Mark commented with ref his "glass mount" ant-
MG>    I'm hoping somebody can give me some hints to cure a problem.  I have 

MG> cellular "look-alike" scanner antenna (glass mount) that I've actually 
been
MG> pretty happy with, except that at low freq., around 40-50mhz, I get a lot 
of
MG> engine or electrical system interference.
MG>    I've run the coax. through the molding over the top of the driver's 
door,
MG> through the dash, and into the center console.  I'm using a Uniden 
BC220XLT
MG> hand-held scanner (that I love).
MG>    Any thoughts would be apperciated.
The problems of using any antenna on frequencies for which they are not
optimized is worse when you complicate the interface between the ant and
the rx/tx. You are observing the effect of feeding an antenna which is far
away from its resonent ( optimum ) use freq with an interface which is
freq dependent in operation. The tru-glass antennas either use a capacitive
coupling thru the glass dielectric ( which makes their coupling effect
inversly related to the frequncy, ie lousy at lower frequencys ( or use
a resonent lc circuit in both of the sections of the antenna interface that
mount to the glass, which works on the frequency to which they are tuned and
reject all other frequency ). I would guess that your antenna uses the 
capacity
coupled approach since that is very cheap to manufacture and is able to
function with a broader range of frequencies. The increase in ignition noise,
etc which you are observing is simply due to your rx having other noise and
signals comming from the antenna reduced by the tru-glass coupler. You will
probably observe the same ignition noise levels if you remove the ant element
from the outside of your vehical and leave the coax and coupler alone to pick
up the ignition noise and feed your scanner. Such ant are useable ( I will 
ot
say suitable for use ) on discrete frequency bands where they are optimized.
THEY ARE NOT BROADBAND ANTENNA! With the effort you have put into your
iostallation you need to consder installing a real tru the metal antenna, or
go to an external magmount compromise, or a series of thru the glass 
compromise
antenna for every frequency you are interested in receiving. Sorry your
results are not meeting your expectations, but the Physics is imutable!
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