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.

--
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 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.

Actually 135 degree phasing and a cardioid pattern.
180 degree phasing results in a figure-8 pattern with
3.3 dB gain.

Was out of town during the holidays and didn't realize
that the earlier statement was limited to a side-by-side
configuration.

In a truck-antenna situation this would require placing the antennas
in a front/back arrangement, not side-to-side.

Yes, but what better place than the top of a metal
trailer? Years ago, I helped a CB friend in AZ install
one on his motor home with about 5 foot spacing if I
remember correctly. Measured front to side ratio was
huge.
--
73, Cecil http://www.w5dxp.com

On Dec 4, 7:11*am, Cecil Moore wrote:
Dave Platt wrote:
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. *

Actually 135 degree phasing and a cardioid pattern.
180 degree phasing results in a figure-8 pattern with
3.3 dB gain.

Was out of town during the holidays and didn't realize
that the earlier statement was limited to a side-by-side
configuration.

In a truck-antenna situation this would require placing the antennas
in a front/back arrangement, not side-to-side.

Yes, but what better place than the top of a metal
trailer? Years ago, I helped a CB friend in AZ install
one on his motor home with about 5 foot spacing if I
remember correctly. Measured front to side ratio was
huge.
--
73, Cecil *http://www.w5dxp.com

That would be some pretty short antennas on top of a semi trailer.

I had an arrangement like that on a school bus turned RV many years
ago. It worked great until I found a low underpass in Waycross Ga.


Jimmie

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.

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

Jim Lux wrote:
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.

The theory is that for mobile CB, one wants radiated energy
distributed up and down the highway, not to the sides.
--
73, Cecil http://www.w5dxp.com