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Re[2]: CB Antennas

The roof helps "amplify" the signal to a roof antenna.  The roof may 
help or hinder a trunk antenna depending on signal direction, and 
antenna size relative to the vertical distance from trunk to roof.  
The original response suggesting the roof was better than the trunk 
was correct, IMO, but the improvement may be slight.  Try each.  
Drilling a hole isn't needed to do the test, just temporarily ground 
the antenna base or cable shield close to the antenna.  An antenna 
expert is welcome to jump in here.

______________________________ Reply Separator _________________________________
Subject: Re: CB Antennas
Author:  Jim Griffin <JGriff@pobox.com> at INTERNET
Date:    9/17/96 7:57 PM

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?



                             Jim Griffin
                        Maryland, USA
"Perception is often stronger than reality!"
                               '92 100S