Not only is the frequency range vastly different, (tho, the SWR LOSS's
would mean quite little extra loss from coax , in receive mode for the
GPS) BUT- the GPS uses a psudo omni directional antenna , read that an
immitation of an isotropic , relative to ALL angles, and polarizations)!
(remember that , with a verticle, a direct overhead signal will be next
to invisible)! and it take something like 4 satellites, to calculate
where you are (these are moveing FAST! Altho, think there is a ham
antenna was designed for such purpose- but SPENDY! Jim NN7K



Dave Platt wrote:
In article ,
Andrew wrote:


I've got a 2 meter antenna for my mobile unit, and I'm wondering if it
could double as a GPS antenna.


Possibly, but I doubt that it would be a good one. GPS uses
frequencies roughly 10x as high as a 2-meter rig. The 2-meter antenna
would probably not be resonant on GPS frequencies, and if it is of a
helical design or has a base-loading coil it might very well be a
_lousy_ antenna on the GPS frequency (the coil or helical winding
could act as a low-pass filter, blocking the GPS signal).

Its radiation pattern would probably have a lot of lobes pointed in
weird directions, and I suspect that the GPS signal from any given
satellite would be subject to a great deal of fading and jitter as the
vehicle and antenna move.

GPS receivers tend to use small "patch" antennas, etched into a piece
of PC-board substrate. These most often have a fairly uniform,
low-gain sensitivity pattern in the skyward direction.

Also, the GPS antennas separate from the GPS receiver are usually
"active" ones - they have a preamplifier very close to the antenna,
and boost the signal before it's fed down the coax to the receiver.
Losses in a typical RG-8X or RG-59 thin coax are quite high at 1.5
gigs, and the already-weak GPS signal could tend to be lost if not
preamplified.

The other problem with using a 2-meter antenna as a GPS antenna would
be that you'd need some pretty good form of switching, to keep the
input of your GPS receiver from being blown out when you keyed up the
2-meter transmitter.


I've got a friend, who 5 years ago
showed me how he had his laptop running MS Streets & Trips hooked to a
GPS receiver. I'm not sure if he had a standalone receiver meant for
laptop use, or somehow used his ham antenna and another receiver. Any
ideas?


I'd guess that it was a standalone receiver. Even back then, there
were GPS receiver the size of a pack of playing cards (or smaller),
which plug into a PC's serial or USB port.