Dale Parfitt wrote:

I wasn't talking about skip either- the majority of V/U weak signal work is
extended ground wave via perhaps enhanced tropo.

Ground wave? Really? I admit I'm not a propagation expert, so I'd
appreciate an explanation from someone who is. Is there really enough
ground wave propagation at VHF/UHF to be useful for any purpose, even
short range communication?

1.Horizontal polarity can take advantage of ground gain reflection that
vertical polarity cannot.

2. In addition, at 900 MHz where a wavelength is just over a foot, even
mounting the antenna at 12" would place the first lobe at 15 degrees,
assuming the car roof completely determines this- and I doubt that it has
much of an effect on far field take off angle.

You'd get that 15 degree lobe only if the roof extends far enough from
the antenna to reflect a wave going out at an angle 15 degrees below the
horizon (plus a bit, because the reflection doesn't actually take place
from a single point). At 12" above the car roof, the 15 degree downward
wave strikes the roof 12/tan(15 deg.) ~ 45 inches from the point below
the antenna. So the the car roof would have to extend at least about 4
feet beyond the antenna in the direction you're sending in order to get
that 15 degree lobe with the antenna 12" above the roof.

Roy Lewallen, W7EL