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GPS Antennas Mountings

PilotjohnS

Well Known Member
OK, there have been great debates in other forums about mounting GPS antennas to various surfaces. Most of these discussions are off track from the original thread topic. So here is my question:

With an RV7(a) / 9(a) slider, will the Garmin antennas fit on the rear fuselage skin so that the slider can slide over them?
Or do the Garmin GPS antennas need to be located aft of the rear of the slider when fully opened?
 
There's plenty of room on the F-7112 top skin for multiple antennas, as long as you don't mount them too far forward: https://www.rv7blog.com/2009/10/31/gps-antennas/

20091031_gps_antennas.jpg
 
I built an RV-7A with a slider and SuperTracks. I placed an Avidyne GPS antenna, a Dynon GPS antenna, and the ELT antenna on the top of the rear fuselage close to the vertical stabilizer, with reasonable spacing between the components to limit interference.

I don't think the canopy would have slid over my GPS pucks if they were further forward.

David
 
Garmin GPS antenna mounting

Garmin install manual of GPS 35 antenna:

"To maintain a constant gain pattern and limit degradation by the windscreen, avoid mounting the antenna closer than three inches from the windscreen"

I believe that includes the canopy. John
 
Garmin install manual of GPS 35 antenna:

"To maintain a constant gain pattern and limit degradation by the windscreen, avoid mounting the antenna closer than three inches from the windscreen"

I believe that includes the canopy. John

Gotta ask "Why?" -- Probably because the aircraft that was used in the certification process had a heated windscreen and that caused some reception issues...(as a wag.)

Otherwise, I don't understand how a micro-stripline/ceramic antenna is going to be adversely affected by the canopy.

We're experimental; so experiment with it.
 
I'm looking into adding a Garmin GPS175, and it uses the GA-35 antenna. The install instructions suggest outside on the top of the fuselage is best. I already have my Dynon GPS antenna up there and the new GA-35 antenna would have to be 6" from that, and not on the same centerline. I thought about mounting it under the cowling, but I don't really have the room there, and it will bake. The installation instructions don't preclude putting it in a wing tip, so I think that is going to be my solution. The right wing tip has plenty of space, no other antennas, and can be kept cool. I just will need to fabricate a mounting bracket and get the antenna leveled and be at least 3" away from the wing skin.
The only downside I can see is my approaches might be 14' off of the centerline to the left.
 
We're experimental; so experiment with it.

Experimenting is great; just be careful to account for all the variables before reaching a conclusion. e.g., a lot of folks throw the antenna somewhere, say ‘it works’ and move on. Then comes the day where the satellite geometry isn’t great, there’s a lot of sunspot activity, and they don’t understand why the gps fails....
 
I'm looking into adding a Garmin GPS175, and it uses the GA-35 antenna. The install instructions suggest outside on the top of the fuselage is best. I already have my Dynon GPS antenna up there and the new GA-35 antenna would have to be 6" from that, and not on the same centerline. I thought about mounting it under the cowling, but I don't really have the room there, and it will bake. The installation instructions don't preclude putting it in a wing tip, so I think that is going to be my solution. The right wing tip has plenty of space, no other antennas, and can be kept cool. I just will need to fabricate a mounting bracket and get the antenna leveled and be at least 3" away from the wing skin.
The only downside I can see is my approaches might be 14' off of the centerline to the left.

Do the installation instructions specify a maximum allowed length of RG400?
Do you have a strobe light out there? Might check it for interference (Look at signal strength page, strobes off. Turn them on, see if any signals go down).
If you use this gps for adsb-out position, you do have to enter the offset in your set-up table.
 
Garmin 35 install

Garmin GPS antenna 35 to another 35: "spacing less than 9 inches center to center results in unacceptable antenna pattern degradation."

Not sure why. John
 
RG400 length

Garmin GPS 35 length: The maximum Db loss must be calculated to be
1.5Db to 6.5Db. It cannot be less than 1.5Db. A 13ft RG400 with connectors is about 2.77Db, within specs in loss. A 14ft wing (9) and 6ft additional length
should be less than 6.5Db for the antenna mounted on the wing. I think max length is 35ft (RG400) with a minimum ground plane radius of7.5inches. John
 
Experimenting is great; just be careful to account for all the variables before reaching a conclusion. e.g., a lot of folks throw the antenna somewhere, say ‘it works’ and move on. Then comes the day where the satellite geometry isn’t great, there’s a lot of sunspot activity, and they don’t understand why the gps fails....

More likely to run into a forced error (i.e. GPS "testing" at WSMR or Kingsbay) than a geometry problem given the size of the constellation these days...

But, your point is well taken. Good scientific method should be applied, generate a hypothesis, test, create repeatable/verifiable data, make a determination thusly...
 
Garmin GPS antenna 35 to another 35: "spacing less than 9 inches center to center results in unacceptable antenna pattern degradation."

Not sure why. John

Ever see a (old fashioned) TV antenna up on a roof, with lots of horizontal aluminum elements with no wires attached (except to one)? Those aluminum pieces are sized to resonate near the TV signal; they pick up the signal, re-radiate it with a phase off-set. Net effect is to make the antenna directional. Same thing can happen with two gps antennas too close to each other. In some directions reception will be better, but in others, worse.
 
Thanks

There's plenty of room on the F-7112 top skin for multiple antennas, as long as you don't mount them too far forward: https://www.rv7blog.com/2009/10/31/gps-antennas/

20091031_gps_antennas.jpg

Thanks for the picture. I will do the same thing, but might forgo the doublers and just use a strip of z-channel bonded to the inside. It is very strong up there since it is between the longerons. i will have to see.

I wish I would have seen this picture a year ago; then I would have put the doublers in when I did the ELT doubler.
 
Full Circle

I really like Matt Burch's picture.
It forced me to actually measure the clearance between the canopy and the GPS antennas.
I have mounted mine on the top of the fuselage, just like Matt's.
Thank you, Thank you, Thank you.
Did i say how happy I am with the feedback?
 

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Supertracks

If you plan the Supertracks, measure the additional length of the canopy slider. It's about an extra 12"
 
Actually

If you plan the Supertracks, measure the additional length of the canopy slider. It's about an extra 12"

Actually, i have the Supertracks slider track installed. I mount the two antennas on either side of the track, one forward and one towards the back of the first "bay".

I have about 14 inches between them.

I made the Garmin doublers and used them to hold the nut plates, but didn't rivet them to the skin. they should be OK.
 
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