Building GPS
Receiving and Transmitting Antenna and Power Supply
The basic idea of the system is to use *two* additional GPS antennas.
Antenna #1 is used to receive the GPS signal from the satellites, and is placed wherever it has a good view of the sky. Antenna #2 is used as a
*transmitting* antenna, and is connected to antenna #1 via coax cable. The transmitting antenna is then placed close to the GPS receiver's own
internal antenna, and the GPS signal is coupled into the GPS receiver's antenna through the air.
The Receiving Antenna
The receiving antenna needs to provide a good strong signal, since the
coupling between the transmitting antenna and the GPS receiver is not as good as a direct connection. This usually means using an "active
antenna", with a built-in preamp, as the receiving antenna. The Lowe or Trimble antennas seem to be ideal for most cases, since their internal
preamp has plenty of gain (about 26 dB). The Garmin GA-26 also works pretty well, though its lower gain (13 dB) means that the signal won't
be quite as strong at the GPS receiver antenna. I have not tried other active antennas, but I would expect that almost any one would work.
Of course, the disadvantage of an active antenna is that it requires
power to operate. Below, I describe how to build a power supply for an active antenna, but this does require an extra "box" in the system.
The Transmitting Antenna
The antenna that "rebroadcasts" the GPS signal must be a passive
antenna, because passive antennas transmit just as well as they receive. Thus, any passive antenna that works well for receiving will
also work as the transmitting antenna. If you had a spare Garmin GPS-45 antenna lying around, it would work well, but it's far too expensive to
buy one for this purpose. The W1GE homebuilt patch antenna also works well, but it's a bit awkward for mounting to your GPS (it's about 5
inches square).
However, all the transmitting antenna needs to do is to couple the
signal into the GPS receiver antenna at close range - it can be taped directly on top of the receiver's antenna if necessary. So the
transmitting antenna really doesn't have to be a very good antenna, and we can give a higher priority to low cost, simplicity of manufacture,
and ease of mounting than to actual performance.
The best design I've found so far is a simple one-wavelength loop
antenna. To build it, start with 190 mm (about 7.5 inches) of stiff copper wire. I used 12 AWG wire to make the loop really stiff, but any
wire thick enough to hold its shape would likely do. You can use insulated or bare wire, but if you use insulated wire strip 1 or 2 mm of
insulation off each end. Then bend the wire into a loop, bringing the two ends to within a mm or so of each other (but not touching). You can
use a round loop if you like, but a square loop fits on top of the GPS better. I made mine a square loop with the gap in one corner, but
putting the gap at the centre of one side should also work.
When the loop is done, find some 50 ohm coaxial cable and strip 2 or 3
mm of it. RG-174 is ideal because it is so thin and flexible, but RG-58 will work as well or better electrically. Then solder the centre
conductor to one end of the loop, and the shield to the other end. If you think you might ever use the loop outdoors in the rain, seal the
entire connection between the coax and the loop with silicone or epoxy, to keep water from getting into the coax. Then install a BNC plug on
the other end of the coax.
The Power Supply
Assuming you use an active receiving antenna, you'll have to supply
power to it. Most active GPS antennas want 5 V at 15-25 mA supplied via the coax that also serves as the signal output cable. If we are going
to use the external antenna "in the field", the power supply needs to be small and portable. A 9 V alkaline battery makes a reasonable power
source, since at these current drains it will last at least as long as
the batteries in the GPS itself.
So, there are a certain minimum set of things that a suitable power supply
must do:
Provide 5 V regulated power from the unregulated battery voltage
- Provide some sort of current limiting so the regulator and other
components aren't fried if someone accidentally shorts the antenna
power output
- Couple the 5 V antenna power into the receiving antenna cable without
loading the GPS signal that is also traveling on the same wire
- Pass the GPS signal from the receiving antenna to the transmitting
antenna
- Block the 5 V DC antenna power from appearing on the transmitting
antenna output, since we want to be able to use a loop antenna
that is a short circuit at DC frequencies.
Here is the circuit diagram of a power supply that meets these needs:
9V Input into 78L05, C1 between 9V and GND
Caps C2, C3, C4 over 5V and GND
L1 in series between C4 (on 5V line) and BNC signal IN
C5 between BNC signal IN and BNC signal OUT
BNC IN GND connected to BNC OUT GND
I built the entire thing into a Hammond die-cast aluminum project box
chosen to be large enough to hold a 9V battery. The rest of the
circuitry isn't very large. The two round connectors labeled IN and
OUT are chassis-mount BNC jacks. They need to be as close as possible
to each other
Some design notes:
The 78L05 voltage regulator is in a plastic TO-92 package; it doesn't
have to dissipate very much power. And it has built-in current limiting
and will shut down if it gets too hot, so it's moderately difficult to
destroy.
However, don't substitute a regular 7805 regulator. The 78L05's maximum
output current is about 300 mA, which isn't enough to burn out most
chokes that you could pick for L1. But a 7805 can provide 1.5 A or more
if the IN jack is shorted, which might well destroy L1. The lower
current limit of the 78L05 is a feature. If you must use a 7805 or
other high-current regulator, you should put a 0.1 A fuse somewhere in
the circuit.
There are low dropout voltage regulators that would work better than the
78L05, since they would get more useful life out of the battery (e.g.the National LP2950, or the Max 630 series). But they are harder to
find than the simple 78L05.
L1 is an inductor whose job is passing DC from the regulator, but
blocking the GPS signal from being loaded by the regulator and its
bypass capacitors. I hand-wound the one in my supply. It is 6 turns of
#26 wire wound on a 3/32 inch drill bit, giving a coil diameter of about
3 mm. The turns are spread apart by one wire diameter or more, giving a
total length of about 6 mm. Since the turns of the coil don't touch,
insulated wire isn't needed. In fact, the number of turns or the coil
diameter don't seem to be critical either - any coil of about this size
should perform reasonably well. Smaller wire than #26 is also
acceptable, provided it can handle the maximum output current of the
regulator.
The remaining components are capacitors. C1 makes sure the regulator
remains stable; it is a 0.33 uF ceramic. Capacitors C2-C4 all act as
bypass capacitors; they filter out high-frequency noise from the output
of the regulator (plus any RF that gets back through L1). Their values
are 10 nF (0.01 uF), 1000 pF, and 22 pF. All are ceramic. (Using three is probably overkill, but safe).
If you can't find a 0.33 uF or larger ceramic capacitor for C1, a 1 uF
tantalum should be fine (check polarity before installing!). Aluminum
electrolytic capacitors do *not* work well here; their AC impedance is
too high.
Capacitor C5 is a 47 pF ceramic capacitor. Since the GPS RF signals
must pass through it, it should be a high-quality low-inductance
capacitor.
For my first prototype, I built a small through-hole PC board and all of
the capacitors were in standard packages with leads. In my second
version, I switched to surface-mount capacitors. The latter are a pain
to handle, but gave me a very small regulator board with no components
at all on the back side, so I could glue it directly to the wall of the
box. However, just about any reasonable construction technique will
work for the regulator circuitry - it's all low-frequency and not
critical.
The thing that *does* require some attention is the RF signal path
between the two BNC jacks. Ideally, this should be a constant 50 ohm
impedance path. For my second version, I built a 50 ohm micro strip signal line from double-sided epoxy-glass PC board material, and mounted
C5 and L1 directly on the board. C5 was a surface-mount capacitor, so
it is just soldered across a short gap in the micro strip signal line.
That's nearly ideal construction.
However, in the first prototype is was sloppier, and just soldered an
axial-lead ceramic capacitor between the two BNC centre pins, not
worrying about maintaining a constant-impedance RF path. It still
worked. I probably got away with it because the distance between the
two BNC connector centre pins was only about 15 mm, which is a small
fraction of a wavelength.
Another possible construction technique is to connect the two BNC
connectors with a length of 50-ohm coax cable, splicing capacitor C5
into the centre conductor at one of the BNC connectors. I haven't tried
this, but it should work.
I think the main design consideration is this: If you're going to just
connect a capacitor between the two BNC connectors, it will cause an
impedance discontinuity, so it's important to keep the non-50-ohm part
of the signal path as short as possible. Mount the two BNC connectors
close together on one side of a box, or on opposite walls of a narrow
box, or on adjacent walls near the corner of a box. On the other hand,
if you want to place the two BNC connectors any significant distance
apart, you need to provide some sort of constant-impedance connection
between them: coax, micro strip line, or whatever.
Finally, however you connect the two BNC connectors together, you should
connect the L1 inductor as close as possible to the "IN" connector. Any
lead length between the BNC connector and the inductor acts as a "stub", which is bad.
Finally, you will probably need to add a switch between the battery and
the regulator circuit (this is not shown in the circuit diagram above).
If you do use one of the micro power voltage regulators, the idle current
drain may be so low that you don't need a switch - just unplug the
active antenna. But the 78L05 draws several mA with no load, so you
need some way of disconnecting the battery.
A useful reference on components for microwave use and micro strip PC
board construction is "The ARRL UHF/Microwave Experimenter's Manual".
5. Putting it All Together
First, connect the receiving antenna to the "IN" jack on the power
supply. Connect the loop transmitting antenna to the "OUT" jack. Turn
on the power.
Then lay the loop antenna over the area of the GPS that contains the
internal antenna. If you built a square loop, it will be about 50 mm
wide, just slightly wider than a GPS 38. Rotate the loop until you get
the best signal. With the corner-fed loop I built, it works best with
the arms of the loop parallel to the sides of the GPS. Once you have
the best orientation, you can tape the loop to the GPS if you want.
That's it.
I find that this system generally gives full-scale signal quality
indications for several satellites on the GPS-38 when used with the Lowe
or Trimble antennas. The signal is definitely better than the GPS-38
itself would receive if it was in the same place as the active antenna.
The 38 shows faster locking and better tracking, much as many people
have observed when using an active antenna with their GPS-45. There
*are* losses in the system, and you shouldn't expect this to work as
well in weak-signal conditions (e.g. in forest) as an active antenna
connected directly to a GPS receiver that does have an external antenna
input (e.g. GPS 45). But it certainly beats the unaided 38 in those
conditions.
Warnings
The transmitting antenna *does* transmit GPS signals, though they are
rather weak and fall off rapidly with distance. The active antenna
provides considerable gain. If you create a situation where the
receiving antenna can "hear" the transmitting antenna, and the overall
gain through the loop is near 1, a variety of nasty effects is possible.
You may reduce the sensitivity of the whole system, or you might
selectively increase it. If the loop gain goes above 1, you might
actually have a GPS-frequency microwave oscillator. This won't work as
an antenna, and could damage or destroy the preamp in the receiving
antenna, or even damage the GPS receiver's front end.
So, keep the receiving and transmitting antennas away from each other.
Summary
This is a viable way of adding an external antenna to a GPS receiver
that doesn't have any provision for one. Performance is good - better
than the internal antenna. But you end up carrying around an extra box.
From: davem@cs.ubc.ca (Dave Martindale)
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