"Channel Jumper" wrote in message
...
gareth;815654 Wrote:

[b]Then run a "shortish" earth braid from the vehicle's chassis to the
antenna.[b]

You are confusing earth ground with a counter poise..

No, I'm not.

To establish a counter-poise, the counter-poise needs to be directly
beneath the antenna.

No, it does not. You are providing the second half of a pendulum

Just hooking some braid to the antenna won't do anything.

Yes it will, taken together with the mass of the vehicle, as I suggested.

On Monday, March 3, 2014 2:28:21 AM UTC-6, gareth wrote:

Just hooking some braid to the antenna won't do anything.



Yes it will, taken together with the mass of the vehicle, as I suggested.

It's possible that connecting to the RV may improve performance
due to the antenna becoming a perverted dipole of sorts.
But as a means of lowering ground losses under a vertical, your
solution will prove to be quite poor.
But that it would actually act like a dipole seems fairly remote
to me.

And there is something that seems to confuse people about
short verticals used on vehicles and such. And I think could
also be applied to short verticals on tripods.
I've fairly much proven to myself that varying the length of the
metal on the ground side of the vertical usually does not convert
it to a different length dipole. Per say..

If this were the case, I would have to modify and re-tune my
mobile whips every time I changed to a different size vehicle,
or added other metal to the vehicle.
If the antenna acted as a dipole, I would expect to need to re-tune
in every change of vehicle or mount.
This does not happen.
These short verticals are still acting like verticals on the ground,
with varying numbers of radials, or metal mass, not perverted dipoles
slightly above the ground.

My mobile antennas are still resonant at the same frequency no matter
what vehicle they are on, big or small, and no change if I add extra
radial wires to the vehicle, or even connect directly to the ocean,
which I have done in the real world when parked next to the Gulf of
Mexico.

What does this tell me? That the metal under a mobile whip is
acting a lot more like a radial system, or even the ground itself,
than the other half of a physical dipole.

So what else does that tell me? That connecting a braid from
a tripod mounted short vertical to an RV is likely to not pan
out too well as far as reducing ground loss under the vertical.
In fact, I predict it to be fairly useless.

On Monday, March 3, 2014 10:51:14 AM UTC-6, wrote:
That the metal under a mobile whip is
acting a lot more like a radial system, or even the ground itself,
than the other half of a physical dipole.

It depends on where it is connected to the vehicle. If connected near the center, the vehicle acts like two opposing radials and radiates mostly omnidirectional. If connected on the rear bumper, it can act like one radial. On my S10 with a rear bumper-mounted 17m hamstick, my signal was quite directional beaming toward the front of the vehicle. I would sit in the parking lot at Intel and aim my S10 mobile antenna system. I often gained an S-unit by aligning the vehicle with my contact. The fact that the S10 was a quarter wavelength long on 17m probably helped.
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73, Cecil, w5dxp.com

On Monday, March 3, 2014 12:52:37 PM UTC-6, W5DXP wrote:
On Monday, March 3, 2014 10:51:14 AM UTC-6, wrote:

That the metal under a mobile whip is

acting a lot more like a radial system, or even the ground itself,

than the other half of a physical dipole.



It depends on where it is connected to the vehicle. If connected near the center, the vehicle acts like two opposing radials and radiates mostly omnidirectional. If connected on the rear bumper, it can act like one radial. On my S10 with a rear bumper-mounted 17m hamstick, my signal was quite directional beaming toward the front of the vehicle. I would sit in the parking lot at Intel and aim my S10 mobile antenna system. I often gained an S-unit by aligning the vehicle with my contact. The fact that the S10 was a quarter wavelength long on 17m probably helped.

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73, Cecil, w5dxp.com

I bet it still acted like a vertical near the ground though, instead
of a dipole low to the ground. For instance, I bet the hamstick
did not require a drastic re-tuning when mounted near the rear vs
mounted in the center.

So I still think in your case, it was acting more like a near
ground mount vertical with a fat radial to the front, rather than a
whip at the rear with the car being the other half of a dipole.

And strictly speaking, I suppose it is. Sorta.. Kinda..

But it's not acting like one in operation. It's acting like any
other short vertical that is near the ground.

I saw a bit of directivity when I had the antenna on the rear trunk
lid of a Monte Carlo in much the same way as your S10. It favored
the forward direction a tad. I don't notice too much directivity
on the trucks, but both have the antenna near the center.

Anyway, as it applies to the tripod vertical, I wouldn't expect
running a braid to an RV as a very effective way to reduce ground
losses below the antenna. And the chances of it pairing up with
the RV to produce a usable "dipole" of sorts are not likely to
pan out. Ground losses will still be high below the whip, and
I bet the RV will more likely resemble a big pile of earth a few
feet away, than a viable radiating element. :/
Unfortunately, the big pile of pseudo earth will be in the wrong
location to be of much help to the tripod mounted whip.

On Tuesday, March 4, 2014 5:22:18 AM UTC-6, Jeff wrote:
Also do not neglect the (considerable) capacitance of the vehicle body
ground to the the real ground.

I was on the top uncovered story of a three-story parking garage made of concrete and steel and the weather was dry.
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73, Cecil, w5dxp.com

On Tuesday, March 4, 2014 10:19:49 AM UTC-6, W5DXP wrote:
On Tuesday, March 4, 2014 5:22:18 AM UTC-6, Jeff wrote:

Also do not neglect the (considerable) capacitance of the vehicle body

ground to the the real ground.



I was on the top uncovered story of a three-story parking garage made of concrete and steel and the weather was dry.

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73, Cecil, w5dxp.com

Makes sense to me. You had a 4 ft wide solid metal appx 1/4 wave
radial system in the forward direction. So what was below the truck
shouldn't have really mattered much. Or at least in that direction.
Which is the whole point of having a dense "radial" system below
a low vertical. And even more critical with a low short vertical.

His is a 7m fishing pole fed with a auto tuner. Would be short on
40 and 80. I bet adding a few top loading wires would help on those
bands, but that's probably out of the question if he can't string up
dipoles and such. Maybe make some "L" spokes to clamp on the top with
a hose clamp. A wire connecting the outer ends of the spokes would be
even better. Would improve the current distribution. Or could add a
center loading coil for 40 and 80. That would help if no top loading.