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.