Re: CB Antennas

[Jim Griffin <JGriff@pobox.com>]

At 05:51 PM 09/13/96 -0400, you wrote:
>I can't resist a clarification to the following excerpt, radiated by 
>Igor Kessel:
>
>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.
>
>

Well, excuse my ignorance, but in layman terms, do I have a good thing going
here or not? By the sounds of it, I am guessing yes. It sounds as if the
roof acts as a reflector, and in doing so, helps to amplify the signal onto
the trunk antenna. Is that what you mean?

Thanks.

Jim

                             Jim Griffin
                      JGriff@pobox.com
                        Maryland, USA
"Perception is often stronger than reality!"
                               '92 100S
~~~~~~~~~~~~~~OOOO~~~~~~~~~~~~~~