Dave Platt wrote:
In article ,
Cecil Moore wrote:

IF the cophased antennas are less than 1/4 wave apart,
there is virtually no change.

I love it when you make an ass of yourself.

The ARRL Antenna Book says that with 1/8WL spacing,
one can achieve 4.1 dB gain with a high F/B ratio.

Cite, please? For which antenna configuration and phasing?

I believe that the high (4.1 dB) figure you are quoting is for an
end-fire array, with the two antennas being fed 180 degrees out of
phase. Good gain, but somewhat tricky to feed and match due to the
low feedpoint impedance and the potential for high losses.

In a truck-antenna situation this would require placing the antennas
in a front/back arrangement, not side-to-side. I'm told that this is
rarely feasible.

The usual two-antenna truck arrangement I've seen is with antennas
side-to-side (one on each rear-view mirror), fed in phase. This is a
broadside array, not an end-fire array.

From all I can see (ARRL Antenna Book, Kraus, Terman), a two-radiator
in-phase broadside array doesn't start to achieve significant gain
(and pattern non-circularity) until you have at least 3/8 wavelength
of separation between the radiators. A 1/4-wave separation yields
only around 1.1 dB of gain, which (by my calculations) works out to
about a 15% increase in useful range in the preferred direction.

My book's at home, but my recollection is that you can't get 4.1 dB of
gain out of a two-radiator in-phase broadside array until you have
more than 1/2 wavelength of distance between the radiators.


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.

Modeling (either computational or on a range) would answer

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