a turnstile doesn't add any gain, it just redistributes it
at the expense of a little ERP. How (or why) is that even worth the
effort for such a cheap and easy antenna?
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