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GPS Antenna Amplifier and Extension

W1GE Patch Antenna for GPS | Receiving and Transmitting Antenna

I had a small h/h GPS that did not work satisfactorily when placed on the dash of my Land Rover Series 3 - so I made an external antenna for it. GPS signals come in at 1500MHz, the wavelength is 190mm.

The receive antenna is an aluminium "patch" antenna about 130mmx120mm. . It has a smaller element about 80x90mm mounted 8mm above this. See the explanation below

The coax from the antenna then feeds into a wide band (0-2010 MHz) satellite in-line amplifier with a slope gain of up to 26dB. This amp costs about R50.00 and is available from any shop that dabbles in TV/sat etc. The amp is small -about two match boxes in line type size and sits about 200mm from the rx antenna. The coax then goes into the Landy and terminates in a simple loop of wire-1 wavelength long. This effectively "retransmits" the GPS signal in the vehicle cab. The h/h GPS lies on top of this loop, which sits on my cubby box/armrest. I get brilliant reception and the system does not tie up the h/h GPS i.e.. I can just pick it up and go walkabout. 
Bruce Molzen E-mail: molzenba(at)sabc.co.za

 

 

W1GE Patch Antenna for GPS

Construction

We have chosen to build the antenna using aluminum sheet and an air dielectric with nylon bolts and washers to support the patch. 

The final dimensions of the patch and its ground plane are shown in the diagram below. The critical dimensions are those of the patch and the location of the feed point. The dimensions of the ground plane are much less critical. Separation of the patch from the ground plane is 5 mm (0.197 in) and is determined by the three No. 8 nylon washers. The sheet aluminium should be at least 1.27 mm (0.05 in) thick to provide sufficient mechanical strength but the exact thickness is not critical. The bandwidth of the two modes of the patch are 8 and 10% respectively which means the critical dimensions of the patch should be held to within about 3%. Having determined the design dimensions of the patch, we will now turn our attention to its construction.

The first step is to cut the aluminium sheet metal pieces needed for the patch and the ground plane. One means of controlling the dimensions of the pieces is to scribe the exact dimensions on the aluminium surface, then cut the piece somewhat larger than the necessary dimensions and grind the edges to obtain the final size.

After the aluminium is cut to size, clamp the patch so that it is centered on the ground plane and drill the four 4.76 mm (3/16")corner holes for the #8 nylon bolts as well as the small 1.59 mm(1/16") hole in the patch for the center conductor of the coax at the feed point. Then separate the aluminium pieces and enlarge the ground plane hole at the feed point to 4.59 mm (3/16") diameter (for RG58/U coax). At this stage round the corners of the patch and the ground plane to a radius of about 4 mm ().157") and then bolt the patch to the ground plane.
The next step is to attach the coaxial cable. A 10 ft. section of RG-58/U will satisfy most applications. Since soldering to aluminium is difficult I attached a 1" square of copper tape to both the top of the patch and the bottom of the ground plane centered at the feed points where the cable should be soldered. The tape that I used has a capacitance of 143 pF or 0.7 Ohms reactance at 1575.42 MHz. A DC connection to the aluminium is not necessary because the capacitive reactance is small enough to be negligible. An alternative to using copper tape to make the solder connections is to use copper or brass rather than aluminium sheet metal.

Harold R. Ward E-Mail: hrward@ultranet.com
Excerpt from QST Magazine, Oct. '95, p. 45

 

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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