"Top" wrote in message
.. .
richard wrote in
:

On Mon, 1 Dec 2008 04:37:39 -0500, "Douglas W. \"Popeye\"
Frederick" wrote:

"Dave Platt" wrote in message
...
I like them two at a time--co-phased. Is there any gain
to be had that way
or am I just skin pipe dreaming at the petro?

If fed in phase and spaced correctly, there could be gain
to the front and to the back with a decreased propogation
to the sides. This is usually desirable if traveling on a
mostly straight stretch of highway. I think the spacing
is a little too far apart for use on most cars.

I believe you're correct. This is a "broadside array"
configuration.

Its gain over a single radiator rises roughly linearly
(measured in dB over a single radiator) up to separations
of around 5/8 wavelength. A separation of 1/2 wavelength
gives around 4 dB gain over a single radiator and a very
nice clean pattern (deep null to the sides) - this is the
spacing most frequently described in the literature (e.g.
Kraus, Terman) for broadside arrays. Gain maxes out at
just under 5 dB at a 5/8-wavelength spacing (at the cost
of a small side-lobe). [Figures are from the ARRL Antenna
Book of a few years ago]

Whether it's worth doing for a vehicle-mobile system is
another question. You need more than .4 wavelengths of
separation to get 3 dB of gain (half a nominal S-unit) -
at 11-meter frequencies that's around 14 feet of
separation, which I think not many vehicles will allow.
Perhaps if you're driving a "wide load" transporter
truck?

At 6 feet of separation between antennas you'd have only
around .2 wavelength, which yields less than 1 dB of gain
over a single radiator. Hardly seems cost-effective.

It might make more sense for 2-meter operation... but as
most 2-meter mobile seems to be repeater-based, you
really want omni rather than shaped-beam-down-the-road
most of the time.

There's also the matching issue. Each radiator in the
array will have a feedpoint impedance different than what
would have if used alone. You'll have to take this into
account when designing the phasing harness, and you may
need an impedance-matching network at the combining point
to establish the 50-ohm load that your transceiver
expects. If you don't match properly your transceiver
won't see the load it expects, and may not deliver full
rated power into the load - you could easily lose more
signal strength this way than the array will gain back.
If you do match properly, there will be some amount of
loss in the matching network.

There ain't no free lunch, alas.

Wow, Richard.

This guy seems 7 ****loads smarter than you.

Wonder if he ever "held an FCC license for radio work"?


If i had said it, you would have a field day accusing me of
all kinds of ****.

Since the late 60's i've been working with CB and have done
all kinds of experiments with antennas on a car. You name
it, I had it.

As he pointed out, the big problem with CB is, you need way
much more space than a vehicle offers to truly get any
usable gain from cophasing.

Do you know the wavelength of 11 meters?
if 27 feet, the normal height of base antenna, is equal to
5/8 or 1/4 wave, then what is 8/8 or 100%? Well over 100
feet.

So to get the true proportion for proper cophasing, the
road aint wide enough and neither is the vehicle.

The only reason truckers run two antennas is because it
looks cooler.

The effectiveness of cophasing in a truck is screwed by the
factory installed crap.


You still didn't give any useful information. No surpise since
you have none. For an average of the cb band running 1/4 wave
the antennas should be spaced 54 inches apart. Use a
commercially produced cophase harness if you can find it. Make
sure you match the SWR and you will out do any other mobile
off the front or rear.

Top




Thanks Top!




--
Popeye
"Best thing for him, really. His therapy
was going nowhere," -Hannibal Lector.

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