In , "Leland C. Scott"
wrote:
"Frank Gilliland" wrote in message
.. .
Forget about all that antenna-modeling software crap and build a
simple field-strength meter. Then go measure it yourself because
that's how it's done in the real world,
Have you personaly done so for the setup in question?
Over the years I have tested many different antenna installations,
both on my own vehicles and at customer request. Here are a few that I
remember:
Just last month I tested a 4' helical mounted on the center of the
roof of a pickup. It showed a little gain to the front and rear, kinda
like what people are trying to achieve with dualies.
Another was a dual-antenna setup on a tractor cab. I don't remember
the make but they were short, center-loaded whips on the mirrors.
Almost no signal to the rear with or without the trailer, moderate
signal to the front and sides.
On the same truck (at a later date) was mounted a single 66" whip from
RS on the passenger-side mirror. This was goofy: it was generally omni
with a little gain to the front-left and right-rear, and there was
-no- expected dead-zone to the left-rear (possibly due to the antenna
height). Pretty good antenna! As far as I know he still uses it.
Tested dual 9' whips on the bumper of a very sweet GMC pickup (1-ton
custom job, diesel, fording package....the works!). Pattern was almost
perfectly omni. EXCELLENT SIGNAL STRENGTH!!!
I also tested my own truck with different antennas. The mount is
centered on the roo-guard. It almost always shows a slight gain to the
sides, but is generally omni.
I have tested more and with different types of radios (lots of VHF),
but I can't remember all of them offhand, and I don't feel like typing
all day. I should mention that I chose the location for the antenna
mount on my truck (on the front at hood level) because it was the
location with the best RF ground, as tested with my GDO. This is the
case for -my- truck. I wouldn't extrapolate that for any other
vehicle. In fact, the other day I was going over the Chevy (S-10) with
the GDO looking for a good spot for an antenna mount, and the spot
that works so well on the Dodge is -not- a very good RF ground on the
Chevy. No antenna modeling software can predict something like that.
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.
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, 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. Oh well, to each his own. As for me, I'm going to try dual 9'
whips on the rear bumper of the GMC.
and that's how the page at
Landshark's link came up with the radiation pattern for the
bumper-mount antenna. A similar pattern can be found in almost any
radio-communications handbook that covers mobile antennas.
The antenna simulation software shows the same thing as single bumper mount
antenna pattern as on the page Landshark posted. That's why I question the
omni pattern for a dual antenna setup. In fact Frank you can look up the
pattern for such a setup, dual antenna, in a copy of the radio engineer's
handbook, and I'm surprised you haven't since you know about it. More than
one Ham has modeled some commercially manufactured antennas and discovered
they don't perform as the ads suggest. When the manufacture was confronted
with the results they modified their claims. The software modeling approach
works or people wouldn't waste their time with it. In fact more antenna
manufactures are doing it since it saves a lot of screwing around making
error prone measurements with a field strength meter, and that's how they
are doing it now in the real world.
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.
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