This page describes modifying a reradiating GPS antenna for use on a GPS which does not have an external antenna adapter. This project came about more as a curiosity to see just what kind of performance can be gained using such an antenna. Two main pages I have used for reference are Heinrich Pfeifer's and Dave Martindale's reradiating antenna pages. I'm also working with Lapaglia, who has already made a similar unit for use with his ipaq and is impressed with his results. My test dummy - a basic yellow Etrex which has to be one of the most common units used.
Things to consider concerning a reradiating antenna:
This is the SM-76 amplified antenna I purchased for this project. I found mine on ebay for less than $27 delivered to the door. I was totally deceived by the picture. This is a very small waterproof unit with a magnetic base, measuring about 1.875" X 2.25" X 0.625". This particular unit is available with 3, 9 or 16-feet cable lengths, with the actual unit I bought having the 9 foot cable. The cable is very thin, less than 1/8 inch diameter and terminates in a standard right angle 50 ohm BNC connector. The antenna uses the inner conductor for both the DC power to feed the amplifier as well as for the RF output to drive the transmitting antenna. Stated input power rating is 2.5 - 5 volts with a current draw of 7 mA with a signal gain of 27-33 db typical.
Below is a basic circuit design I put together so I'd have something to play with as soon as the antenna unit got here. Power is supplied by 2 AA batteries, as opposed to the designs from the pages mentioned at the top of this page which get power from a 12 V power source. Using the batteries makes the circuit design much more simple and portable.
Battery power is sent to the center conductor of the BNC jack via a small hand wound inductor. I made this coil by winding 6 turns of #26 AWG magnet wire over a 3/16 drill bit. This inductor acts as an AC block, keeping the Rf energy from the amplifier from getting into the power supply. I originally thought this inductor was not necessary since I was running battery power, but without it the RF energy feeds though the batteries causing them to get warm, which we can assume is not a good thing!
The transmitting portion of the circuit consists of 2 parts. A 47pf capacitor which acts as a DC block to the transmitting loop, without it the batteries would effectively be shorted by the low resistance of the transmitting loop. The transmitting antenna loop I am using as a test dummy is 190 mm (one wavelength) of # 22 AWG magnet wire. This wire is too flexible for a permanent design, but works well for testing purposes.
Initial test: In my house. I can typically get a lock inside the house half of the time. It just so happened 12 satellites were "in view" during this test. The basic circuit was used and held about an inch above the GPS.
Same test, next morning. 7 satellites in view.
Still inside house. I changed C1 from 47 pf to .33 µf to see if that helps. I am looking for formula for "ideal" value, but can't seem to find it in my books. Results are impressive for this test though.
Similar test as last one with .001 µf capacitor. As you can see, there are times when the antenna may actually lose a signal the GPS could pick up, yet add others. I have emailed Dave Martindale as to how he arrived at his values, and the text of that email is here.
I've decided to build a shielded enclosure and came up with this for a first design. I used a short piece of 1/2 inch copper tubing and 2 end caps, one of which has been shortened to facilitate soldering of the circuit board directly to the BNC connector. The entire assembly is less than 3 inches long, and could be shorter. The second cable is for external batteries, with a longer tubing there would be room for AA batteries, they fit perfectly in the tubing. As an alternative, there is room for button cells inside as-is, and with the low current draw it could be an option. I am using a surface mount capacitor (200pf, smallest I could find) now to keep the impedance down some.
First test, using the new setup. Again, indoors. Outdoor tests soon. If it does this in heavy tree cover, it's worth something...
Finally the weather cleared so I made it outside for a test. All day today I have had a good signal, even without the aid of the reradiating antenna, but had to try it out anyway. These results were taken in an open area with a decent view of the sky. The antenna is supposed to work without a ground plane, but for this test I tried it both with and without a ground plane (hood of truck).
My first use of the antenna at a cache site when the signal was poor. Most of the Satellites were near the horizon, and could only get 3 satellites to lock without the reradiating antenna. I was about 50 feet from the cache without the antenna, and with it got me within 15 feet.
On to the actual transmitting antenna. The length should be 190 mm as stated before, but that leaves options for different shapes. From what I have read, it's not so much the shape of the loop that makes a difference, it is the area within it. A circle will yield the most area for a shape with a 190mm circumference, about 2874 sq mm; a square loop would yield about 2256 sq mm. However, I have twisted my loop into several shapes and it still seems to work quite well, except for shapes that leave very little area inside. As with all high frequency circuits, very sharp corners and loops should be avoided as it causes higher losses and unwanted inductance.
At this point it also makes sense to find a suitable way to attach the transmitting loop. For the Etrex line, Heinrich Pfeifer's way of using a pfranc connector as a support for a rigid wire loop is one good method. I'll play with that idea another day.
I came up with this brainstorm, tried it out, and it seemed to work as well as the ones I have been using. It is a standard Garmin stretch fit cover with a very fine (35 Gauge, approximately .005") nichrome wire sewn into the center of the skin of the cover. The dotted line shows where the wire runs to make a 190 mm loop. For connectors, I tried nickel plated brass snaps typically used on clothing. Since the nichrome wire is not able to be soldered by conventional means, I wrapped it tightly several turns around one of the loops, then coated it with a conductive ink pen. I then let it dry and coated it with a drop of superglue. For the other connector, I used a small piece of PCB board, drilled two though-holes corresponding to the connectors on the GPS cover, stuck the wires though, placed the other half of the snap and soldered together. I then reinforced the cable with some hot glue on the back side. It seems quite durable and the electrical connection is stable.
Additional notes sent by others:
I read of your experiments with a re-radiating antenna with interest.
I may be contacted through geocaching.com, user name brdad.
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