An external antenna for the GPS-38, Magellan 2000, or Eagle Explorer ==================================================================== 1. Introduction My first GPS was a Garmin GPS-38. I soon discovered that there were a number of circumstances where I wanted to use an external antenna with it, either to have the antenna physically separated from the display, or because I wanted a higher-performance antenna. The GPS-38 has no external antenna jack, so some other means was needed to get the signal into it. After some experimenting, I came up with the following system. It works very well. 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. 2. 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. In theory, you could use a good passive antenna as the receiving antenna, eliminating the need for the power supply. However, the receiving antenna needs very broad angular coverage to "see" satellites anywhere in the hemisphere above it, and you cannot get high gain and broad angular coverage at the same time from a passive antenna. The W1GE homebuilt patch antenna is about the best passive antenna I've seen for signal strength (at least for satellites above 45 degrees elevation) and it's still pretty marginal as a signal source for this system. So, you'll probably end up using an active antenna. Fortunately, they're not too expensive - about $65 US for the Lowe one, for example. 3. 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. 4. 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 travelling 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 --- + +---------+ L1 / \ ----+----| 78L05 |----+-------+-------+---@@@@@@-+---+-+ | IN | +---------+ | | | | \ / | | | | | C5 | --- | C1 | | C2 | C3 | C4 --- | --- | --- --- --- --- --- --- | --- --- --- | / \ | | | | | +---+-+ | OUT | | | | | \ / - | | | | | --- ----+---------+---------+-------+-------+----------------+ 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 labelled IN and OUT are chassis-mount BNC jacks. They need to be as close as possible to each other (see below). 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 microstrip 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 microstrip 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, microstrip 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 micropower 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 microstrip 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. 6. 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. 7. 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. Dave Martindale