[aprssig] Re: Duplexer or Diplexer

Stephen H. Smith wa8lmf2 at aol.com
Tue Sep 6 03:53:59 CDT 2005

n0yxv at gihams.org wrote:

> I found it much cheaper to use two VHF antennas on my mobile. One for 
> APRS and one for Voice.
> Anybody have a problem using two antennas? I'm not sure but when I had 
> one single magmount 2 meter on my cars trunk I think I got better 
> distance than when I switched to two trunk lip mount antennas.

You are probably recalling correctly.  When the whips are right up 
against the greenhouse of the car on the forward edge of the trunk lip, 
the radiation pattern is being blocked and distorted much more severely 
then when the antenna is in the center of the trunk lid, or center of 
the roof.  To get a reasonably uniform radiation pattern in every 
direction, you have to have a significant area of horizontal sheet metal 
in every direction (approximately a  quarter wave radius - about 19" at 
VHF, about 6" at UHF - or more) from the base of the antenna.

> Somebody suggested that it might be a grounding issue so I peeled the 
> paint back under the antenna mount and ran a separate grounding wire 
> from one of the antennas trunk lip mount to a ground that I found in 
> the trunk. Would size of the ground wire make a big difference? 
> Currently I'm just using two 20 gage wires twisted together for the 
> ground run. What would be the best position for the antennas?

This may be a ground for DC or 60 Hz AC power.  It IS NOT a ground at 
VHF RF.   To be effective at VHF, the ground (i.e. connection to a 
substantial area of sheet metal)  MUST be at the immediate base of the 
antenna, not many feet or inches away to do anything.  Furthermore, any 
ground lead used for RF purposes must be a heavy, wide conductor, such 
as flattened braid from a larger-size coax or copper strap a good 
fraction of an inch wide, to have low-enough self-inductance at VHF 
frequencies to be useful.  

For a trunk-lip mount, this means the mount's set screws MUST bite 
through the paint and contact bare metal at the point where it is 
clamped down.  ( A magnet mount achieves the same effect with capacitive 
coupling through the much larger larger SURFACE area of it's base in 
contact with the painted sheeet metal below, rather than direct metallic 
contact. Even then it often isn't as consistent as punching a hole and 
installing a permanent NMO mount that is in direct metallic contact with 
the sheet metal surface.

One way to work around the ground problems is to use 
ground-plane-independent antennas.   These are antennas designed around 
half-wavelength elements (38" long) rather than quarter-wave (19") or 
5/8-wave sections.   They are widely used on fiberglass-bodied cars, 
fiberglass caps for pickups,  van conversion ambulances with fiberglas 
bubble tops, boats and motorcycles where a  horizontal sheet metal 
surface doesn't exist.

Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      14400    [Think bottom of the 2M band]
Home Page:          http://wa8lmf.com

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