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Sealing antennas ..

bkervaski

Hellloooooooo!
Testing
I'm working on installing antennas today. The comm and transponder antennas come with gaskets, easy. The Garmin WAAS/GPS antenna has an o-ring but it's inside the screws so it will have to be sealed.

Should I seal these with something that should be painted or just wait until after paint and just use some silicone?

Thanks and watch out for the Black Friday crazies!
 
I wouldn't want to install any 'protrusions' before paint. Just something else to mask off & protect from the paint, and the next item mounted there might be smaller, exposing unpainted surface.
 
My take on it having done many, attended training courses at the AEA and talking to industry experts.

Throw away all gaskets. After installation seal around perimeter with either Pro Seal (best PITA) or Permatex Right Stuff gasket maker (Autozone, etc...)

More specifically:
Again. Throw away the gaskets. Remove the paint from under the antenna footprint (outside of skin). Remove primer from inside of skin in doubler footprint if the skins are primed. Clean and alodine both inside and out. Rivet doubler to skin making sure rivet holes edges are bare metal. If antenna doubler is primed or painted on non-contact surface run a piloted burnishing brush with a cordless drill on the interior surface of the doubler to remove any paint where the attachment washers and nuts make contact as applicable. Install antenna being mindful not to over-torque the hardware (see specific manufacturer instructions). Mask around base of antenna with fine line tape both above and below the base. Apply a bead of Pro Seal (yuk) or Permatex Right Stuff (yay) gasket maker around the perimeter of the antenna base keeping material within the fine line tape. Use finger wet with soapy water to trowel a smooth radius of the stuff into the interface. Pull fine line tape while the sealant is still fresh and touch up. Any excess sealeant can be wiped off with a soft rag, applying soapy water as necessary.

P.S. Ditch the gaskets. If you feel you really need one, then buy one of the aftermarket gaskets compoosed of gell substrate with wire mesh imbedded in it. The mesh provides electrical bonding to the skin. Beware. They are pricey. I have purchased such from Av-Dec H-Tak gaskets in Fort Worth, TX. They aren't used to selling to GA as they typically supply military contracts, but they can cut them to match for most any commercially available antenna. In the final analysis it's really not worth going this route.

https://www.youtube.com/watch?time_continue=94&v=0Kf4aG4OLmo

If you worry about corrosion on and exposed prepared surfaces or hardware coat these with Pro Seal or Right Stuff.

Summary:
Ditch the gasket
Prepare the interior and exterior skins for direct contact with the antenna base and doubler.
Protect bare surfaces with alodine.
Seal final assembly around the exterior antenna perimeter with Right Stuff or Pro Seal.

Live happily ever after never having to worry about electrical bonding, ground plane, poor radio reception, or metal corrosion. Also cures the heartbreak of Psoriasis, the embarrassment of halitosis and the indignation of the guys at muscle beach kicking sand in your face.

I think.

Jim
 
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TBH, I didn't do any of this, and have had zero issues with reception or transmission.

I just used the gaskets that came with the comm antennae, and mounted both Garmin and Dynon GPS antennae on a shelf under the cowling (which was painted). Maybe I got lucky, but they all work perfectly fine. (Archer VOR/ILS/GS antenna in the wingtip has reasonably good performance; 1m stripped coax bonded to the inside of the lower cowl for MB does great).

I'd advise just mounting them and trying them before going whole-hog on all the cleaning/burnishing/bonding stuff. It's likely that the mounting structure (platenuts) on the inside will give you a good ground plane.

If it doesn't, *then* I'd try more "aggressive" solutions.

Oh, and the COMM, XPDR and ADS-B antennae were removed prior to paint then reinstalled, no change in performance.

(ETA: I bought those wire-mesh-embedded gaskets for the standard COMM antennae base fairing profile, but never installed them. Pricey little f***ers.)
 
Jim,

I know I'll get flamed for saying this but here goes. For the antennas we use this process is not required or desired. Air Force spook planes and such are a different consideration.

Use the gasket or don't, it has no practical impact on antenna performance. The only ground needed is via the mounting screws so I do use star washers for those on the inside of the plane. After paint (as in there is paint under the antenna base) adding a small bead of clear RTV around the antenna base is fine to keep out water and such. If you like, you can take the same clear RTV and coat the antenna connector on the inside to keep out water and oil (I do this for my outside antenna coax connectors and the seal lasts for decades).

For GPS antennas you can mount them on fiberglass- the required ground is part of the antenna. This is from the Garmin install manual: "No special precautions need to be taken to provide a bonding path between the GPS antenna and the aircraft structure".

Carl
 
+1

Carl beat me to it, on all points. The star (toothed) washers will bite through the paint & supply all the ground needed. If you want to get really anal, apply a little dielectric grease to the area under the star washers before assembly. That will keep moisture/corrosion away from the 'bite' points.
 
Garmin GPS antennae

Jim,

.......
For GPS antennas you can mount them on fiberglass- the required ground is part of the antenna. This is from the Garmin install manual: "No special precautions need to be taken to provide a bonding path between the GPS antenna and the aircraft structure".

Carl

That's what I thought too... but the GTN6XX/7XX Installation Manual I looked at said something different.

Ensure that the GPS/NAV/COM antennas are electrically bonded to the aircraft. Follow the aircraft manufacturers? instructions for the NAV and COM antenna installations, or obtain other FAA approval.

The GPS/WAAS antenna requires a minimum ground plane radius of 7.5 inches around the perimeter of the antenna. Refer to Figure 2-7. For metal aircraft, the surrounding metal skin on which the antenna is mounted supplies the ground plane. For non-metal aircraft, the ground plane can be composed of heavy duty aluminum foil tape, such as 3M P/N 438 or other adhesive backed dead soft aluminum foil minimum 0.012 inches thick.


I guess I should add some foil under the planned 'roof-top' GPS location on my -10
 
Be Careful With Type of Silicone Used

Just one additional recommendation - Do NOT use regular silicone/RTV sealant. Per the G3X Installation manual, page 22-10:
"CAUTION
Do not use construction grade RTV sealant or sealants containing acetic acid.
These sealants may damage the electrical connections to the antenna. Use of
these type sealants may void the antenna warranty.​

The Garmin recommended sealant is "Dow Corning 738 Electrical Sealant or equivalent".

Early in my career as an Electrical Engineer I learned the hard way that the off-gassing from normal silicone/RTV sealant during curing can cause a lot of very costly damage to electrical devices. So this is one caution I would recommend you not ignore.

Cheers
 
Just one additional recommendation - Do NOT use regular silicone/RTV sealant. Per the G3X Installation manual, page 22-10:
"CAUTION
Do not use construction grade RTV sealant or sealants containing acetic acid.
These sealants may damage the electrical connections to the antenna. Use of
these type sealants may void the antenna warranty.​

The Garmin recommended sealant is "Dow Corning 738 Electrical Sealant or equivalent".

Early in my career as an Electrical Engineer I learned the hard way that the off-gassing from normal silicone/RTV sealant during curing can cause a lot of very costly damage to electrical devices. So this is one caution I would recommend you not ignore.

Cheers

I believe the GE Silicone II series commonly available at your hardware/home store is non-corrosive and doesn't smell of vinegar.

https://www.builditsolar.com/References/Caulks/Silicone II ALuminum & Metal - TDS.pdf

Disadvantage - it's non-paintable.
 
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I agree the onerous prep and bonding process I outlined is not necessary for most GPS antennas. Despite what the installation instructions say practical experience has proven that GPS reciever antennas do just fine without. Hence the great results with antennas mounted under the fiberglass cowl just forward of the firewall on minimal bracketry.

However, for communication antennas that transmit there is no such thing as too much bonding. Especially as transmitter power goes up. The day after installation they will all seem great. However, a few years down the road with increased resistance what might still work for a 5 Watt radio (Icoms, Xcom, TY91 etc) might break down for a 10 Watt transmitter (GTRs, 430's etc.). There is a reason the likes of Comant etc don't include a gasket with their comm antennas. It's not about selling it separately for more money. It's an attempt to have field installers get the best bonding possible.

If you are a merchant builder or plan to flip your RV after a couple of years the incentive for proper long-life bonding of transmitting antennas dimenishes

I picked up a an aircraft last year from a completion center for a friend and the state of their antenna installation and aiframe bonding (along with many other items) made me think they were a utility trailer factory rather than airframe craftsmen. Many small utility trailer factories have a competive approach that if their trailer makes it out the front gate they did it right. If they were to pack wheel bearings, deburr holes and wire correctly they wouldn't be in business long due to the tight margins. I got that same feeling from that builder assistance center (the electronic tach had exactly 40.00 hours on it upon delivery).

I have fought enough transceiver antenna troubles in the past that the thorough bonding approach has cured in the long haul.

Jim
 
That's what I thought too... but the GTN6XX/7XX Installation Manual I looked at said something different.

Ensure that the GPS/NAV/COM antennas are electrically bonded to the aircraft. Follow the aircraft manufacturers? instructions for the NAV and COM antenna installations, or obtain other FAA approval.

The GPS/WAAS antenna requires a minimum ground plane radius of 7.5 inches around the perimeter of the antenna. Refer to Figure 2-7. For metal aircraft, the surrounding metal skin on which the antenna is mounted supplies the ground plane. For non-metal aircraft, the ground plane can be composed of heavy duty aluminum foil tape, such as 3M P/N 438 or other adhesive backed dead soft aluminum foil minimum 0.012 inches thick.


I guess I should add some foil under the planned 'roof-top' GPS location on my -10

I can only assume this add is to address the known GTN-650 GPS RFI issues.

Carl
 
My take on it having done many, attended training courses at the AEA and talking to industry experts.

Throw away all gaskets. After installation seal around perimeter with either Pro Seal (best PITA) or Permatex Right Stuff gasket maker (Autozone, etc...)

More specifically:
Again. Throw away the gaskets. Remove the paint from under the antenna footprint (outside of skin). Remove primer from inside of skin in doubler footprint if the skins are primed. Clean and alodine both inside and out. Rivet doubler to skin making sure rivet holes edges are bare metal. If antenna doubler is primed or painted on non-contact surface run a piloted burnishing brush with a cordless drill on the interior surface of the doubler to remove any paint where the attachment washers and nuts make contact as applicable. Install antenna being mindful not to over-torque the hardware (see specific manufacturer instructions). Mask around base of antenna with fine line tape both above and below the base. Apply a bead of Pro Seal (yuk) or Permatex Right Stuff (yay) gasket maker around the perimeter of the antenna base keeping material within the fine line tape. Use finger wet with soapy water to trowel a smooth radius of the stuff into the interface. Pull fine line tape while the sealant is still fresh and touch up. Any excess sealeant can be wiped off with a soft rag, applying soapy water as necessary.

P.S. Ditch the gaskets. If you feel you really need one, then buy one of the aftermarket gaskets compoosed of gell substrate with wire mesh imbedded in it. The mesh provides electrical bonding to the skin. Beware. They are pricey. I have purchased such from Av-Dec H-Tak gaskets in Fort Worth, TX. They aren't used to selling to GA as they typically supply military contracts, but they can cut them to match for most any commercially available antenna. In the final analysis it's really not worth going this route.

https://www.youtube.com/watch?time_continue=94&v=0Kf4aG4OLmo

If you worry about corrosion on and exposed prepared surfaces or hardware coat these with Pro Seal or Right Stuff.

Summary:
Ditch the gasket
Prepare the interior and exterior skins for direct contact with the antenna base and doubler.
Protect bare surfaces with alodine.
Seal final assembly around the exterior antenna perimeter with Right Stuff or Pro Seal.

Live happily ever after never having to worry about electrical bonding, ground plane, poor radio reception, or metal corrosion. Also cures the heartbreak of Psoriasis, the embarrassment of halitosis and the indignation of the guys at muscle beach kicking sand in your face.

I think.

Jim

+1 for Jim's method, its the best way to avoid issues.
 
Nice video Gator. Here are some things I observed...

They rightly ditched the gasket. +1

They explained how the star type lock washers provide the electrical bonding to the aircraft skin for a good ground plane by biting into the skin. Then they turn around and put on a floating interior doubler so the washers bit into it, not the skin. Think about that. A way around this would be by using a riveted doubler per AC 43-13 so the rivets would establish multiple redundant electrical bonding to the skin. Removing the interior skin paint/primer under the footprint of the floating doubler shown in the demonstration would be a close second.

Their sample panel appears to be painted on both sides. Again I would expect at a minimum the removal of the inside skin surface paint/primer under the footprint of the doubler or at a maximum combining this with removal of the exterior finish down to bare metal under the footprint of the oval antenna base. Both Comant and Cessna dictate that the antenna base and aicraft skin be bare metal. This is also taught to technicians attending installation classes at the AEA. Hence the "ditch the gasket" motto.

The fellers in the video use a siliconized latex from DAP. Anytime I hear the word silicon I think of acids that could trigger corrosion on aluminum. Maybe this stuff is safe for aluminum but it would require due diligence on the part of the installer. Permatex Right Stuff gasket maker (your local auto parts store) does not contain silicon and works.

They go on to demonstrate globbing a bead of the DAP around the periphery of the antenna base prior to mounting then mounting and clamping up the antenna providing for squeeze out. This can set up trouble since when there is "squeeze out" there is also "squeeze in" over a portion of the metal antenna plate towards the center connector of the antenna. The areas with the squeeze in will insulate the metal surfaces of the antenna base and the aircraft skin, and in the process mechanically shim up the whole faying surface. Resistance between the two begins to increase from day 1. If the sealant contains silicon curing acids you also just started the first phase of corrosion, the avoidance of which was the whole point.

And fine line tape combined with extra masking would have avoided the sloppy dry finger wipe and smear job the good old boys demonstrated.

I would propose as an alternative practice with respect to transmitting antennas to install the antenna dry to final torque. Then apply one's favorite and appropriately compatible sealant around the base of the antenna (preferably with said masking). After trowling a bead and sealeant still fresh carefully remove any masking and clean up excess with wet towel (in the case of Right Stuff) so the edges can flow out naturally. No contaminants between the skin and antenna. Again the method preferred by antenna manufacturers and the AEA.

Jim
 
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There's no value in exposing any skin to the base of the antenna, except at the 'bite points' of the star washers. There won't be enough clamping force to maintain a gas tight joint between the base and the skin anywhere except at the screws themselves. If you use the technique in the video, but add a pair of star washers between the antenna base and the skin, you're done with grounding and you get a formed gasket using the caulk.

I've used that caulk for a lot of projects, and I don't remember any of the distinct acetic acid odor you get from the corrosive version of silicone caulk. Worth checking to be sure, but I'd bet it's similar to the 'sensor safe' silicone that cures without the acetic acid.

Biggest question I have about the video is why they didn't rivet the doubler. Not for grounding, but for strength. Maybe better's the enemy of good enough, in this case, but with a longer comm antenna on a fast a/c, riveting would make the area stronger. (If they'd riveted the doubler, the star washers under the nuts would have provided all the grounding needed.)
 
There's no value in exposing any skin to the base of the antenna, except at the 'bite points' of the star washers. There won't be enough clamping force to maintain a gas tight joint between the base and the skin anywhere except at the screws themselves. If you use the technique in the video, but add a pair of star washers between the antenna base and the skin, you're done with grounding and you get a formed gasket using the caulk.

......)

Also note that a star washer at the other end of the screw is probably a good idea too. Some antenna manufacturers specify them, and others seem to ignore this end.

washers-external-countersunk.jpg


It needs to be a countersunk star washer and the antenna also needs to be checked to see if it was designed for 82 or 100 degree flat head screws. Again, that seems to vary by manufacturer.
 
GPS Antenna Ground Plane

Great explanation. Question do the GPS pucks (antennas)
need to have the same ground plane and grounding just as effective as this write up? My Dynon GPS antenna came with a rubber gasket.


My take on it having done many, attended training courses at the AEA and talking to industry experts.

Throw away all gaskets. After installation seal around perimeter with either Pro Seal (best PITA) or Permatex Right Stuff gasket maker (Autozone, etc...)

More specifically:
Again. Throw away the gaskets. Remove the paint from under the antenna footprint (outside of skin). Remove primer from inside of skin in doubler footprint if the skins are primed. Clean and alodine both inside and out. Rivet doubler to skin making sure rivet holes edges are bare metal. If antenna doubler is primed or painted on non-contact surface run a piloted burnishing brush with a cordless drill on the interior surface of the doubler to remove any paint where the attachment washers and nuts make contact as applicable. Install antenna being mindful not to over-torque the hardware (see specific manufacturer instructions). Mask around base of antenna with fine line tape both above and below the base. Apply a bead of Pro Seal (yuk) or Permatex Right Stuff (yay) gasket maker around the perimeter of the antenna base keeping material within the fine line tape. Use finger wet with soapy water to trowel a smooth radius of the stuff into the interface. Pull fine line tape while the sealant is still fresh and touch up. Any excess sealeant can be wiped off with a soft rag, applying soapy water as necessary.

P.S. Ditch the gaskets. If you feel you really need one, then buy one of the aftermarket gaskets compoosed of gell substrate with wire mesh imbedded in it. The mesh provides electrical bonding to the skin. Beware. They are pricey. I have purchased such from Av-Dec H-Tak gaskets in Fort Worth, TX. They aren't used to selling to GA as they typically supply military contracts, but they can cut them to match for most any commercially available antenna. In the final analysis it's really not worth going this route.

https://www.youtube.com/watch?time_continue=94&v=0Kf4aG4OLmo

If you worry about corrosion on and exposed prepared surfaces or hardware coat these with Pro Seal or Right Stuff.

Summary:
Ditch the gasket
Prepare the interior and exterior skins for direct contact with the antenna base and doubler.
Protect bare surfaces with alodine.
Seal final assembly around the exterior antenna perimeter with Right Stuff or Pro Seal.

Live happily ever after never having to worry about electrical bonding, ground plane, poor radio reception, or metal corrosion. Also cures the heartbreak of Psoriasis, the embarrassment of halitosis and the indignation of the guys at muscle beach kicking sand in your face.

I think.

Jim
 
Thanks, I've watched that video. It is for regular antennas. I am asking specifically about the GPS low profile antenna's. I don't think they transmit. And some of the new ones are coming with rubber seals on the mounting side.

I'm sure sealing for weather is important. And I can ground the nut side. But the antenna side is not going to contact metal.
 
No. With GPS antennas they seem to just work fine by themselves. The lower mounting star washers work great bonding gps antennas to aluminum skins. I think the rigor specified by TSO'd GPS antenna installation manuals is a belts and suspenders approach coming from the FAA approval process. The metallic base of a GPS antenna, in most cases, yields acceptable reception.

The more critical antenna bonding will always be associated with transmitting antennas, like communications transceivers.

Jim
 
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