Experience Using An External Re-radiating Antenna

[jmason4531]

I use my GPSr (a Garmin eTrex Legend) while hiking, which is typically under a dense canopy of foliage. I attach the GPSr to my belt, which means that the unit is oriented vertically. This is a challenging environment for good satellite reception. Not surprisingly, I frequently lose satellite reception. In an effort to improve reception, I purchased a PC-Mobile re-radiating antenna (consisting of an active antenna and a power/transmitting unit). I had to use a re-radiating design because the Legend does not have an external antenna connector. My thought was that I could mount the active antenna on my backpack to keep the antenna properly oriented toward the sky as I walked and minimize shielding from my body and thus improve reception.

 

My experience using this antenna has not been what I expected. What I expected was that satellite reception would be improved by the use of the re-radiating antenna in all conditions. This is not what I have observed.

 

I confirmed the operation of the re-radiating antenna by attempting to receive satellite signals with the GPSr inside my house. Without the re-radiating antenna, the GPSr was not able to see any satellites. With the external antenna placed outside and the GPSr still inside, the GPSr was able to receive satellite signals, thus demonstrating proper re-radiating antenna function. However, when the GPSr can receive signals directly, use of the re-radiating antenna attenuates those signals, resulting in poorer reception. Nor is there an appreciable increase in the number of satellites received. I have tried placing the re-radiating element at different places around the GPSr, with the best location determined to be directly over the GPSr internal antenna (no surprise there). Whenever the re-radiating antenna is removed, satellite reception improves.

 

So what to do? One thing I can do is attach the GPSr to my backpack so that its internal antenna is pointed toward the sky. But what I'm wondering is there something I'm missing, something I can do to make the re-radiating antenna idea work better? Your thoughts would be appreciated.

 

Jeff

[+tirediron]

That doesn't sound like the performance gain that I would expect from a re-rad. One thing that does come to mind however is the location of your AE. Depending on your direction of travel, and the relative location of satellites in your area, your hat-stand could be blocking out a fair amount of signal. What I do with the remote for my 76CS is to put it on my hat. I have a Tilley hat with a small pocet at the crown (which is flat by design). Into that I slip a 3x3 square of 18AWG steel and the let the magnet in the AE "clamp" onto that. Looks a little daft, but it's effective.

 

For carrying my Legend, I found it most effective to clip it in a more or less horizontal position on the top of the shoulder strap of my back-pack.

[+PDOP's]

I don't have any direct experience with them but have you read this review?

[+blindleader]

The intended place for the re-radiating loop is attached directly to the etrex in such a way as to overpower the signals arriving directly from the satellites. etrex owners will have to tell you exactly where that is. But I suspect having the loop anywhere within a couple inches of the internal antenna should make a major difference.

 

I don't know what to say about the loop actually degrading reception. It sounds as if the amplifier is either turned off, or not working. If I turn mine on with my Meridian it immediately maxes out all the satellite strength bars. Don’t expect it to get more satellites than without it. If there’s any signal at all, the etrex alone will pick it up at least intermittently. The amplifier can only amplify what’s there.

 

edited to keep it short and to the point

Edited September 13, 2004 by blindleader

[+NightPilot]

I also have the reradiating antenna and a Legend, and my experience has been different. You do need to have the reradiating loop directly over the internal patch antenna, held by some sort of elastic. The rubber band provided doesn't last all that long, but I've found that the blue rubber bands found on vegetables in the supermarket work pretty well. The ones on broccoli are about the right size. With the loop held directly over the patch antenna, I always get very much improved signal strength. Putting it somewhere else on the GPS doesn't help much, though. You do have the batteries installed, the amplifier turned on, and a strong blue light, I assume?

[appletree]

I use the PC-Mobile antenna with my Vista and it always works great. Never any doubt, the strength bars zoom up quickly. I also use the brocolli rb, but I cut it in half so it doesn't press any buttons. I took the metal plate out of the antenna (actually, I bought 2 and left one intact) so it really weighs next to nothing. I use a small metal clip to attach the cable to the back of my shirt collar.

 

My 2 units work so well, I would encourage you to check yours out carefully and contact PC-Mobile if it continues to dissapoint you. I would strongly suggest trying a second unit before giving up on it. I have been very impressed with the signal strength my units produce.

[jmason4531]

Thanks to all who have responded.

 

Yes, the batteries are installed, the blue light is bright, and I have put the re-radiating loop directly over the internal antenna. My indoor experience did suggest that the unit is functioning. However, from the experience of others, it seems that I may indeed have a duff unit.

[+Lucanos]

Hi All,

 

As a quick aside - I notice that PC-Mobile offers the amplifier with 2 different antenna types. Being the GAS3Tx & the GAS76x.

 

Which of the two do you guys have?

(Just wanting to ensure that if when I purchase one I can be confident that it will work)

 

Thanks

Luke

[+NightPilot]

I have the MCX. That's probably the best, since it's the smallest and lightest, unless you plan to use the antenna alone, connected to a GPS that uses BNC. BNC is much larger and heavier, and a much older design. If you only plan to use the antenna connected to the amplifier as sold, then I would get the MCX.

 

The only metal in the antenna are the magnets, and they aren't that heavy. The bottom seal is only something like Mylar, very thin, light and flexible. I keep the magnets in mine, because I mostly use it on top of my cachemobile. A large washer permits attaching it to lots of stuff.

[appletree]

Actually, there is a 1.2 oz bright silver metal component into which the magnets fit on the bottom and the antenna assembly on the top. It was not easy to pry loose from the plastic cover but, after peeling off the mylar seal, I was able to do it without damaging anything. I placed the antenna back into the plastic cover and secured it with a piece of closed cell foam. I taped the two small magnets inside and re-sealed the bottom. I have noticed no loss of signal strength and I have less than half the original weight hanging from my collar. No reason to go to the trouble unless you will be wearing it on your head or neck, which I do.

[+NightPilot]

That metal would likely be the ground plane, and it generally makes a difference in reception, unless you put the antenna on some other metal that will act as a ground plane. Well, it's your antenna.

[Prof. Y. Lupardi]

Most of the active antennas use a patch antenna. Those come in a complete package containing (I think that is because of physics laws) a built-in ground plane. So any metal you see that is not an integral part of the ceramic? antenna is for other purposes.

 

But if not: put the antenna on your head. The water in your head will function as a groundplane.

Edited September 15, 2004 by Prof. Y. Lupardi

[jmason4531]

The saga continues...

 

I contacted PC-Mobile about my concerns with the performance of the re-radiating antenna. Here is their reply:

 

"The problem is, when you use the re-radiating antenna, you should not allow your

GPS to receive GPS signal. If both GPS and re-radiating are receiving signal at the same time, miscalculation will occur and it may not even display proper location.

 

If you use it outdoors, wearing the GPS on a waistbelt will effectively block GPS signal by your body thus allowing the re-radiating antenna to work.

 

If you are experimenting, try as you did before outside the house, then use a piece of metal above the GPS to block siganl to the GPS and you should see the improvement."

 

Any comments from the peanut gallery?

 

Jeff

[+Lucanos]

Makes sense...

I guess (harking back to HS Physics here) the unltimate solution would be to enclose your GPSr in a Faraday Cage or some sort (maybe fine metal mesh).

 

Dunno how you'd do that and still make it visible & easy to carry, maybe a wire mesh?

No doubt there are posters here with a better understanding of RF Engineering than I who can comment.

[+NightPilot]

You don't need a Faraday cage. You hand will work well enough, and you only need to cover the top of the antenna - not the entire GPS.

 

I think PC-Mobile is being overly conservative. The loop from the reradiating antenna covers a lot of the antenna anyway, and I haven't noticed a problem with both antennas receiving. I don't often have the Legend placed so that it's receiving good signals, though. It's usually inside my car or in my pocket. I admit I haven't done an exhaustive scientific study of this.