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2 GPS antenna under cowling?

LyleM

Active Member
I want to mount 2 GPS antennas on a shelf mounted to the firewall under the cowling. One would be from a Garmin ADS-B transponder, and the other from a Garmin GPS NAV unit. Will this work and if so how close can they be to each other?
 
I think this will work Lyle. Friend of mine has an -8 with two antennas mounted on a tray under the cowl.

My real reason for answering your question, however, is to point out something I learned when I mounted just one antenna under the cowl: be sure to mount the antennas sufficiently far forward of the firewall. Just how far, I can't say so hopefully someone else will chime in, but at one time I had a GPS antenna under the cowl but only about two inches ahead of the firewall. I found that depending on the direction of flight the weather updates could be very infrequent. For example, one trip from Dallas to Oshkosh a few years ago, there was considerable rain along the route in North Texas and through Missouri. As I approached an area that looked very ominous, my Sirius Satellite Weather had not updated for 45 minutes and I was facing some very dark clouds ahead. My theory is that the satellites are basically in southern orbits and consequently the canopy, roll bar, and firewall, forward baggage area block the antenna from "seeing" the satellites when headed north (low angle). The opposite was true when headed southbound. I ended up in this particular instance circling in order to get an update and eventually landed in Missouri to wait out some of the closer storms. Not long after that trip, I removed the antenna and mounted it back on top of the instrument panel where it was originally. No issues since. I now have two GPS antennas on top of the instrument panel (one ADS-B and one for NAV), right next to each other and no interference between them.

So just a thought to look into how long a shelf to build and how far away from the firewall to place the antennas. Check with some others but IMO I don't think you'll have an issue with interference.

Chris
 
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You might want to check with the manufacturer of your antennas... the installation manual for at least one of their other products contains the following warning:

Due to the excessive temperature environment and large areas of signal blockage caused by the fuselage, mounting the antenna under the engine cowling (forward of the firewall) is not recommended and likely will not provide adequate GPS reception.
 
I've got 2 antennas on a shelf on my 7, one for the G3x the other for the GNX375. Has worked fine for the first 100 hours. They're about an inch apart from each other, and about 3 inches forward of the firewall. I've monitored the GPS status and have never seen less than 12 satellites in any portion of flight, normally it's a lot more. I realize I'm not following manufacturers suggested installation, but so far I've seen no degradation in performance in either system.
 
I would never mount a IFR GPS antenna under the cowl, extremely harsh environment for such a critical item. GPS antennas are ‘active’ units with built in amplifiers, like all electronics , they don’t like heat or high energy spark producing devices (like ignition systems) in close company.

Of course if the loss of a particular GPS source doesn’t effect the outcome of a flight then go for it.
 
Back in the days before WAAS, i had GPS antennas mounted on a shelf under the cowl, foreword of the firewall, and things worked fine. When we went WAAS, we got lots of drop-outs - I think because the signal levels were more critical and there was blockage from the firewall when the antenna needed to “look” aft. I generally lived with it until ADS-B came along - when the GPS solution drops out, your ADS-B gets flagged, and when that happens enough, you get a letter from the FAA saying you are non-compliant, and have to fix it!

I have since moved all antennas out from under the cowl, and have them where the antenna manufacturers recommend…..

Paul
 
The original builder of my airplane put the IfR GPS antenna here under the cowl and it’s worked fine for 500 hours, first on his 430W and now on my IFD440. I’m sure it’s hot under there but so far-so good. I do have a couple of other GPS inputs (AFS 5400 EFIS with GPS puck on the glare shield and a backup G5), not to mention Foreflight on 2 or 3 iOS devices.


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Just in case you don't already know, A garmin GA-35 teardrop style antenna will fit on the tailcone aft of the canopy skirt. At least on the RV7 slider The canopy will clear it with about 1/2" to spare as it retracts. I assume the 8 is similar.

This is where both stein and my avionics shop recommended installing it. Since it's feeding an IFR GPS I elected to follow that advice.


Here's a cut and paste from Steins website;

GA-35 – WAAS GPS Antenna
This antenna should be mounted on the top of the aircraft, either in front of or behind the canopy. It should not interfere with opening the canopy. If there are two GPS antennas being installed they can be mounted in-line or offset. Mount them at least 6 inches apart. Typically used with Garmin GTN units.
GA-26X – Internal Mounted GPS Antenna (formally GA-26C)
This antenna can be mounted on the glare shield or on a mounting plate behind the panel. Typically used for the PFD and/or MFD gps source.
GA-57x – Combo XM/GPS Antenna
Garmin GPS + a Garmin XM receiver. This antenna can be mounted under or above the aircraft along the fuselage. Mount at least 6 inches away from other antennas. Typically used with G3X system with xm weather.
 
Garmin antenna mounted on the turtledeck of my previous RV-8:

i-HRF2XJr-L.jpg
 
I have a shelf in front of the firewall with two GPS antennas (G3X touch & GTX335 with gps) in close proximity. Just my data point so far, but I have not seen any negative issues yet, passed the ADS-B test, seeing a display of 7ft accuracy in flight. (early days still).
 
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Garmin antenna mounted on the turtledeck of my previous RV-8:

Did the same for the GTN-650 antenna on the RV-8A and RV-8. The SkyView GPS antenna is happy on top of the glare shield - covered with a piece of dark cloth to prevent glare on the canopy.

Carl
 
For your consideration.

I have a 7 with two GPS antenna mounted under the cowling. The temps measured in operation show under the recommended limit, and they are high enough for minimum shading from the rear but better view forward and to the side. 250 hrs. No dropouts ever, but have not flown everywhere.

One issue though - I failed the 35 on an IFR plan - thankfully near the airport and VFR at the time. It failed and blocked all the other GPS under the cowl as they were too close together. A failure mode unaccounted for in the locations. Next time the 650 breaker will get pulled.

Although I reinstalled in the same location, if the time clock were rewound - it would go aft of the cockpit where a failure would not have overpowered and jammed the other GPS antenna.
 
The builder of my RV7A mounted the two non-WAAS gps antennas on separate mounting brackets under the cowl about 3-4” forward of the firewall. Two years ago I mounted a WAAS antenna between the two other gps antennas for my Garmin 335 adsb with internal gps. The airplane now has over 540 hours on it and the two nonWAAS gps antennas are still working and my WAAS gps antenna has over 100 hours without problems.

However, if I was to do it again I would not place them where they’re at. The environment gets really hot and like others have said electronics don’t like heat. Frankly I recommend placement on top of the aircraft behind the cargo panel where you can easily use doublers and get access if needed. Remember, putting ascetics above functionality and reliability is always a poor trade off.
 
Bach There, and A Fantasy

I was chary about putting a GPS antenna under the cowl. Nice to get it far forward in our tailheavy RV-6. Not too hard to keep it cool enough… until you park the airplane. So I mounted mine on a simple bent aluminum bracket on the fore and aft bit right behind the roll bar, under the canopy. Works great and I hope its a reasonable compromise for Wt. & Bal. considerations. At least its really light. I’ve had a Wild Thought about mounting it under the forward deck, between the firewall and the structural bulkhead forward of the instrument panel. I’d have to cut a fairly generous sized hole, maybe a round 6” diameter one, in the skin to add a fiberglass radome, and I suppose that radome would need to be fairly thick to handle the structural load of the missing aluminum. And of course it would beed to be removable from outside the airplane because I’m getting old and crotchety about going under the panel these days. Yeah, a lot of work for not much gain. But kinda cool, maybe?
 
Wire Routing

How are you guys routing your coax to rear your mounted GPS antenna? Are you taking it down the top side rails or under the floor? Are you going up the rear seat bulkhead through the baggage compartment or taking it all the way back the the last baggage compartment bulkhead, up the back side and then forward on the top of the baggage compartment? Any pics???

My plan is to go under the floor and up the rear seat bulkheads. Unless someone has a better idea.

Thanks guys.
 
RV8 GPS Antenna Location

All three GPS antenna's -- work great here...

Right over the panel... Easy wiring !!

Painted them too.. No Problem. (Walmart.. Flat black paint).. yes did testing to show no. issues with painting

Note location of temp sensor..

See log link below for more details


Al


AL9nZEUEXLNBDXbF3Vrc5ivdoFP_T_b1C1jgOdoN4mki-f-D1YwjtnRcMfr2H5N-uZakDDl1RuWJDHH1nmOZCxzI8gSNpgAvyCz93I1eu2U7eR6Q9DsFAL7zfBNiJts_90-spRaR6YI3DJHiF37FF1ezo7LH=w750-h500
 
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You may get the report of worked fine from many but it really depends on what is meant by 'fine'

On my last build, I put my G3X antenna under the cowl, maybe a couple of inches from the FW and I do pick up satellites and position without any issues but upon investigating a different issue, I was told by Garmin that my GPS data is not as reliable as it needs to be and since the AHAR drives some of its calculation based on GPS position, I was advised to look into it.

I installed the antenna on the glareshield and the data was clean and as reliable as it can be expected.
Also be mindful that for WAAS antenna, there is a minimum and maximum cable run, at least for Garmin, so you may end up with a coil of coax under the penal.
 
How are you guys routing your coax to rear your mounted GPS antenna? Are you taking it down the top side rails or under the floor? Are you going up the rear seat bulkhead through the baggage compartment or taking it all the way back the the last baggage compartment bulkhead, up the back side and then forward on the top of the baggage compartment? Any pics???

My plan is to go under the floor and up the rear seat bulkheads. Unless someone has a better idea.

Thanks guys.

The RV-8 design provides for a wire run on both port and starboard sides under the canopy rail. Look at the holes at the top of the side ribs - these accept snap bushings.

On the current RV-8 I used these holes for:
- Static tubing to the panel (feeds the panel mounted analog airspeed instrument)
- SkyView network cable
- GTN-650 GPS antenna coax
- Wire to connect wing mounted OAT sensors to the ADHARS modules
- ELT power, GPS position serial, and ELT “phone” control cable

Under floor cables include:
- Power from the rear mounted PC-625 battery and associated control wiring. Forward PC-625 is mounted in the forward baggage well.
- ADS-B receiver (single four #22 conductor shielded wire)
- Autopilot pitch servo cable
- Tail NAV/Strobe cable
- AOA and Pitot tubes

Carl
 
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