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?

Douglas W Adair wrote:
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?


Any time you double-illuminate the far field you are creating lobes and
nulls. A single omnidirectional radiator is optimal for a moving
station, unless you have means to steer the lobes whilst steering the
vehicle.

They do look bad-ass, but just connect one of them and you'll have
better overall performance (theoretically).

Dave wrote:
Douglas W Adair wrote:
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?

Any time you double-illuminate the far field you are creating lobes and
nulls. A single omnidirectional radiator is optimal for a moving
station, unless you have means to steer the lobes whilst steering the
vehicle.

They do look bad-ass, but just connect one of them and you'll have
better overall performance (theoretically).

You could phase them for favoring the direction of travel, I suppose.
That might be the idea.

On Nov 30, 11:08*am, Dave wrote:
Dave wrote:
Douglas W Adair wrote:
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?

Any time you double-illuminate the far field you are creating lobes and
nulls. *A single omnidirectional radiator is optimal for a moving
station, unless you have means to steer the lobes whilst steering the
vehicle.

They do look bad-ass, but just connect one of them and you'll have
better overall performance (theoretically).

You could phase them for favoring the direction of travel, I suppose.
That might be the idea.

http://nimbusters.org/forum/read.php?board=8&id=647774

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.

"Douglas W Adair" 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?

On Nov 30, 4:40*pm, "Hal Rosser" wrote:
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.

"Douglas W Adair" wrote in ...



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?- Hide quoted text -

- Show quoted text -

I figure this is for an 18 wheeler on CB. In that case the dual
antennas make the patern more omni. Back in the 70s I belonged to a CB
club and we did some test of the effects of auto body styles on
radiation patterns. We discovered body stle and mounting location were
as important if not more so than the antena you were using.

An 18 wheeler with a single antenna mounted on a mirror has a really
ragged radiation pattern. Two antennas makes it a lot less ragged,
still a far way from being omni-directional.


Jimmie

I always wondered whether a short/loaded magbase antenna on the trailer
roof (so it doesnt hit bridges etc) would work better than a mirror mount...

How much roof to bridge etc clearance is there normally?

I would have a thought a DDRR would have been good too but I read
something recently that mentioned performance has never been as good as
expected.

Thoughts?

Cheers Bob


JIMMIE wrote:

An 18 wheeler with a single antenna mounted on a mirror has a really
ragged radiation pattern. Two antennas makes it a lot less ragged,
still a far way from being omni-directional.

On Sun, 30 Nov 2008 08:05:31 -0500, "Douglas W Adair"
wrote:

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?


Not much on a big truck.
Since most of the usable cophased signal is blocked by the cab and
trailer. I've found a single antenna works just as good.

What hurts the system the most, is the factory installed crap.
They generally use the smaller 75ohm cable and use splice after splice
to make it work. I generally get my own cable, bypass their stuff.

One thing to remember in cophasing, cable length DOES make a
difference. If they are not equal, things will be out of whack. Unlike
in a single antenna where length is not an issue.

Oh and for all you loudmouths out there, the only reason they say you
must have x amount of feet, is to sell the damn cable.

In reality, the shorter the cable, the better off you are.

Have you held an FCC license for radio work?
I have.

On Nov 30, 10:02*pm, richard wrote:
On Sun, 30 Nov 2008 08:05:31 -0500, "Douglas W Adair"
wrote:

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?

Not much on a big truck.
Since most of the usable cophased signal is blocked by the cab and
trailer. I've found a single antenna works just as good.

What hurts the system the most, is the factory installed crap.
They generally use the smaller 75ohm cable and use splice after splice
to make it work. I generally get my own cable, bypass their stuff.

Since the feed point impedance of most of those antennas is really way
less than 50 ohms 75 ohm cable may not be the best choice to make a
phaasing harness. Most of the time 50 ohm cable works better

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.

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