Hi Fellows.

I tried hooking a 2 mtr/440 MHz antenna to my Garmin GPS 90. Zip, nada!
I did this while doing a bunch of work on a passive filter to separate the
energy to direct it to the right place. With this result I discontinued the
project as futile.

I wish to dispel the myth that an antenna with a null overhead can not be
used as a GPS antenna. The satellite overhead does not stay in the very
narrow null long enough to be a operational problem. I used with good
effect a simple quarter wave vertical over a ground plane and my Garmin
locked up nicely. Translation: Stick a bent up paper clip into the BNC
jack as a vertical and the thing still works! Not as strong but no reason
to give up if your 'real' antenna gets busted.

For the ease of you folks who want to duplicate my experiment, my
math shows that the proper length should be 1 7/8 inches, or 47.6 mm
for the metric minded.

Ed WA5SWD

"G.Beat" wrote in message
news:etYFc.7440$JR4.7037@attbi_s54...

"Gary S." Idontwantspam@net wrote in message
...
On Sat, 03 Jul 2004 20:53:32 GMT, "G.Beat"
wrote:

You should investigate the usage of a mobile antenna, such as the GPSx
Survivor by Antenex, 2000-205 Bloomingdale Road Glendale Heights,
Illinois
60139
http://www.antenex.com/

GPSx Survivor - Model GPSDVHF
It comes complete with 14 feet of RG174 terminated with an SMB connector
for
GPS and 14 feet RG58A/U terminated with N connector. It is mounted
through
a
5/8" diameter mounting hole and secured with an included heavy stainless
steel jam nut.

Sounds like a good solution.

However, I would not call that a dual band antenna, but two different
antennas mounted in the same package.

How is the active GPS antenna powered?

Gary -

Correct, this is 2 antenna is one mounting - which is important for some
mobile installations (e.g. public safety and utility vehicles) AND since
the
desired characteristics of each antenna are entirely different.

For GPS at 1.5 GHz -- the antenna pattern needs to see the entire sky.
Why?
GPS receivers require a minimum of 4 satellites in view to perform a
position lock - and the constellation of GPS satellites are always moving
(orbit) in you visible sky. Look at any new car (e.g. 2004 Honda Accord
with Navigation option) - you will see the GPS antenna housing at the
center
of the hood back - for maximum sky visibility.

For 2 meter mobile - the desired antenna pattern is toward the horizon.
This is why 5/8 wave antennas are popular - since their pattern at the
horizon is superior to almost all other antenna types (e.g. 19" 1/4 wave
or
38" 1/2 wave antennas).

Power to the antenna is an excellent question. There are 2 types of GPS
antennas - passive and active.

"Passive" GPS antennas (also called substrates) do not include a Low Noise
Amplifier (LNA) and are best suited to GPS applications requiring a short
distance, 1 inch to 3 feet, between the GPS receiver and antenna. Passive
antennas require an LNA on the GPS receiver (such as the GT+ "R4" part
number series) or mounting a separate LNA close to the RF input connector
of
a receiver that does not include an on-board LNA (such as the GT+ "R3"
series and the M12).

"Active" GPS antennas include a built-in Low Noise Amplifier (LNA) which
provides sufficient gain to overcome coax cable losses while providing the
proper signal level to the GPS receiver. This LNA requires power from the
GPS receiver to operate -- the GPS receiver antenna connector (just like
the
older style UHF TV pre-amps from the 1970s).
The GPS voltage standard use to be 5 volts -- BUT this was recently been
changed to 3.3. volt by most manufacturers.

In order to prevent damage to the GPS receiver or antenna -- YOU MUST
MATCH
the GPS receiver and antenna.
Some antenna manufacturers can handle both voltages - BUT NOT ALL.

The company that I obtain my GPS supplies form is Synergy Systems, LLC in
San Diego. They are the largest distributor of Motorola GPS receivers -
and
have been a supported of TAPR, Amateur Radio APRS usage as well as
educational experimenters (GPS for Scholars program).
http://www.synergy-gps.com/

I always check their excess inventory sale for inexpensive GPS items
(Acrobat PDF)
The Oncore units are OEM boards and require a TAC-2 or equivalent
interface
board for operation
http://www.synergy-gps.com/Excess_Inventory.pdf

Greg, w9gb