richard wrote in
:

On Mon, 1 Dec 2008 23:55:02 -0800,
(Dave Platt) wrote:

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.

Ya gots to understand with whom you are trying to
communicate. "Top" is the master know it all who has
absolutely no background in electronics. He just drives a
truck and thinks that gives him the knowledge. You've heard
of "Billy Big Rigger"? You just met the dude. Top just goes
along with what other truckers have said over the years.

I have the actual working experience to back me up with.
The only thing Top knows about CB is how to yack on it.



Dave let's agree to our own experiences.

Now to Richard

You forgot who you are talking to again. Nothing unusual for
you to get things wrong. I don't drive a truck unless you are
talking about a pickup. As far as electronics backround I was
in communications in the Army. I spent a good bit of time in
the field in sitiuations where I had no backup so I had to
know enough to make things work when they broke. I don't mean
simple backpack radios either. Then there was the year I spent
teaching radio wave propagation. Another year teaching
programming small and meduim size telephone switches. In 2005
(well after I had retired) I spent the year assembling boards
to build MRI machines. If you think I have no electronics
knowledge then as usual you brain is no bigger than you little
toe.