In article ,
Jim Lux wrote:

Guys.. this is nowhere near a "two 1/4 monopoles over a uniform ground"
that you're seeing in the handbook.

The antennas are 30cm or so from a big metal box (the tractor), and
possibly in close proximity to a even bigger metal box (the trailer).

Before one starts going on about whether you get any gain from two
antennas 1/8 wave apart or whatever, look at whether it has any
practical benefit in terms of, for instance, filling in nulls.

That potential (and quite possibly real) benefit has already been
mentioned a couple of times in this thread.

So would practical experience. I suspect that there's enough "real"
benefit from the dual whip configuration that it persists.

I strongly suspect that you're right, and I think I've been trying to
make that very point. The benefit to a typical two-antenna truck
setup isn't that it provides a lot of gain (and thus a
non-omnidirectional pattern), because in fact it doesn't (the spacing
is too small). Rather, the practical pattern is going to be closer to
a true omnidirectional pattern than can be achieved by a single
side-mounted antenna alone. The final pattern may actually have
*less* gain in a few directions, but may have shallower nulls and thus
a more consistent overall coverage.

It's one of those weird situations, in which the reason for the
benefit that can be perceived is actually just the *opposite* of what
one may believe at first!

I must admit a bit of scepticism in re Cecil's suggestion to use a
90-degree-phase-shifted endfire array feed... at least, for most car
and trucking applications. Granted, it'd give quite a bit of gain in
the forward direction... but at the expense of a deep null in the
rearward. It'd be great for speaking with the guy a few miles in
front of you... but you'd lose the ability to talk well with the guy a
few miles in back of you. If every vehicle used this sort of antenna
pattern, overall coverage would probably be worse (4 dB of gain in the
forward direction wouldn't compensate for a 10 or 20 dB null in the
rear direction of the next vehicle).

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave Platt wrote:
I must admit a bit of scepticism in re Cecil's suggestion to use a
90-degree-phase-shifted endfire array feed... at least, for most car
and trucking applications. Granted, it'd give quite a bit of gain in
the forward direction... but at the expense of a deep null in the
rearward. It'd be great for speaking with the guy a few miles in
front of you... but you'd lose the ability to talk well with the guy a
few miles in back of you.

You would also lose interference from the rear. Simply
arrange a switched phase-shifting feedline that can aim
the beam forward or backward. It's like a Yagi mounted
on a motorhome.
--
73, Cecil http://www.w5dxp.com