In article ,
Top wrote:

Cophase being omindirectional? You need to do some more
reading before you try to correct anything.

The directionality of a broadside array (with the two radiators fed
exactly in phase) depends very strongly on the separation between the
two antennas. For separations of 1/4 wavelength or less, there's very
little directionality - the pattern is very close to omnidirectional.

Every dual-antenna truck setup I've seen has been a side-by-side
mounting (e.g. one on the left mirror and one on the right), and the
harness feeds them both in-phase. I've been assuming that this was
what was being meant by "co-phase".

If so, I stand by my statement that two CB antennas, fed in phase
through a co-phase harness (i.e. no phase difference between the two),
and separated by only 54 inches, produces a nearly-omnidirectional
signal. The two antennas need to be further apart, before the pattern
becomes significantly directional.

Take a look at the NEC plots at http://www.cosjwt.com/index.php?a=20
to see... the 4.5-foot separation model produces a pattern which is
almost circular. There is little gain towards the front and back, and
very little loss off to the sides. These plots seem to jibe well with
other references I've read (Terman, Kraus, and the graphs in the ARRL
Antenna Book).

The other alternative is an end-fire array, with the antennas fed
signals of opposite phase - with these then there can be significant
directionality even with close spacing of the antennas. In a
truck-antenna system, this would require placing the antennas one in
front of the other, separating them by several feet, and inverting the
phase of the signal sent to one of the two antennas (perhaps by having
the feed coax to one antenna be 1/2-wavelength longer than the other).
You could get several dB of gain this way... but the close spacing
will cause the antenna feedpoint impedance to drop a lot, and some
form of matching network will certainly be required to keep the radio
happy and develop maximum power from the transmitter.

The two bottom plots on the site I mentioned above, show the effect of
feeding the antennas with signals of different phase. In these
examples, the pattern is being skewed off to one side - the difference
in feedline length is converting the antenna from a broadside array to
an end-fire array. With the right amount of phase shift, you end up
with a cardioid pattern, with a broad lobe in one direction and a very
deep null in the other.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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