GPS Antenna Question

[Constancia]

I'm running a Garmin 128 fixed mount GPS on my boat. Have a new Garmin 29 antenna mounted up on the T-top. Gets great reception. I also have a Garmin GPS 76map handheld as a back-up unit and occasionally use it, as well.<br /><br />I bought the BNC to MCX cable adapter that will allow me to plug the handheld into the external antenna on the T-top. Haven't tried it yet, but it should work just fine. <br /><br />So here's my question: If I installed a T-connector on the coax BNC connector between the antenna and the fixed mount unit, could I run the handheld off it at the same time? In other words, run both GPS devices off the same antenna at the same time? I haven't tried this yet, but don't see why it wouldn't work, unless it screws up impedance or something by adding a second device to the cabling. Anybody running two devices off the same antenna? Or should they only be run one device-one antenna?<br /><br />The reason for this is that the handheld captures more data and allows me to upload/download from Blue Water charts on my computer. The fixed mount doesn't. <br /><br />Thanks!<br />Stuart

 

[JB]

Re: GPS Antenna Question

Don't think so, Stuart.<br /><br />It would disrupt the impedance match between the receiver and the antenna.<br /><br />One or the other should be okay, but not both.

 

[Constancia]

Re: GPS Antenna Question

Thanks, JB. I kind of suspected that might be the case. The handheld has its own antenna and seems to do fine reading satellites while laying on my console, but the antenna on the T-top gets better reception and generally sees more satellites. So... of course I had a great big brain fart thinking I might be able to run both off that antenna. <br /><br />The fixed mount has a bigger display and is much easier to use for navigation. But the handheld is more versatile with better functionality.<br /><br />Kind of like a boat, eh? There hasn't been one made yet that fits everything you want might to use it for!

 

[swist]

Re: GPS Antenna Question

Depending on the frequency and antenna/receiver design, some situations are less sensitive to impedance mismatches than others. Since most GPS units show signal strength on a per satellite basis, it wouldn't hurt to try to see what the results would be.<br /><br />Of course, if JB knows for a fact that this won't work at all, then I defer....

 

[Boatist]

Re: GPS Antenna Question

I do not think I would try it. The reason is the units supply power to the antenna. If the power levels are not the same then I think you could blow up one unit. Most fixed mounts run on 12 volts where handhelds run on 2.8 to 3.0 volts. For sure before you try it call your manufacture.

 

[Constancia]

Re: GPS Antenna Question

Well, in the interest of not screwing anything up, I'm going to email Garmin Tech Support and see what they say about it. Of course, they are in business to sell antennas, so...<br /><br />The difference in voltages between the units - I kind of doubt this because either unit can use the external antenna. The only thing coming coming out of the antenna is a coax cable, which would be strictly signal, not voltage to power the antenna. If this was a risk, they wouldn't sell a BNC to MCX connector cable so that you could plug the handheld into the external antenna.<br /><br />On edit - In talking with a resident guru here at work, he verified that there is a voltage sent through the coax that powers the antenna. He said you could put a capicator in line from one of the devices to the center conductor of the coax to shunt the voltage from one of the devices and then it would probably work fine. The antenna module has a voltage regulator circuit that will limit the voltage to what it needs, so there could indeed be different input votages. However, that's more engineering than I want to deal with, so I'm not even going to try it. I can just leave the GPS on the console and let it use its own internal antenna. <br /><br />Thanks for the input, guys!<br />Stuart

 

[ahein]

Re: GPS Antenna Question

You might be interested to know that I bought an "emergency" antenna on eBay for $20 bucks, and it works better than my Garmin GA-29 antenna! The Garmin antenna finally crapped out after four years so I tryed my cheap $20 dollar unit. I was surprised to get all twelve satellites showing good signal strength and WAAS accuracy to within 9 feet. The antenna unit is waterproof, is about 2" square by 1/2" tall, and comes with long cord and a BNC plug on the end. I've been using it for the last 3 months without any problems and it's right on the money. It could be an inexpensive way to allow you to use your handheld at the same time as your fixed-mount.<br />Just do a search on ebay for "GPS BNC antenna" and should find what your looking for. Several people selling them. Good luck.

 

[Constancia]

Re: GPS Antenna Question

Thanks, ahein - I'm an Ebay Eholic, so I'll check that out. That's small enough I could mount it very easily somewhere. Wish I would have know about these earlier -- my GA 28 antenna bit the dust on my last trip. It was five years old. I didn't realize that they had a lifespan to them, but my buddy told me he's experienced the same thing, generally 3 to 5 year, then kaput. A back up antenna would have been nice. At that price, I can buy a couple and keep one in my spares box.