Tom Horne wrote:
Roy Lewallen wrote:

I haven't read the article, but are different mobile antennas being
compared on different vehicles, or the same vehicle? It's amazing how
many people don't realize that the vehicle is fully half the antenna,
and may in many cases play a more important role in determining
overall radiating efficiency than the supposed "antenna". So it's
impossible to draw any conclusions about mobile antennas based on
comparisons done when they're mounted on different vehicles. It's as
much a test of the vehicle's effectiveness as a radiator as it is the
antenna's.

Roy Lewallen, W7EL



Are you being serious when you say that much of the difference in the
mobile rigs performance may be the vehicle on which it and therefore
it's antenna are mounted? IS the body of my half ton cargo van doing a
substantial amount of the job of radiating my signal? If I mounted the
same antenna with the same mount on my Saturn should I see a difference
on a field strength meter that is the same distance to the centimeter
from the antenna over the same parking lot with the antenna over exactly
the same spot?

You betcha...

In fact, move that antenna around on the SAME vehicle and you'll get
huge differences in field strength in any particular direction.

The vehicle is more than half the antenna. Not only does it sort of
form the "other half" of the dipole, it's also a big capacitor coupling
to the earth underneath.

Figure a car is about 3 meters long and 2 meters wide (6 square meters)
and it sits about 15 cm off the ground.

C = epsilon * A /d
We can use the permittivity for free space, 8.8 pF/m
C = 8.8 * 6/.15 = 350 pF (approx)

On 40m, that's an impedance of about 11 ohms, which is fairly small,
when you think about it.

On 10m, the impedance to the ground is even lower (3-4 ohms), so it
starts to look like a vertical over a ground plane.