By Mark Kesauer, N7KKQ
An Inexpensive External
GPS Antenna
If you operate APRS or just need an external antenna for your GPS
receiver, here s one that is easy to build yet offers surprisingly good
performance in a compact size. Best of all, it uses commonly available
components and materials.
his antenna design is based on a
classic turnstile configuration (for
Tcircular polarization) two
dipoles are placed on the same plane but
rotated 90° from each other. These dipoles
are then spaced ź wavelength above a
ground plane. A ź wavelength parallel-
plate transmission line (printed circuit-
board 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 diam-
eter of the container lid is 4 inches, ap-
proximately the same width as the hobby
tin/brass sheet.) Mark the exact center of
the base plate. This is where the parallel-
plate 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 paral-
lel-plate section later.
Select an 8-foot length of RG-58/U,
RG-174 or RG-188 coax. Attach a male
Table 1
BNC connector to one end (or whatever
Materials
compatible connector is used on your
Hobby tin (K&S #254) or brass sheet (K&S #251) (0.010 thick).
particular GPS receiver). I used a
Sheet of single-sided, glass-epoxy PCB material (FR-4 or G10 .0623 thick, enough to
solderless connector but removed the
make two 2-inch pieces 0.2503 wide.
screw and then soldered the center con-
Solderless right-angle male BNC connector (RadioShack 278-126) or the appropriate
ductor directly into the screw hole. If your type for your GPS receiver.
8-foot RG-58/U (Radio Shack 278-1314), RG-174 or RG-188 coax.
GPS unit has a BNC antenna connection,
8-inch #14 solid bare copper or brass wire.
you can use an Ethernet coax cable found
Empty 8-oz cream cheese container.
at most computer stores. Just make sure
Misc Clear 5-minute epoxy or superglue.
they are 50 &!. They ll already have the
Clear spray lacquer, #600 fine sandpaper.
BNC connectors crimped on each end.
K&S Engineering, 6917 W 59 St, Chicago, IL 60638; voice 773-586-8503; fax
Just cut in the center, trim to length and
773-586-8556; www.ksmetals.com/.
you ll have enough for two antennas. The
RadioShack, www.radioshack.com/.
GPS frequency is 1.57542 GHz so the
36 October 2002
Figure 1 Close-up view of the coax
connection to PCB transmission line and
support.
longer the coax, the greater the loss. Use
no more than 8 feet less if you don t
need the length.
To make the parallel-plate transmission
line, cut two 2-inch lengths of single-sided
printed circuit board material that are
0.250-inch wide. Make sure it is glass-ep-
oxy (FR-4 or G10 type material) and that
it is 0.062-inch (1/16 inch) thick.
On one of the PCB strips, cut the cop-
per foil with a sharp hobby knife or
Dremel tool, as shown in Figure 1. This
will be the active section of the paral-
lel-plate where the other non-modified Figure 2 The active side of the Figure 3 The ground side of the
transmission line. transmission line.
strip will be the ground side, as shown
in Figure 2. The 45° cut on the active side
is known as a microwave turn which
allows the signal to effectively turn 90° #14 wires to the ground side of the paral- 8-ounce cream cheese container makes a
to the coax. Glue the two strips together lel-plate section. Position as shown in Fig- practical radome for the antenna. More
(copper outside) and set aside to dry. ure 4. Do the same with the active importantly, it helps protect the internal
I ve found it easier to cut the PCB side you may need a helping third hand workings from mechanical damage.
strips a bit wide and glue them together as it s difficult to hold the soldering iron, I usually don t paint the container but
first. Then I just file both edges to antenna and position the wires all at the I do remove the silk-screened label by
the correct dimensions. A light sand- same time. using an automotive rubbing compound.
ing with #600 sandpaper finishes off Measure each leg of the horizontal wires It takes some effort but it does come off.
the edges and removes any burrs. and trim to 1.51 inches from the center Just follow the manufacturer s instruc-
Double-sided 0.125-inch thick PCB junctions. Next, trim both the 45° wires to tions. Be careful not to apply too much
material could be used but can be diffi- 1.82 inches from the center junction. If all pressure to the lid when you rub the la-
cult to obtain for the average hobbyist. went well, you should have approximately bel off. It s made of a different plastic
Conversely, by using a single 0.063-inch ½ inch between the tips of the 45° wires than the container and stretches easily.
thick double-sided material we would be and the base. If not, carefully resolder or You should be able to snap the base
working with a rather small and fragile bend the wires to this dimension. plate into the lid of the container. It s a
structure (half the thickness equates to Using a fine saw or a Dremel tool, re- tight fit so just work your way around the
roughly half the width). This might not move the excess length of the transmis- lid until the entire base plate is flush with
hold up during handling and operation. sion line just above the wire junctions. the lid bottom You might have to cut a
By using the two sections glued together, Sand the exposed junction to remove any notch in the lip of the lid to allow the coax
we ve solved the problem by creating our burrs and check for a short circuit. to exit the unit cleanly. Carefully align the
own 0.125-inch thick material. Note that we ve purposely kept the coax with the lid notch and snap the cover
Solder the transmission line section to transmission line section length long, onto the lid. It s normal for the top of the
the base plate keeping it as square and plumb until after construction. The thin copper transmission line assembly to slightly
as possible. Drill or melt a hole in the plas- foil tends to separate from the glass ep- raise the bump on the container bottom.
tic container the same diameter as the coax. oxy during heavy duty soldering. The
Theory of Operation
Feed the end of the coax through the hole longer length acts as a heatsink to pre-
and attach the coax to the transmission line serve the bond between the copper foil In a normal turnstile, we would have
active side as shown in Figure 4. and the glass-epoxy base. a double dipole configuration with both
Measure 1.78 inches up from the base dipoles on the same plane but rotated 90°
Final Assembly
end of the parallel-plate section and scribe from each other. Additionally, the second
a line in the copper foil. Solder one of the I ve found that an empty, upturned dipole is fed 90° out of phase with an-
October 2002 37
other ź wavelength of coaxial cable (see
Notes 2 and 3). This creates some diffi-
cult assembly problems since you would
have to isolate the second dipole section
from ground while maintaining the tight
distance and spacing requirements. Due
to the size constraints, this second dipole
connection would require a very small
diameter coax that might be difficult to
work with and even harder to obtain. With
this antenna, we cheat a bit and use a self-
phased quadrature type feed.
To obtain circular polarization without
a coaxial phasing line, the shorter dipole
is cut so its impedance is 50 j50 &!. The
longer dipole is fashioned into an inverted
V shape and cut so its impedance is low-
ered to 50 + j50 &!. With the combined
asymmetrical dipoles and with them
spaced slightly closer than ź wavelength
to the ground plane, the antenna s imped-
ance is near 50 &! with a much more
omnidirectional pattern, an important
consideration for reception of GPS satel-
lites close to the horizon.
Operation
Connect the antenna to the GPS re-
ceiver and watch the signal-strength indi-
cator. 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 maxi-
mum 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
Figure 4 Side (A) and front (B) views of the parallel-plate transmission line and
radiating elements. active elements touch ground. [Be ad-
Figure 6 The
finished GPS
antenna with
radome ready to
snap into place.
Figure 5 View of the parallel-plate
support with elements attached.
38 October 2002
My thanks to Zack Lau, W1VT, of the
vised that some GPS receivers with in- way to solve this problem is to locate the
ternal patch antennas have an antenna receiver somewhere cooler and place this ARRL Lab for his advice and expertise.
switching circuit. This circuit disables the external antenna on the dash. Once that s
Notes
internal antenna when an amplified ex- done, you ve protected your expensive re-
1
Tom Hill, WA3RMX, A Triband Microwave
ternal antenna is attached. The receiver ceiver while sacrificing a $10 antenna.
Dish Feed, QST, Aug 1990.
senses current flow that is intended to This design was not intended for out- 2
Zack Lau, W1VT, A Simple 10-Meter Satel-
power the amplifier of the external an- side use, hence the lack of external lite Turnstile Antenna (RF), QEX, Nov 2001.
3
Zack Lau, W1VT, A Simple 10-Meter Satel-
tenna. If your receiver has this feature, mounting suggestions. Any prolonged
lite Turnstile Antenna (Feedback), QEX,
you will want the switch to activate and exposure to the elements will degrade the
Jan 2002.
disable the internal patch antenna. Plac- antenna s unprotected metal parts. My
ing 1 k&! to 5 k&! across the ground and suggestion if you plan on using this unit
Mark Kesauer, N7KKQ, received his Novice
center conductor of the coax should be outside, at least spray the metal parts with
license in 1969. He has held the call signs
sufficient. Check with the manufacturer clear lacquer and seal the exposed coax
WN8CGM, KA5ZCH and his present Extra
of your GPS receiver. Ed.] end with RTV. Otherwise, don t waste a
class call sign. Mark belongs to QCWA, Ten-
If you are mobile, most GPS receivers lot of time weatherproofing it. Because
Ten International and has been a longtime
will do a fair job of receiving signals these antennas are so cheap and easy to
ARRL Member. Mark holds an Engineering
through the windshield of a car. During the build, if one does deteriorate throw it degree and has been a Computer Aided De-
summer, however (and especially out here away and build another. Perhaps you signer (printed-circuit boards) for 29 years.
Mark may be contacted at n7kkq@arrl.net.
in the Southwest), the GPS gets baked might want to keep a couple of spares on
while sitting in the hot sun. Obviously, one hand, just in case.
ing PowerPoint slides for a videocon- Features include silent, diode-based T/R
ference and much more. A separate switching; a built-in remote control port with
FEEDBACK
instructor s manual is available as well. true RS-232 levels; low receive-mode cur-
John has been a ham since 1959 and is rent drain for enhanced portability; all basic
The impressive 13-star flag flying
chairman of the ARRL High-Speed Digi- K2 features, including dual VFOs, multiple
proudly from page 20 of Sep 2002 QST is
memories, split TX/RX operation, RIT/XIT,
tal and Multimedia Working Group.
actually a Bennington Flag, not a Ben
full-break-in CW, built-in memory keyer,
Videoconferencing Skills is available for
Franklin flag. Our thanks to several read-
narrow IF crystal filtering, excellent receiver
$11.50 (the Instructor s Manual is $15)
ers who pointed out that the original can
dynamic range and IF-derived AGC.
from: Thomson Learning, Order Fulfill-
be seen at Vermont s Bennington Museum
The K2/100 shares a number of K2 op-
ment, 10650 Toebben Dr, Independence,
( www. benni ngtonmuseum. com/
tions, including the KSB2 SSB adapter,
KY 41051; tel 800-354-9706 (8 AM-6 PM
flaghistory.html).
KNB2 noise blanker, K160RX 160-meter
Eastern); www.swlearning.com.
In the Apr 2002 QST article AMRAD
adapter with second receive antenna jack,
Low Frequency Upconverter, there is a
KAF2 audio filter/real-time clock and the
short across R10 in Figure 2. That short
MH2 Heil/Elecraft microphone.
should be removed. Also, early versions
Price: The K2 sells for $589 and the
of the PC board used a slightly different
NEW PRODUCTS
KPA100 100-W Integration Kit (internal),
crystal oscillator circuit. That circuit
which completes the K2 as a K2/100, sells
THE K2/100 HIGH-PERFORMANCE
worked fine except it occasionally would
for $349. For more information, point your
not start. Several of those boards were HF TRANSCEIVER KIT FROM
Web browser to www.elecraft.com or e-mail
shipped from FAR circuits. If you have ELECRAFT
sales@elecraft.com.
one, they will exchange it for the later cir-
cuit if you wish. The older circuit is shown
&!
NEW 50-&!
&! COAX
&!
&!
in the hand-drawn schematic on the
FROM CABLE X-PERTS
AMRAD LF Web page at www.amrad.
Cable X-Perts, Inc, is pleased to intro-
org/projects/lf/ if you wish to use the older
duce a new version of their CXP1318FX, a
circuit.
50-&! low-loss coaxial cable. Manufactured
with a gas-injected foam polyethylene
dielectric and a 19-strand center conduc-
tor, this cable is designed to give excep-
STRAYS
tional flexibility and reliability. Other
enhancements include a double shield
VIDEOCONFERENCING BOOK
(100% bonded-foil and 95% tinned copper
FROM K8OCL
braid) and noncontaminating and direct-
Elecraft s landmark K2 kit transceiver is
John Champa, K8OCL, has written a
burial jacket. Nominal attenuation (per
now available in a 100-W model. The com-
new book titled Videoconferencing Skills.
100 feet) is said to be: at 150 MHz, 1.6 dB;
pact K2/100 is based on the K2, with the
The book offers students and instructor s
450 MHz, 2.9 dB; 1200 MHz, 5.0 dB, and
same features and same world-class re-
detailed information about how to present 2400 MHz, 7.5 dB. Available in bulk and
ceiver performance. It has the portability
dynamic and forceful videoconference ready-made lengths with UHF (PL-259)
and efficiency of a QRP transceiver with a
and N connectors. For more information,
meetings. The ten lessons in Videocon- 100-W punch when you really need it.
ferencing Skills provide an activity-driven see www.cablexperts.com or via email at
Created by Elecraft co-founders Wayne
cxp@cablexperts.com. Cable X-Perts,
approach with three to five activities in
Burdick, N6KR, and Eric Swartz, WA6HHQ,
Inc, 225 Larkin Dr, Ste 6, Wheeling, IL
each lesson, instructional illustrations, an the K2/100 uses an integral heat sink as its
60090-7209, tel 800-828-3340; Fax 847-
introduction to the equipment used in top cover, thus retaining the same form fac-
520-3444.
videoconferencing, instruction in prepar- tor as the base K2.
October 2002 39
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