[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
"APRS 101" Explanation of APRS Path Selection & Digipeating
Updated APRS Symbol Chart
New/Updated "Rev G" APRS http://webs.lanset.com/wa8lmf/aprs
Symbols Set for UI-View,
UIpoint and APRSplus:
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the aprssig