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Paint under antenna?

rmartingt

Well Known Member
What is everybody doing regarding antenna installation and paint? Specifically, are you pulling them off and painting underneath them (on the outside)? Are you priming the outside and installing them permanently, then just masking for exterior paint? Bare metal/alodine and seal?

I'm working on avionics and wiring (so I can keep putting off the canopy :p) and I'd like to "permanently" install some of the antennas in the tailcone and under the baggage floor, if that can be done without having to come back and remove them later if/when I decide to paint.
 
Wherever possible mount the antennas with nutplates. Obviously you can't for the ones with studs. Also make the connector holes large enough to pass the connector on the coax and leave enough slack to be able to pull the connector outside. You will have to remove them eventually, if not for paint then replacement, and you will be glad to do it from one side.
 
Prime the inside, both the skin and both sides of the doubler plate. I rivet the doubler plate to the inside of the skin. Mount the antenna to the outside skin without further priming - assuming this will only be for a short time before final paint.

Final paint remove all external antennas, prime and paint the plane.

For the antenna to airframe ground nothing more that star washers on the antenna studs are needed. For antennas using screws, it is a good idea to install nutplates on the inside.

Carl
 
I rivet on a doubler and then rivet on nutplates. As long as you use the taperred star washers on the antenna mounting, you can then paint the entire fuse bottom and install antenna later. Ground path is via the screws to plates to skin. Most ant assemblies tie the tapered portion of the screw seats into the ant ground.

Larry
 
Personally I prefer to mount on bare skin given the option, it's the preferred method.

From 43.13 2b:

b. Th e electrical bonding of the antennas to the aircraft skin of a metal aircraft is best accomplished by direct metal-to-metal contact of the antenna base to the skin. A resistance of no more than 0.003 ohms between the antenna base plate and skin should be achieved.
NOTE: To achieve this electrical bonding, the aircraft paint in the mounting area will need to be removed and the surface covered with an oxide film (i.e., aluminum conversion coat) to protect aluminum against corrosion in accordance with MIL-C-5541B.

c. An alternate method for providing electrical bonding to metal aircraft skin is through the antenna mounting screws, which attach to a backing plate inside the aircraft, making electrical contact with the backside of the skin. To ensure good contact, remove any interior paint in the area where the backing plate is placed and coat this area in accordance with MIL-C-5541B to minimize corrosion.
 
Wherever possible mount the antennas with nutplates. Obviously you can't for the ones with studs.

Of course, those are the ones I'm really concerned about... I have two blade-style transponder antennas, one aft centerline (at the next bulkhead behind the elevator bellcrank) for the ADS-B receiver and one under the baggage floor for the transponder. Both have integral studs. My baggage floor is done with nutplates so it is accessible, at least.

The rest are easier to get to and can use nutplates.
 
Personally I prefer to mount on bare skin given the option, it's the preferred method.

From 43.13 2b:

b. Th e electrical bonding of the antennas to the aircraft skin of a metal aircraft is best accomplished by direct metal-to-metal contact of the antenna base to the skin. A resistance of no more than 0.003 ohms between the antenna base plate and skin should be achieved.
NOTE: To achieve this electrical bonding, the aircraft paint in the mounting area will need to be removed and the surface covered with an oxide film (i.e., aluminum conversion coat) to protect aluminum against corrosion in accordance with MIL-C-5541B.

c. An alternate method for providing electrical bonding to metal aircraft skin is through the antenna mounting screws, which attach to a backing plate inside the aircraft, making electrical contact with the backside of the skin. To ensure good contact, remove any interior paint in the area where the backing plate is placed and coat this area in accordance with MIL-C-5541B to minimize corrosion.

Walt,

Not the first time we find ourselves on different pages, on the same issue.

While your reference is exactly correct, it does not reflect reality for our applications and should be viewed as “make it so it can be checked using DC units of measure”. In my previous life we called this “make it sailor proof”. Striving for a perfect DC ground when an RF ground is the objective results in unnecessary bare aluminum being exposed to the elements - even if top coated after installation.

I note that use of nutplates does what you suggest without scraping paint - so that option makes us both right. With the backing plate primed, along with the interior fuselage skin, setting the backing plate rivets will achieve your perfect DC ground. Measure if in doubt.

So if anyone is thinking they need to scrape off paint to mount these antennas, install the antenna as I suggest then check it with an antenna analyzer to verify proper operation. I do this for all antennas and feed lines as standard practice.

Carl
 
Walt,

Not the first time we find ourselves on different pages, on the same issue.

While your reference is exactly correct, it does not reflect reality for our applications and should be viewed as “make it so it can be checked using DC units of measure”. In my previous life we called this “make it sailor proof”. Striving for a perfect DC ground when an RF ground is the objective results in unnecessary bare aluminum being exposed to the elements - even if top coated after installation.

I note that use of nutplates does what you suggest without scraping paint - so that option makes us both right. With the backing plate primed, along with the interior fuselage skin, setting the backing plate rivets will achieve your perfect DC ground. Measure if in doubt.

So if anyone is thinking they need to scrape off paint to mount these antennas, install the antenna as I suggest then check it with an antenna analyzer to verify proper operation. I do this for all antennas and feed lines as standard practice.

Carl

Reality is you can choose to believe what you read on VAF or just go the manufacturer/and or FAA guidance and use that. I'm not making this stuff up. I'm pretty sure Carl or anyone else on VAF doesn't have the testing facilities/equipment to test antennas like the manufacturer does, but may I'm wrong? Or perhaps that's the way you did it, so now you recommend everyone else do it that way because yours works ok?

Will it work if you just bolt it on probably, but maybe not as good as it could... don't forget any receiver/xmtr is only as good as the antenna, the cable and it's installation. I believe we've been down this road before on ignoring antenna ground plane requirements as well if I recall.

I learned a long time ago I'm not the smartest guy in the room so I'm always looking to "experts" on how to do things, the 'expert' in this case is the antenna manufacturer unless someone can prove them wrong.

In case anyone wants to read what Comant has to say (one of the largest aviation antenna manufaturers) review their document in the link or the quick view I copied from it below.

PS: I get the practical aspect of installing on a painted aircraft, but on an unpainted aircraft why not follow best practice.

http://delta-omega.com/download/comant/AntennaInstall.pdf

In short here is what Comant has to say:

Aircraft Skin Surface and Mounting Preparation
The electrical bonding of the antenna to the aircraft ground is highly important. If this is not done properly, antenna performance characteristics may become distorted and nulls may appear in the antenna radiation pattern. This, in turn, may cause erratic navigational readings or signal drop out. The electrical bonding of the antennas to the aircraft skin is best accomplished by direct metal-to-metal contact of the antenna base to the aircraft skin. To accomplish this, the aircraft paint in the mounting area will need to be removed and the surface alodined to protect aluminum against corrosion. An alternate method for providing electrical bonding is through the mounting screws, which attach to a backing plate inside the aircraft skin. Remove any interior paint in the area where the backing plate is placed to assure a good ground. Coat this area with alodine to minimize corrosion. To test the electrical bonding of the blade to the aircraft, a reading of .003 ohms between the antenna base plate and ground should be achieved.

Important Warnings !
Failure to follow Installation Instructions as provided by Comant, and/or supplemental information and guidance provided
by the FAA when installing antennas could result in poor systems performance, a void in warranty, and/or damage to the
aircraft. Comant is not responsible for improper installations
 
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Why do the antenna manufacturers supply a cork gasket if they want you to bond directly to the outer skin? Are they just covering their ***?
 
The majority of the ELT antenna are not install properly either. Most have their own ideas which they think are better than the manufacturer or the FAA/FCC. If things are not done properly it is another door open for the insurance companies to deny a claim. Eg. If the ELT batteries are dead and the plane just had an annual signed off the insurance company could deny the claim even though it had nothing to do with the accident.
 
If things are not done properly it is another door open for the insurance companies to deny a claim. Eg. If the ELT batteries are dead and the plane just had an annual signed off the insurance company could deny the claim even though it had nothing to do with the accident.

This is called ‘strict liability’ and is NOT the law in most US states. I don’t know about Canada.
 
In short here is what Comant has to say:

Aircraft Skin Surface and Mounting Preparation
The electrical bonding of the antenna to the aircraft ground is highly important. If this is not done properly, antenna performance characteristics may become distorted and nulls may appear in the antenna radiation pattern. This, in turn, may cause erratic navigational readings or signal drop out. The electrical bonding of the antennas to the aircraft skin is best accomplished by direct metal-to-metal contact of the antenna base to the aircraft skin. To accomplish this, the aircraft paint in the mounting area will need to be removed and the surface alodined to protect aluminum against corrosion. An alternate method for providing electrical bonding is through the mounting screws, which attach to a backing plate inside the aircraft skin. Remove any interior paint in the area where the backing plate is placed to assure a good ground. Coat this area with alodine to minimize corrosion. To test the electrical bonding of the blade to the aircraft, a reading of .003 ohms between the antenna base plate and ground should be achieved.
This "alternate method" is common practice in antenna installation. The only way you will be able to tell the difference is inside a controlled environment (Lab) with highly sensitive test equipment that most amateur builders don't have access to. Sometimes good enough is well, good enough.

;)
 
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This "alternate method" is common practice in antenna installation. The only way you will be able to tell the difference is inside a controlled environment (Lab) with highly sensitive test equipment that most amateur builders don't have access to. Sometimes good enough is well, good enough.

;)

True for a new antenna on an already painted aircraft, but on an un-painted aircraft (per the OP) I would just install the antenna, mask it off obviously, and paint around it.
 
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