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Antenna Install Question

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Dean Pichon

Well Known Member
When installing an antenna in our metal aircraft is it important or advisable to remove the paint from the internal surface around the location of the back-up plate? (I not sure what is meant by "clean" in the installation instructions for my COM anenna.

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Thanks in advance.
 
Yes Dean, the inside skins should be scotchbrited to a bare finish in order to get your contact patch which establishes your ground plane.

Remove all paint from the inside surfacce!

Hope this helps!

:) CJ
 
Interestingly also note that this is one area were hardware store variety screws with a 82 degree countersink angle are used rather than the 100 degree aircraft hardware.
 
Dean,

I know most will consider this sacrilege, but you need not get carried away with removing paint. There are a lot of antennas on cars that are attached via mag mounts - providing a boat load thicker layer of paint and anti-scuff tape between the antenna and skin than you will have.

The thin layer of paint on the inside of the fuselage will capacitively couple the antenna to the fuselage skin at comm frequencies so it does not care if you have a DC ground or not. That said, you could clean up around the screw holes so the screws (thus antenna base) connect to aircraft ground.

So - clean up around the screws holes on the inside, install, check the screw head for DC ground and you are done.

Carl
 
Carl Froehlich said:
Dean,

I know most will consider this sacrilege, but you need not get carried away with removing paint. There are a lot of antennas on cars that are attached via mag mounts - providing a boat load thicker layer of paint and anti-scuff tape between the antenna and skin than you will have.

The thin layer of paint on the inside of the fuselage will capacitively couple the antenna to the fuselage skin at comm frequencies so it does not care if you have a DC ground or not. That said, you could clean up around the screw holes so the screws (thus antenna base) connect to aircraft ground.

So - clean up around the screws holes on the inside, install, check the screw head for DC ground and you are done.

Carl
Click to expand...

It is sacrilege.
Most of those magnetic mounts on cars are for receivers. Modern receivers are so sensitive, with low S/N ratios, that a coat hangar will work. But for our low power transmitters, you want something decent. The important ground path is from the outside of the connector to the skin. Yes, the screw making contact with a tiny area on the inside of hole thru the skin will work - when new. But in a few years dissimilar metal corrosion will build up an insulating layer, and you'll be left wondering why your once-good antenna now stinks. And don't rely on capacitive coupling; it introduces a phase shift which will screw up the 50 ohm impedance match. (the better magnet mounted car antennas use internal components to fix this - our aircraft whips do not). No need to go overboard, but make sure there is a bare metal path from the nut and washer to the backing plate, and from the other side of the backing plate to the skin. If the backing plate is riveted to the skin (before paint/primer) so much the better.
 
I replaced the retractable antenna on my 84 Corvette (fiberglass body). The new one came with a bunch of wires they wanted hooked to the metal around the base of the antenna inside the fender. I didn't hook them up, closed it up and figured all would be ok. I now know why they are there.

I can only get two stations close to home and they have to be relatively close to my location. Haven't wanted to pull it all apart to finish the installation per the instructions. Luckily, my phone hooked via USB provides all the music I care to hear.
 
Bob,

We are just on different pages WRT antenna theory so I'll drop out.

For all, do what you want - it is your airplane. I recommend however that if you are not verifying antenna function after install you may end up chasing your tail trying to figure out what is wrong.

Talk your local EAA chapter into buying one of these: http://www.mfjenterprises.com/Product.php?productid=MFJ-259C

Verify the antenna at the antenna base, and at the radio end of the coax. This is especially critical for any wingtip antenna. If you just follow the installation instructions you will be disappointed.

Carl
 
BobTurner said:
It is sacrilege.
No need to go overboard, but make sure there is a bare metal path from the nut and washer to the backing plate, and from the other side of the backing plate to the skin. If the backing plate is riveted to the skin (before paint/primer) so much the better.
Click to expand...

Hi Bob,

Thanks for the detailed post. I do feel that my antenna performance has degraded with time. I recently returned the my radio to Garmin for service, but found no noticeable performance change. I will remove the antenna, remove the paint from the inside of the belly skin where the doubler sits and then alodine the surface. Would you recommend any sort of conductive "goo" between the doubler and skin?

Thanks,
 
Carl Froehlich said:
Bob,



For all, do what you want - it is your airplane. I recommend however that if you are not verifying antenna function after install you may end up chasing your tail trying to figure out what is wrong.

Talk your local EAA chapter into buying one of these: http://www.mfjenterprises.com/Product.php?productid=MFJ-259C

Verify the antenna at the antenna base, and at the radio end of the coax. This is especially critical for any wingtip antenna. If you just follow the installation instructions you will be disappointed.

Carl
Click to expand...

Hi Carl,

I did not do this when I initially installed the antenna and now wish I had - for a reference baseline if for no other reason. This time, I have vowed to make an SWR measurement both before and after I remove the paint. I just need to borrow, rent, or buy a device.

Thanks,
 
I recall reading somewhere that alodine is not conductive. Conductive goo is not recommended. But I have used plain old grease on electrical connections that are subject to corrosion. The pressure of the fasteners forces the grease out to allow metal to metal contact. The grease keeps water and air and contaminants out and helps to prevent corrosion.
 
Mich48041 said:
I recall reading somewhere that alodine is not conductive. Conductive goo is not recommended. But I have used plain old grease on electrical connections that are subject to corrosion. The pressure of the fasteners forces the grease out to allow metal to metal contact. The grease keeps water and air and contaminants out and helps to prevent corrosion.
Click to expand...

Alodine (aka iridite, aka chemical conversion coating) on aluminum is conductive.

It is anodizing that is an insulator.
 
az_gila said:
Alodine (aka iridite, aka chemical conversion coating) on aluminum is conductive.

It is anodizing that is an insulator.
Click to expand...

Hi Gil,

Alodine/chemical conversion coatings come in two classes in accordance with MIL-DTL-5541. Class 1A is more corrosion resistant and less conductive. Class 3 is less corrosion resistant and more conductive. Once I remove paint from the belly skin, I can out brush or "pen" treat the area and will not be able to control whether I end up with Class 3 or 1A. My question is, if I alodine and end up with a Class 1A coating, is that better or worse than bare aluminum. I am inclined to believe an Alodine is better than none, but...

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Thanks for the input.
 
Several good ideas
As Carl said, if you can borrow an SWR meter, that's an excellent way to gauge the overall 'health' of your system.
I wouldn't put grease on the scotchbrited aluminum; but some around the cleaned area, to help seal out the elements, can't hurt. You're hoping the clamping force of the washer will seal this area.
Put an ohmeter between the outside of the BNC connector, and a good airframe ground (maybe a screw in a nutplate) when you're done. You shouldn't see more than a few ohms.
 
Dean Pichon said:
Hi Gil,

Alodine/chemical conversion coatings come in two classes in accordance with MIL-DTL-5541. Class 1A is more corrosion resistant and less conductive. Class 3 is less corrosion resistant and more conductive. Once I remove paint from the belly skin, I can out brush or "pen" treat the area and will not be able to control whether I end up with Class 3 or 1A. My question is, if I alodine and end up with a Class 1A coating, is that better or worse than bare aluminum. I am inclined to believe an Alodine is better than none, but...

......

Thanks for the input.
Click to expand...

I think it will be in the "don't worry about it category"...:)

When they use microhms as a unit for the better coating - text from a plating outfit -

The electrical contact resistance of aluminum chromate conversion coatings is low, with Class 3 coatings per MIL-C-5541 being the lowest. Typical contact resistance of Class 3 aluminum chromate conversion coatings is less than 5,000 microhms per square inch as applied. The surface roughness and flatness of the test pieces can have a significant impact on this value.
 
Antenna install in wing inspection plate

I have seen two bent whip antennas installed in the inboard wing inspection plates on left and right wings, it seems easy for both install and maintenance but I wonder about the effectiveness of the ground plane.

Can anyone who has this configuration comment on how it performs?
 
Ian Coates said:
I have seen two bent whip antennas installed in the inboard wing inspection plates on left and right wings, it seems easy for both install and maintenance but I wonder about the effectiveness of the ground plane.

Can anyone who has this configuration comment on how it performs?
Click to expand...

The wing is metal and therefore a large ground plane.
 
Raymo said:
The wing is metal and therefore a large ground plane.
Click to expand...

I think he was concerned with how good the electrical connection is between the inspection plate and wing skin. If it were me, I'd remove any paint from under the screw heads.
 
BobTurner said:
I think he was concerned with how good the electrical connection is between the inspection plate and wing skin. If it were me, I'd remove any paint from under the screw heads.
Click to expand...

Gotcha. I agree, make sure the panel is well grounded to the wing. This can be done by removing paint between the panel and skin. Apply alodine if corrosion is a concern.
 
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