"Ian Jackson" wrote in message
...
In message
,
JIMMIE writes
On Jan 31, 5:01 am, Ian Jackson
wrote:
In message , Chevy454
writes

In message , dave
writes
But why the trunk lid? Surely the roof is better? On a vehicle, every
inch above the ground helps.

The car is being used for 2M fox hunting. The Doppler antennas are on
the roof. The MFJ-1728B is on the trunk lid.

Are you saying that the 6 dB gain of this antenna is not high enough
for a 20 degree tilt to make a difference?

Ken KC2JDY

Noted about the roof already being occupied.

Obviously, if the antenna is 'leaning backwards' at 20 degrees, the
radiation in the forward direction is going to point upwards more than
it would if ie was vertical and, of course, to the rear, the radiation
will point more downwards. However, even with the antenna dead vertical,
the shape of the body of the vehicle will distort the radiation pattern
considerably. I've had a quick Google, and this is one site which
provides some info:
http://www.k6mhe.com/files/mobile_vhf_ant.pdf
There will be lots of others.

This also might be of interest:
http://74.125.77.132/search?q=cache:...st.com.au/Ante
nnas/overview.html+antenna%2Bradiation%2B%225/8+wavelength%22&hl=en&ct=cl
nk&cd=25&gl=uk

As I said, you probably won't notice much difference between vertical
and 20 degrees. But the only way to find out will be to try it.
--
Ian

I think I remember from some place that tilting a vertical monopole
operating against a counterpoise increases the take off angle in both
the direction it is tilted and the direction opposite.

I welcome correction if I am in error.

Jimmie

No. You are correct. At VHF/UHF, with a vertical antenna over ground, the
strongest radiation is always at an angle upwards (say 10 to 40 degrees,
depending on the type of antenna). If you tilt the antenna over, the angle
of radiation is therefore lower in the direction of the tilt, and higher
in the opposite direction. On a vehicle, a whip with a 'sporty' rearwards
rake will probably radiate somewhat better towards the back. However, I
wouldn't lose too much sleep about it.

As for the increased radiation in the direction of a counterpoise (or
radial), it is in that direction that the counterpoise provides a better
'mirror image' of the antenna. This augments the radiation in that
direction.
--
Ian

.... but, surely, there will be more counterpoise to the front of a whip
mounted on the boot/trunk lid so this should favour low-angle radiation
towards the front - shouldn't it? If true, the result of tilting the whip
back could be closer to onmi-directional at low elevations!

Chris