28-09-2012, 05:31 PM
An Inexpensive External GPS Antenna
[attachment=34763]
This antenna design is based on a
classic turnstile configuration (for
circular polarization)—two
dipoles are placed on the same plane but
rotated 90° from each other. These dipoles
are then spaced ¼ wavelength above a
ground plane. A ¼ wavelength “parallelplate”
transmission line (printed circuitboard
material) serves as the connection
method and mounting post for the dipoles.
Construction
Start with the base plate. Cut a 4-inch
diameter circle out of thin hobby tin or
brass. (It happens that the inside diameter
of the container lid is 4 inches, approximately
the same width as the hobby
tin/brass sheet.) Mark the exact center of
the base plate. This is where the parallelplate
transmission line assembly is
attached (see Figure 1).
Cut two 4-inch lengths of #14 solid
copper or brass wire and bend each in the
exact center at 90°. Make the radius of
the bend as small as possible. Set these
aside, they will be soldered to the parallel-
plate section later.
Select an 8-foot length of RG-58/U,
RG-174 or RG-188 coax. Attach a male
BNC connector to one end (or whatever
compatible connector is used on your
particular GPS receiver). I used a
solderless connector but removed the
screw and then soldered the center conductor
directly into the screw hole. If your
GPS unit has a BNC antenna connection,
you can use an Ethernet coax cable found
at most computer stores. Just make sure
they are 50 Ω. They’ll already have the
BNC connectors crimped on each end.
Just cut in the center, trim to length and
you’ll have enough for two antennas.
Operation
Connect the antenna to the GPS receiver
and watch the signal-strength indicator.
You should see an improvement over
the supplied stock antenna. You can tweak
the antenna by bending the wires up and
down gently and watching the results on
your GPS unit. Be careful of the solder
joint—it’s rather fragile. Adjust for maximum
displayed signal. Repositioning the
antenna may also improve reception. With
this antenna, I routinely receive five to
eight satellites on my Garmin II receiver.
If you are using a GPS unit that sends
dc voltage volts up the coax to power an
external preamp or amplified antenna,
don’t worry. Since the elements are not
grounded or shorted, there is no dc path.
Just be careful not to let either end of the
active elements touch ground. [Be ad-
preparvised
that some GPS receivers with internal
patch antennas have an antenna
switching circuit. This circuit disables the
internal antenna when an amplified external
antenna is attached. The receiver
senses current flow that is intended to
power the amplifier of the external antenna.
If your receiver has this feature,
you will want the switch to activate and
disable the internal patch antenna. Placing
1 kΩ to 5 kΩ across the ground and
center conductor of the coax should be
sufficient. Check with the manufacturer
of your GPS receiver.—Ed.]
[attachment=34763]
This antenna design is based on a
classic turnstile configuration (for
circular polarization)—two
dipoles are placed on the same plane but
rotated 90° from each other. These dipoles
are then spaced ¼ wavelength above a
ground plane. A ¼ wavelength “parallelplate”
transmission line (printed circuitboard
material) serves as the connection
method and mounting post for the dipoles.
Construction
Start with the base plate. Cut a 4-inch
diameter circle out of thin hobby tin or
brass. (It happens that the inside diameter
of the container lid is 4 inches, approximately
the same width as the hobby
tin/brass sheet.) Mark the exact center of
the base plate. This is where the parallelplate
transmission line assembly is
attached (see Figure 1).
Cut two 4-inch lengths of #14 solid
copper or brass wire and bend each in the
exact center at 90°. Make the radius of
the bend as small as possible. Set these
aside, they will be soldered to the parallel-
plate section later.
Select an 8-foot length of RG-58/U,
RG-174 or RG-188 coax. Attach a male
BNC connector to one end (or whatever
compatible connector is used on your
particular GPS receiver). I used a
solderless connector but removed the
screw and then soldered the center conductor
directly into the screw hole. If your
GPS unit has a BNC antenna connection,
you can use an Ethernet coax cable found
at most computer stores. Just make sure
they are 50 Ω. They’ll already have the
BNC connectors crimped on each end.
Just cut in the center, trim to length and
you’ll have enough for two antennas.
Operation
Connect the antenna to the GPS receiver
and watch the signal-strength indicator.
You should see an improvement over
the supplied stock antenna. You can tweak
the antenna by bending the wires up and
down gently and watching the results on
your GPS unit. Be careful of the solder
joint—it’s rather fragile. Adjust for maximum
displayed signal. Repositioning the
antenna may also improve reception. With
this antenna, I routinely receive five to
eight satellites on my Garmin II receiver.
If you are using a GPS unit that sends
dc voltage volts up the coax to power an
external preamp or amplified antenna,
don’t worry. Since the elements are not
grounded or shorted, there is no dc path.
Just be careful not to let either end of the
active elements touch ground. [Be ad-
preparvised
that some GPS receivers with internal
patch antennas have an antenna
switching circuit. This circuit disables the
internal antenna when an amplified external
antenna is attached. The receiver
senses current flow that is intended to
power the amplifier of the external antenna.
If your receiver has this feature,
you will want the switch to activate and
disable the internal patch antenna. Placing
1 kΩ to 5 kΩ across the ground and
center conductor of the coax should be
sufficient. Check with the manufacturer
of your GPS receiver.—Ed.]