GPS Antenna Ground Plane

Hey - I am getting ready to mount the GPS antennas for my 650 and G3X and reading the Garmin manual, it says that "Although no ground plane is required, the antennas typically perform better when a ground plane is used."

Right now, I am leaning towards mounting both of them on top of the the FG cabin top... which obviously would not provide a ground plane. Has anyone had issues with GPS antennas mounted on the FG casbin top, as opposed to further aft on the AL fuselage?

Thanks,
Jason

My main G3X and GTN 650 antennas are mounted on top of the cabin between the doors with no ground plane. No issues in 6 years of IFR flying whatsoever.

I put mine (G420W antenna) behind the baggage bulkhead, because:
1. No holes to drill in composite cabin top, which, iirc, Vans frowned on;
2. Ground plane;
3. At that time, there was a minimum coax length (loss) specified. I thought I might as well put that extra coax to use.

I also placed my GPS antennas aft of the baggage bulkhead on the top of the tail cone. I talked to Van's builder support and they said Van's advises against drilling any holes in the fiberglass cabin top that are not specified in the build plans, since the cabin top is a critical structural element and additional holes would weaken it. Also, placing it on the tail cone provides a ground plane for the antenna. I also have one of my comm antennas and my ELT antenna on the tail cone.

Just my experience. Since this is an EAB aircraft, you are of course free to place the antennas where you wish.

Cheers,

Ground plane NOT needed...

GPS antennas (GA 35, GA 36, etc.) do not require a ground plane as they are NOT a "monopole" type antenna.

Don't believe me? Mount it to a piece of cardboard and watch it pick up the satellite constellation.

Does your GPS in your phone need a ground plane?

Carl

Additionally...

Since the aft cabin/fuselage of the RV-10 is fiberglass (and by extrapolation not RF-Opaque) - consider fabricating a shelf of sorts and mount all the GPS, SiriusXM antennas on the inside of the airframe...

bjdecker said:

GPS antennas (GA 35, GA 36, etc.) do not require a ground plane as they are NOT a "monopole" type antenna.

Don't believe me? Mount it to a piece of cardboard and watch it pick up the satellite constellation.

Click to expand...

I guess you'd be willing to bet you life on that because that's exactly what you're doing, and advising others to do the same is irresponsible, or perhaps you think you know more than the manufacturer of the equipment? Flying an LPV approach to minimums in actual bad weather is probably not the time to test your 'cardboard theory'.

The Garmin GTN STC install document for the GTN/GA35 requires a minimum radius of 7.5", might be smart to take their advice vs what you read on the internet or VAF.

Since the aft cabin/fuselage of the RV-10 is fiberglass (and by extrapolation not RF-Opaque) - consider fabricating a shelf of sorts and mount all the GPS, SiriusXM antennas on the inside of the airframe...

Click to expand...

PS: as far as mounting them all together like you suggested the following is from the manual:

NOTE:
The internal GTN unit COM does not interfere with its own GPS receiver. However,
placement of the GTN GPS/SBAS antenna relative to transmitting antennas (including the
GTN unit COM antenna) is critical.
b. No closer than two feet from any antennas emitting more than 25 watts of power. If an EMC
check reveals unacceptable interference, a different GPS antenna location can be selected.
c. No closer than nine inches (center to center) from other antennas, including passive antennas
such as another GPS antenna or XM antenna.

Bob Archer says...

Bob Archer says...

Ref section "TENNA TIP #3" in "Antennas for Aircraft" PDF at http://www.aeroelectric.com/articles/Antennas/Antennas_for_Aircraft.pdf

"The installation of all these types of (GPS) antennas in a non-conductive composite aircraft, or even fabric, is simple in that none of them require ground planes and they are so small and light that they do not require much room or structure to support them. All that is required is that they be mounted in the top part of the aircraft looking up in such a manner that they can see most of the sky through the non-conductive skin when installed. In metal or conductive aircraft of course they must be installed on outside of the conductive skin of the aircraft or installed inside a fiber-glass fairing of some type. On tandem type aircraft with canopies the top of the roll over structure is a great location. We will just leave the details to the builders."

johnbright said:

Bob Archer says...

Ref section "TENNA TIP #3" in "Antennas for Aircraft" PDF at http://www.aeroelectric.com/articles/Antennas/Antennas_for_Aircraft.pdf

"The installation of all these types of (GPS) antennas in a non-conductive composite aircraft, or even fabric, is simple in that none of them require ground planes and they are so small and light that they do not require much room or structure to support them. All that is required is that they be mounted in the top part of the aircraft looking up in such a manner that they can see most of the sky through the non-conductive skin when installed. In metal or conductive aircraft of course they must be installed on outside of the conductive skin of the aircraft or installed inside a fiber-glass fairing of some type. On tandem type aircraft with canopies the top of the roll over structure is a great location. We will just leave the details to the builders."

Click to expand...

I suggest you contact Bob Archer and see if he agrees with your implication that his rather ancient article, where he talks about hand held GPS and the likes of Loran, would apply to the installation of certified GPS approach equipment.

I'm just not sure what's going on here, is everyone just trying to find a way around the recommended installation guidance offered by the manufacturer, continually searching the internet for reasons not to do as suggested... weird.

I guess the nature of EAB is; because I'm building the airplane I can do it however I want, and I'm smarter than everyone else.

Right or wrong, here is what I did...

Walt said:

I'm just not sure what's going on here, is everyone just trying to find a way around the recommended installation guidance offered by the manufacturer, continually searching the internet for reasons not to do as suggested... weird.

I guess the nature of EAB is; because I'm building the airplane I can do it however I want, and I'm smarter than everyone else.

Click to expand...

Walt (and others),

If reading articles published in/by/for IEEE, ARRL, etc. qualifies as "searching the internet" count me in; the library is pretty inefficient.

Personally, I want to understand WHY things are documented/specified the way they are (e.g. WHY does the Garmin STC specify a ground plane, when the text elsewhere suggest it's "nice to have" or not needed at all, based upon the actual type of antenna and how antennas work.).

Your description of EAB and the reasons we build is inaccurate and needing correction; I build because I wanted to understand why & how things work. Then, how I can improve a system or systems in aesthetics and efficiency, while applying my experience and education. I'll submit the Wright Bros, Lockheed Bros, Mooney Bros, Curtis, Northrop, VanGrunsven, Rutan, Hamilton, Neibauer, Patey and many many others as examples of this spirit.

Build on.

bjdecker said:

Walt (and others),

If reading articles published in/by/for IEEE, ARRL, etc. qualifies as "searching the internet" count me in; the library is pretty inefficient.

Personally, I want to understand WHY things are documented/specified the way they are (e.g. WHY does the Garmin STC specify a ground plane, when the text elsewhere suggest it's "nice to have" or not needed at all, based upon the actual type of antenna and how antennas work.).

Your description of EAB and the reasons we build is inaccurate and needing correction; I build because I wanted to understand why & how things work. Then, how I can improve a system or systems in aesthetics and efficiency, while applying my experience and education. I'll submit the Wright Bros, Lockheed Bros, Mooney Bros, Curtis, Northrop, VanGrunsven, Rutan, Hamilton, Neibauer, Patey and many many others as examples of this spirit.

Build on.

Click to expand...

That's all fine and good... for you and your aircraft.

I suggest you stop telling folks that based on your "research" doing it your way is the correct way and basically the folks at Garmin don't know what they are talking about. I'm pretty sure the testing, engineering and flight test work to develop the STC went a bit beyond your "mount it on a piece of cardboard" test.

"If you are the smartest person in the room, you are in the wrong room."

bjdecker said:

Personally, I want to understand WHY things are documented/specified the way they are (e.g. WHY does the Garmin STC specify a ground plane, when the text elsewhere suggest it's "nice to have" or not needed at all, based upon the actual type of antenna and how antennas work.).

Click to expand...

The understanding of "why", in the case of GPS antennas, requires a pretty thorough understanding of electromagnetics in the L-band. There aren't a whole lot of folks who can distill all the complexities of how flat patch antennas working at L-band interact with all the "stuff" that's around them. One of the items that falls into this "stuff" category is the ground plane.

Having done a fair bit of work in this domain, all I can say is that much of what goes on appears to the layman as being pretty much "pure freakin' magic". I have been fortunate to have worked with some very, very smart people who were able to boil all the complex math involved in antenna theory down to a few simple "rough rules of thumb". Without those guys I would still be dumb as a post and heavily reliant on "magic" as the explanation for how these antennas work.

Suffice it to say that proximity to a ground plane generally causes flat patch antennas operating at L1 to exhibit a more even and predictable radiation (or reception) pattern, particularly at low elevation angles down closer to the horizon.

In the case of a GPS receiver antenna, having a more consistent radiation pattern near the horizon is critical. Satellites which are near the visible horizon have the greatest propagation distance to cover, with their signals being subject to greater atmospheric attenuation and to much greater influence from disturbances as compared to the same satellite operating directly overhead. The GPS signal is pretty tiny, thus an extra dB of antenna gain down close to the horizon might mean the difference between whether or not we have sufficient satellites in view to allow a GPS-guided approach to take place.

The GPS constellation and its characteristics are pretty much known commodities these days. Likewise for GPS receiver and antenna characteristics. The gain/loss budgets are well understood. Still, it is recognized that we want to avoid ugly "corner cases" where a combination of negative variables including such things as solar flares might put us in a position where we can't use GPS to make an approach.

It's because these "corner cases" really DO exist that we are asked to make our GPS antenna installations as optimal as we possibly can make them. Sure, a sub-optimal installation might work for 99% of the people 99% of the time. One has to ask oneself if we're willing to put ourselves into that outlier position where it might be our installation that isn't quite good enough to allow our GPS navigator to lead us down through the billowing storm clouds.

And that's WHY a ground plane is recommended for GPS antenna installations.

Canadian_JOY said:

It's because these "corner cases" really DO exist that we are asked to make our GPS antenna installations as optimal as we possibly can make them. Sure, a sub-optimal installation might work for 99% of the people 99% of the time. One has to ask oneself if we're willing to put ourselves into that outlier position where it might be our installation that isn't quite good enough to allow our GPS navigator to lead us down through the billowing storm clouds.

And that's WHY a ground plane is recommended for GPS antenna installations.

Click to expand...

And this is the key. A very knowledgeable GPS guy walked me through this a while back. "You won't miss a ground plane if the satellite geometry is good wherever you are at the moment. But for those occasions when, for a few minutes, the satellite geometry is poor (i.e. there aren't several satellites well above the horizon), a ground plane adds a lot of performance.

Oh my. And I forgot to bring the popcorn…..