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Two receivers from one GPS antenna?

f14av8r

Well Known Member
I expected to answer this question with a quick google search and / or VAF query. No luck. So, now I'm posing the question to all of you.

My hangar neighbor has a Garmin GTX-327 that he is replacing with the Stratus ESGI system. The ESG requires a GPS antenna input. Appareo (Stratus) provides an antenna with their unit. He also has a Garmin 430 (non-WAAS) which is connected to a Garmin antenna. As we ponder the install, I questioned if we could just split off the GPS signal from his existing 430 puck antenna or, perhaps replace the 430 antenna with the provided Stratus GPS antenna and supply both the 430 and the Stratus ESG from the same antenna.

I don't have any experience sharing GPS antennae among receivers. Do any of you know if it is acceptable to split a GPS antenna across multiple receivers?
Thanks!
Randy
 
Not a direct technical answer, but I'm pretty sure ADSB Out needs a certified WAAS position source. A non-WAAS 430 won't do that.
 
WAAS Source vs Antenna

Not a direct technical answer, but I'm pretty sure ADSB Out needs a certified WAAS position source. A non-WAAS 430 won't do that.

Hi Charlie,
I see your point but, I'm not sure that all antennas are "position sources." In some systems, I think the antenna may be just an antenna. In others, like the Dynon Skyview system for example, the puck is both an antenna and a receiver. In others, I think the antenna is just that, an antenna, and the avionics is actually doing the work. I may be completely wrong about that and, at least for me, this is one of the points of confusion - thus my question. I look forward to hearing what the rest of the experts have to say.
Thanks!
Randy
 
Some GPS pucks do everything; you feed them power & they output a digital data stream. With those it's typically easy to split signals. Others may output IF (intermediate frequency), meaning that they have both 1st stage gain and at least one frequency downconverter in the puck. Still others (rare) may send raw, but amplified, 1.5 Ghz down the coax. There may be some that are simple antenna patches, with no electronics inside at all, but I wouldn't bet on seeing one since moving an unamplified 1.5 Ghz nanowatt level signal through a run of coax is not an easy task.

The 430 definitely sends DC power through the coax to the puck, so something active is obviously happening in the puck.

And you still don't have a WAAS signal; upgrading a 430 to WAAS requires replacing the GPS puck. And, we still don't know what the Stratus has for a GPS.
 
The answer to this one is easy. Does the Appareo ADS-B install manual say you can split the antenna wire and share it? If not, you can't, even if it would technically work.

The FAA expects us to follow the manufacturer's instructions when installing ADS-B. If you don't, demonstrating ADS-B compliance is up to you, and that involves a lot more than passing an automated flight check once.
 
The ESG transponder has a built in, approved (for ADSB-out) gps receiver.

As others have said, gps antennas come in all kinds, and what happens depends on what's really in there if you try to split it. That includes what the splitter does to any incoming power.
I suspect it's academic. Read the ESG approval paperwork. I'll be there's one (or maybe more) antennas that have to be used, for the box to qualify for 2020 ADSB-out rules. And I'll bet there's no mention of splitting the signal.

I'd just find a way to mount 2 antennas - and not too close to each other.

Edit: I see previous post said the same thing.
 
The safe solution

You guys are all correct I'm sure. Obviously, the safe and surely compliant solution is to simply install the antenna that Appareo sent along with the Stratus ESG box and make a direct connection. But, as this is EXPERIMENTAL aviation, I'm always looking a little outside the box. So, I'm trying to figure out if I can share an antenna (either the existing Garmin or the supplied Stratus) across both devices.

I found a few GPS splitters on the market. They are all in the $300 range and all pass DC voltage from one receiver to the antenna and block DC voltage from the other receiver. They also place a dummy load in the blocked line to fake the blocked receiver into thinking its seeing a valid antenna. The question of course is if both devices are looking for the same amplified signal coming back down that coax! More research required!
 
Ant

The 430 WAAS antenna is different than the one for the original 430. You have to have the correct antenna for proper function. The WAAS antenna, IIRC, uses a threaded connector instead of the older BNC. There is a minimum length for the lead to the antenna as well.
 
I found a few GPS splitters on the market. They are all in the $300 range and all pass DC voltage from one receiver to the antenna and block DC voltage from the other receiver. They also place a dummy load in the blocked line to fake the blocked receiver into thinking its seeing a valid antenna. The question of course is if both devices are looking for the same amplified signal coming back down that coax! More research required!

Hi Randy,

I used one of the GPS Splitters (GPS Networking LDCBS1X2) to split a Gilsson GPS antenna (MCX1M09B090-AWMG) between my Navworx ADS600-EXP ADS-B Transceiver and my Garmin GDL-39 ADS-B receiver. While another GPS antenna was cheaper than the GPS splitter, I didn't have free room for yet another antenna (I already have 3). My setup has worked flawlessly for the past year. I purchased the splitter on Ebay for under $100

DSCN4708.JPG


I think splitting the GA-56 non-WAAS antenna would probably work for you just fine. While I am not an antenna expert, the specs on the WAAS and non-WAAS antennas from Garmin are nearly identical in terms of frequency range, gain, noise, etc.

GA-56 (non-WAAS) installation manual: http://static.garmin.com/pumac/930_InstallationInstructions.pdf
GA-35 (WAAS) installation manual: http://static.garmin.com/pumac/GA35GPS_WAAS-antenna_AntennaInstallationInstructions.pdf
 
You guys are all correct I'm sure. Obviously, the safe and surely compliant solution is to simply install the antenna that Appareo sent along with the Stratus ESG box and make a direct connection. But, as this is EXPERIMENTAL aviation, I'm always looking a little outside the box.

Unfortunately, ADS-B OUT is not experimental. These are signals sent out which are used to manage the national airspace system, not just your airplane. Things get different when that's true and the category of the aircraft doesn't matter.

It's not legal to fly into 2020 airspace unless you have a 91.227 compliant system on board. The manufacturer has made a statement that if you install it the way they say, you will be compliant. Go away from their instructions, and now it's up to you to determine. How exactly are you going to determine that you meet 91.227 and all the performance requirements of the TSO (hundreds of pages)?
 
Randy - I hate to put this in such stark terms... Just...don't...do...it.

Yes, there might be splitters and things available, but the bottom line remains the same - it is extremely inadvisable to try to get very sensitive (not to mention expensive) GPS receivers to all work through a single antenna.

The DC power that's fed up the center of the coax in the Garmin and others is used to power pre-amplifiers in the antenna. The GPS signal from the satellites is tiny tiny tiny. The pre-amplifier boosts the signal to a level which allows us to use a few feet of reasonably small coax cable to install the antenna in our aircraft. Without the pre-amplifiers in the antennas we'd be allowed about three feet of coax the size of a sink drain to connect the receiver to the antenna. Each antenna has a specific amount of amplification, or "gain" to match the input signal requirements specific to each GPS receiver. Some want 30dB of gain in the antenna, some want 24dB of gain, some want only 10dB of gain. What's good for one GPS receiver isn't good for another.

At the end of the day, what are you saving by splitting antennas? Not much. What are you risking by splitting GPS antennas? Lots. It's just not worth it, IF it's feasible at all. And you will only know if it's feasible after you've spent $300 on a splitter and found it doesn't work.

Again, I apologize for sounding so negative, but the facts are the facts. Your best solution for an aviation application using disparate GPS receivers is to use the manufacturer's recommended antenna and coax for each receiver.
 
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