CB Antennas

[KSMITH1@mailgw.sanders.lockheed.com]

I can't resist a clarification to the following excerpt, radiated by 
Igor Kessel:

<<<< begin excerpt >>>>>

From: a6561TB@gnn.com (Igor Kessel)
Date: Fri, 13 Sep 1996 01:01:54
Subject: Re: Radio S**t CB stuff

Hi  Jim:
>... the position of the antenna is just about the
>most important factor. The best spot would be in the center of the 
roof

Absolutely!

>since the area surrounding the antenna is used as well.
> So, in the middle,the whole car would be used as an antenna,
> helping to pick up signals.

Sorry, no. A car is a ground. Antenna is well insulated from it. Its 
the high position, unobscured the by the car's sheet metall, which 
acts as a shield, that's what counts. That is why same ant works bad 
on the trunk.
 
>>>>> end of excerpt <<<<<

Well, the obscuration of the trunk location by the roof is a valid 
construct, but, if I recall my electromagnetic theory of 30+ years 
ago, the metal car roof has a much more important effect than being a 
ground.  The conductive roof acts as a ground plane, in effect a 
mirror to the wave, and by reciprocity, to the antenna pattern.  This 
makes the quarter-wave whip into a half-wave vertical dipole.  I have 
forgotten the impact of this, but I suspect that an infinite ground 
plane provides a 3 dB (factor of 2) improvement over the whip antenna 
alone suspended in space (far above the ground).  At CB frequencies, 
the wavelength is larger than the car, but the conductive earth is 
within a wavelength, so the gain is probably closer to 2 than to 1.

Mandatory word: QUATTRO  

***                 ...Kirby    (Kirby A. Smith)                 ***
***              ksmith1@mailgw.sanders.lockheed.com             ***
***              [=]    kirby.smith@lmco.com                     ***
***  Opinions expressed herein are entirely those of the author. ***