? on Antenna Doubler

I will be adding a second Comm antenna to my belly. My other antennae have doublers that are riveted to the skin. Given that the plane is painted, I would prefer not to use rivets to avoid the touch up work. Can I get away with using an .063 doubler and gluing it to the skin. I just don't know how much movement of the doubler relative to the skin there would be.

Does anyone have experience with this?

Thanks,

Larry

I don't know the answer to your question (my guess is a sandwich of skin-fiberglass/epoxy-doubler would hold up okay) but make sure there is a good, direct electrical connection from coax braid to the skin (not just the doubler if it is electrically insulated).

Do you have access under the floor (or otherwise in the fuselage) to the location where you plan to put the antenna? If so could the doubler go on the inside of the skin? Pretty well hidden that way and can help ensure good electrical connection.

I'm not sure glueing on the doubler is a good idea because of ground plane bonding, as said. There is a fair amount of vibration in that area too, from engine exhaust turbulence. Riveting a doubler (4" X 6" .040) on the inside floor would only expose unpainted rivets - on the bottom of the plane, where it will only be seen by you, when you are cleaning the oil off your belly.

Antenna doubler

Rivet doubler in place then paint the rivet heads with a artist brush.

Don Broussard
RV9 Rebuild in Progress
57 Pacer

You might also be able to locate some fingernail polish that is very close to the paint of the plane.

THanks for the comments. I had planned for the doubler inside the skin. I hadn't thought about the ground aspect and the glue would make it more difficult to get a good electrical bond. I'll just put in a few rivets.

Larry

Or an alternative would be bond the doubler to the skin with proseal of similar and then remove the paint from the skin under the base of the antenna and install with no gasket (which offers improved ground plane contact than just the 4 screws).

How do you handle anti-corrosion between the skin and doubler? Should a primer be removed and the pieces alodine treated before riveting them together? Or is it okay to have primer inbetween the doubler and skin with the rivets and attach bolts providing enough ground?

majuro15 said:

How do you handle anti-corrosion between the skin and doubler? Should a primer be removed and the pieces alodine treated before riveting them together? Or is it okay to have primer inbetween the doubler and skin with the rivets and attach bolts providing enough ground?

Click to expand...

Prime the skin and the doubler. Rivet the doubler to the skin. Install the antenna. No alodine required or desired. Setting the rivets will provide more than enough electrical bonding between the skin and doubler. The antenna bolts will be the other link - again more than enough bonding.

RF ground is very different from DC ground.

Carl

Am I crazy?

I must be missing something.

I thought the doubler was for strength only.

If the doubler is inside the skin, and the skin is a good grounding plane, then why would the doubler need a good electrical contact with the skin?

If the coax shield has a good electrical connection to the skin then you don't have to worry about the doubler. But many connectors ground to the inside (e.g., doubler) skin. If so you need for the doubler to have an electrical connection to the outside skin.

majuro15 said:

How do you handle anti-corrosion between the skin and doubler? Should a primer be removed and the pieces alodine treated before riveting them together? Or is it okay to have primer inbetween the doubler and skin with the rivets and attach bolts providing enough ground?

Click to expand...

I cleaned off a 1/2" circle of primer around each rivet hole on the side of each sheet facing each other (skin and doubler) on my other antenna. My antenna gets it's ground connection from the interior side of the skin and and I was looking for a safety margin.

I read some of these posts and for the life of me can't seem to understand many of them.

Rather than a bunch of opinions being touted , a quick google search will take you to quite a few documents from reliable resources, like antenna manufactures, that tell you how to do it.

Here's an exerpt from a Cobham document on antenna installation:

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."

In post #8 I described an easy and painless way to install an antenna on a low speed aircraft that follows the above guidelines and will ensure good results, but if making things complicated makes you feel better than go for it.

Of course it goes without saying, the 'Bible" 43-13, 2B Chapt 3, Antenna Installation should be consulted and is the 'official' FAA guideline.

+ 1 for pro-seal

I have not had to do it, but I have seen a few that have been Pro-sealed to the back of the skin and have held very well.
Hope this helps, Yours, R.E.A. III #80888

Doubler Shape

Walt or others
Thanks for the input. But one clarification on an antenna doubler would be helpful. It is probably an interpretation question. Is the doubler for the comm antennas just a flat plate riveted to the skin or does it have to bend up and also rivet to the ribs? I see both on construction logs. Seems like if the comm antenna is mounted between the floor ribs, the ribs already keep the skin from vibrating and buckling. and so just a plate doubler would be enough. Or are the more forces than the skin can handle and so the doubler should rivet to the rib also?

Also what thickness sheet is good for doublers? Is .024 or .032 enough or does this function require 0.64?


thanks

I'll take mine w/ribs, please.

sglynn said:

Walt or others
Thanks for the input. But one clarification on an antenna doubler would be helpful. It is probably an interpretation question. Is the doubler for the comm antennas just a flat plate riveted to the skin or does it have to bend up and also rivet to the ribs? I see both on construction logs. Seems like if the comm antenna is mounted between the floor ribs, the ribs already keep the skin from vibrating and buckling. and so just a plate doubler would be enough. Or are the more forces than the skin can handle and so the doubler should rivet to the rib also?

Also what thickness sheet is good for doublers? Is .024 or .032 enough or does this function require 0.64?


thanks

Click to expand...

I tried an experiment of each, no ribs and w/ribs. AC43-13 says w/ribs and it is far, far stiffer, so that is how I did all of mine. Rib attachment on at least one side, two if possible. It took lots of templates and joggles, though. I think these are .040" , the AC discusses the thickness.

Don't put it here though, the skin overlap on the bottoms is an issue, mount one bay aft.

Walt said:

.....

In post #8 I described an easy and painless way to install an antenna on a low speed aircraft that follows the above guidelines and will ensure good results, but if making things complicated makes you feel better than go for it.

......

Click to expand...

Follow post #8 as stated above, but add a couple of rivets under the area that the base of the antenna covers.

These few rivets will ensure electrical bonding. Omit the proseal from the rivet area.

az_gila said:

Follow post #8 as stated above, but add a couple of rivets under the area that the base of the antenna covers.

These few rivets will ensure electrical bonding. Omit the proseal from the rivet area.

Click to expand...

I don't think that is a good idea UNLESS you are using flathead rivets AND insure that the countersink for the rivets makes them either dead flush or even a bit oversunk.

A far better course of action would be to place four rivets just OUTside the footprint of the antenna base. Then you aren't counting on those mounting screws cutting into the base metal of the antenna for your conductivity. The base of the antenna isn't going to contact the outer skin anyway due to the paint already there, but there should be enough capacitance from the base to the actual skin to make a decent ground plane. You only need a couple of hundred picofarads to be a dead RF short at the VHF com band. jw

Jim

jweir43 said:

I don't think that is a good idea UNLESS you are using flathead rivets AND insure that the countersink for the rivets makes them either dead flush or even a bit oversunk.

A far better course of action would be to place four rivets just OUTside the footprint of the antenna base. Then you aren't counting on those mounting screws cutting into the base metal of the antenna for your conductivity. The base of the antenna isn't going to contact the outer skin anyway due to the paint already there, but there should be enough capacitance from the base to the actual skin to make a decent ground plane. You only need a couple of hundred picofarads to be a dead RF short at the VHF com band. jw

Jim

Click to expand...

I somewhat disagree...

The original goal of the OP was to not mark the existing paint work. Some countersunk rivets slightly "undersunk" would electrically connect the skin and the doubler. The antenna would then electrically connect to the doubler via toothed lock washers on the four (maybe 3) antenna mounting screws. This is one case were I prefer toothed lock washers over nut plates.

Note that some antenna manufacturers specify these countersunk lock washers under the screws at the head end -



Also note that some older antennae might use 82 degree hardware rather than the standard aviation 100 degree screw heads.


Hide these few rivets under the antenna mount and all goals are met.

Just another question and clarification regarding this process. I am making the antenna doubler for my Delta Pop COM antenna's. they are going under the seats on my -10.

I currently have my fuselage primed with AZKO.
I am planning on making doublers using the method outlined in green (Thanks Mark Cooper "Charlie Waffles")

Can those pieces be primed?
Do I need to remove the primer from the inside of the bottom skin of the fuse?
Do I need to put any conductive grease/paste...etc between the doublers if they are not primed

Thanks in advance

mciaglia said:

Just another question and clarification regarding this process. I am making the antenna doubler for my Delta Pop COM antenna's. they are going under the seats on my -10.

I currently have my fuselage primed with AZKO.
I am planning on making doublers using the method outlined in green (Thanks Mark Cooper "Charlie Waffles")

Can those pieces be primed?
Do I need to remove the primer from the inside of the bottom skin of the fuse?
Do I need to put any conductive grease/paste...etc between the doublers if they are not primed

Thanks in advance

Click to expand...

See my prior post #14, I would suggest you follow the guidlines provided by the FAA AC or the equipment manufacter's to achieve the best results rather than some of the methods posted here.