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From: hpubvwa!ssc!Tad.Cook@cs.washington.edu
Newsgroups: comp.dcom.telecom
Subject: Re: Information Needed About Cellular Antennas
Message-ID: <15463@accuvax.nwu.edu>
Date: 17 Dec 90 18:24:32 GMT
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In article <15427@accuvax.nwu.edu>, srm@dimacs.rutgers.edu (Scott R.
Myers) writes:

> I need an education on antennas.  I'm purchasing a transportable and
> want to install an antenna on my car for greater signal strength when
> driving.  I have a fair understanding of the different types (roof
> mount, trunk mount, glass mount, etc.).  What I want to know is the
> layman's explanation of Db's.  I see ads for 3db trunk mounts and 5db
> glass mounts.  The impression I'm given is the 5db glass mounts are
> very acceptable for signal strength.  How do they compare to roof and
> trunk mounts?  Just what are Db's anyhow?

'Dbs' are decibels.  It is an expression of a ratio of two power
levels.  The db gain figure is against a reference antenna, which is
probably a 1/4 wave (about three inches long at cellular freqs) ground
plane.

Decibels are on a logarithmic scale.  For power, you can figure it as
decibels = ten times the log of the ratio of the two powers.  So 3 db
gain is the same as doubling your power output.  5 db is the same as
multiplying your power by about 3.16.  But remember that antenna gain
also helps the received signal.

There is quite a bit of loss through the glass on the glass mounted
type, particularly if it is near any defrosting elements.  The best
mount is in the center of the roof with a hole drilled in the roof.
The worst is when you use a glass mount on one of those side windows
on a van, so that the antenna is below the roof line.

I like the looks of these trunk mount ones that are on a long sleeve,
that puts them high above the trunk of the car.

> I have a second part to this question.  I have read about the cabling
> used in antenna installs (RG-58).  Based on what I'm reading, there is
> significant signal loss with that type of coax run long distances (ie.
> trunk to front passenger side seat.).  Would using another guage of
> coax reduce the amount of signal loss from the cable?  Any other
> recommendations to cut down on signal loss as well as the best antenna
> configurations.  Thanks in advance.

RG58 has quite a bit of loss at 900 MHz.  RG8/U would be better,
although it is larger and harder to install.  There is a mil spec
grade of RG8 that is best, although with just a few feet of line it
may not make much difference.  At 900 MHz there can be a lot of loss
in improperly installed fittings and antennas, so best to use a
professional installer.  I wonder about some of these deals that I see
for phones "$200, installed" at the local auto supply.


Tad Cook   Seattle, WA   Packet: KT7H @ N7HFZ.WA.USA.NA   Phone: 206/527-4089 
MCI Mail: 3288544        Telex: 6503288544 MCI UW  
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