What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

GPS Antenna Ground Plane?

Auburntsts

Auburntsts

Well Known Member
Folks,
I'm in the process of installing my GPS antennas (G3X and GTN 650) and the location of choice is on the cabin top. Question is whether a ground plane is required or not? If so, what's the best (read easiest) solution? I've searched here on VAF and on other web sites and only have managed to confuse myself. Garmin's install manuals aren't much help either. The 6XX/7XX AML manual says a 7.5" ground plane is required, while the 650 Rev B and G3X manuals only say it's recommended. What say you?
 
I asked this previously and the answer is no, a ground plane is not required for GPS. I think one snarky comment was even "do you see a ground plane on your hand held GPS receiver?"
 
CharlieWaffles said:
I asked this previously and the answer is no, a ground plane is not required for GPS. I think one snarky comment was even "do you see a ground plane on your hand held GPS receiver?"
Click to expand...

Although I agree with the answer, the comparison to a handheld is not really valid.
You don't have a ground plane on a handheld com radio, but it would work much better with one.
 
The GTN install manual I have states that the WAAS GPS antenna should have a minimum of 7.5" around its perimeter. For metal aircraft, the surrounding skin serves as the ground plane.
Send me a PM & I'll email you the PDF install manual I have.

Sam

Edit...never mind, I should have read your post a little closer. :(
 
TX7A said:
The GTN install manual I have states that the WAAS GPS antenna should have a minimum of 7.5" around its perimeter. For metal aircraft, the surrounding skin serves as the ground plane.
Send me a PM & I'll email you the PDF install manual I have.

Sam

Edit...never mind, I should have read your post a little closer. :(
Click to expand...


Todd,

Your GTN 650 may work without a ground plane, but it certainly will work better with one. I would ensure that you have the 7.5" ground plane per the manual. This is why the most popular location is on the empennage just behind the cabin cover. Putting it on the cabin cover just increases the complexity.

Mark,

The GTN 650 isn't a hand held.

bob
 
Mel said:
Although I agree with the answer, the comparison to a handheld is not really valid.
You don't have a ground plane on a handheld com radio, but it would work much better with one.
Click to expand...

Agreed. The ground plane/counter poise helps the performance of any balanced antenna. The handhelds use semiconductors to make the handheld believe it is connected to a balanced antenna when it really isn't. The performance is degraded as a result.

Phil
 
Ahh, the law of unintended consequences. My earlier build choices are coming back to haunt me now. I've got a comm antenna a few inches back on the tailcone now that was originally on the belly that I moved to make room for a cargo pod, so I'm pretty much stuck figuring out how to install the 650's GPS antenna on the cabin top. Of course as luck would have it I just finished installing my overhead console. Access to where I was going to put my GPS antennas wasn't an issue until this ground plane business reared up. :(
 
If you dont use a metallic paint, could you place the ground plane inside the overhead console under the canopy and place the GPS antenna on the outside? Just wondering what the Z-axis has to be for the antenna to benefit from a ground plane.
 
CharlieWaffles said:
If you dont use a metallic paint, could you place the ground plane inside the overhead console under the canopy and place the GPS antenna on the outside? Just wondering what the Z-axis has to be for the antenna to benefit from a ground plane.
Click to expand...

I thought about that, but what I've read today seems to lead me to believe that not having the ground plane directly under the antenna causes more problems than it solves because the "Z" distance makes the antenna appear larger than it really is and affects the "tuning" or something to that effect. All I got was it was bad.

So I'm thinking the easiest solution at this point is to rout out a ground plane on the top (about .020) and glass in either an aluminum plate (or circle of wire mesh) cut to the required diameter. It maybe overkill, but this is my IFR GPS so I'm willing to do what Garmin saysto insure I get the specified performance. Note: I'm only doing this for the 650 and not the G3X's MFD (GDU 375) GPS/XM antenna since that is a VFR install and I'm less concerned about it. That and the fact the G3X PFD (GDU 370) screen has it's own suction cup type antenna.
 
Before you make any mods, I would talk with garmin. I do not have a ground plane on either of my two gps and xm antennas. Last time i took one of these antennas apart it was a stripline spiral and would not require an external ground plane. Mine work just fine centered on the cabin cover. While I have not flown yet, I continue to receive a good gps signal inside my wood frame garage. The GPS frequency is 1575 MHz, which is a full wavelength of .19 meters, so a quarter wave ground plane would be 1.8" radius. Im not sure where the 7" comes from.
 
The crude physics here is that you want the ground plane to be close to the antenna, "close" in terms of a small fraction of a wavelength.
GPS operates around 1.5 GHz which puts its wavelength at 20 cm. So you'd like to see the spacing between the ground plane and antenna very small compared to 20 cm. I recall the cabin top is about 2 cm thick, or about 0.1 wavelength. That's close but I'd like it to be a bit smaller if possible. There's also the odd discontinuity that the bottom of the antenna is, I think, metal and already part of the ground plane(?), so there'd be a "step down" to the inside.

IF I were doing this I don't think I'd rout any of the cabin top out. I'd find some electrically conducting paint to spray all over the top near the antenna. Fairly thick under and near the antenna, thinner as you work your way out 20 cm or more. At these wavelengths a very thin conductor is all you need. You need to make sure the base of the antenna makes good electrical contact. I'd sand or build up that area to be perfectly flat prior to applying the conducting paint. Then outside of the antenna footprint, overcoat this paint with primer and your final paint color.

Do you think you'll need a doubler on the inside for mechanical support?

Note to Bill: you'd really like the ground plane to be as large as possible, a quarter wave is a bit small. The 7" is a full wavelength. This assumes that the antenna is designed to work with a ground plane. Like you, I thought I had heard that one wasn't really necessary.
 
Last edited:
I have 430w with gps ant just above and behind center brace w/o gnd plane. 68 hrs and never a degraded signal. I have SL-40 com ant above rear seats with 22" x .016" ground plane bonded to underside and no problems there either.
 
Lots to ponder. I think to start off with I"ll mount without a ground plane and see what happens during phase 1. My -10 won't be painted until after my 40hr flyoff (I'll have a 40hrs due to my engine) so if I need to make a mod or move the antenna I can do it without worring about cosmetics. In the meantime I'll continue my research and see if the light bulb goe on. :D
 
Last edited:
Actually, the Commant GPS antenna installation instructions state that a ground plane is not required for composite installations.
 
You must log in or register to reply here.
Back
Top