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Old 08-26-2022, 03:34 PM
LyleM LyleM is offline
Join Date: Jan 2008
Location: Milan, MI
Posts: 76
Default 2 GPS antenna under cowling?

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?
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Old 08-26-2022, 03:59 PM
chrispratt's Avatar
chrispratt chrispratt is online now
Super Moderator
Join Date: Oct 2005
Posts: 831

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 Pratt (2022 VAF DUES PAID)
RV-8 Flying, 900+hours
Lycoming O-360-A1A, Hartzell CS
52F (Northwest Regional, TX)

Last edited by chrispratt : 08-26-2022 at 04:01 PM. Reason: phrasing
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Old 08-26-2022, 04:15 PM
mburch's Avatar
mburch mburch is offline
Join Date: Feb 2005
Location: Oregon
Posts: 1,456

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.
Matt Burch
RV-7 (last 90%)

Any opinions expressed in this message are my own and not those of my employer.
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Old 08-26-2022, 04:48 PM
agent4573 agent4573 is offline
Join Date: Aug 2019
Location: Mountain view
Posts: 614

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.
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Old 08-26-2022, 05:18 PM
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Walt Walt is offline
Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 6,913

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.
Walt Aronow, DFW, TX (52F)

EXP Aircraft Services LLC
Specializing in RV Condition Inspections, Maintenance, Avionics Upgrades
Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
FAA Certified Repair Station, AP/IA/FCC GROL, EAA Technical Counselor
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RV7A built 2004, 2000+ hrs, New Titan IO-370, Bendix Mags, MTV-9 prop
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Old 08-26-2022, 05:32 PM
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Ironflight Ironflight is offline
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Join Date: Jan 2005
Location: Dayton, NV
Posts: 13,229

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 F. Dye
Editor at Large - KITPLANES Magazine
RV-8 - N188PD - "Valkyrie"
RV-6 (By Marriage) - N164MS - "Mikey"
RV-3B - N13PL - "Tsamsiyu"
A&P, EAA Tech Counselor/Flight Advisor
Dayton Valley Airpark (A34)
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Old 08-26-2022, 05:41 PM
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MacCool MacCool is offline
Join Date: Aug 2020
Location: central Minnesota
Posts: 1,340

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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RV-9A, 2011, bought flying
IO-320D1A (factory new), C/S
Dual Pmags
IFR equipped
AFS 5400/3500, G5, IFD440 navigator,
bunch of other stuff

Last edited by MacCool : 08-26-2022 at 05:51 PM.
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Old 08-26-2022, 06:32 PM
N82VM N82VM is offline
Join Date: May 2017
Location: Warrenton, VA
Posts: 226

I read this earlier this week
The GPS under the cowl is a better look, but Iím going with the advice from the kit-planes article
RV-8A Project
2022 Dues Paid
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Old 08-26-2022, 07:22 PM
LyleM LyleM is offline
Join Date: Jan 2008
Location: Milan, MI
Posts: 76

Thanks guys, I will mount both behind the passenger. One on each side of the rail.
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Old 08-26-2022, 07:47 PM
Desert Rat Desert Rat is offline
Join Date: Aug 2018
Location: Wichita KS
Posts: 1,051

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.
Terry Shortt
AGI, CFI, CFII, MEI, A&P, Janitor
RV7 Empennage & Wing done
Fuselage almost done
Avionics almost done
90% done, 90% to go
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