Grounding of VHF Comm Antenna

I am installing a bent whip on belly of my RV-9A. Comant supplies a cork gasket for mounting between antenna & skin. In the next breathe, they state that the antenna is best mounted with antenna base in contact with skin - if this is the case and one uses the gasket, is grounding achieved through mounting screws? I installed nut plates on my doubler . . . is grounding achieved through the rivets I used to secure my doubler??? Maybe I replace the cork with a metal impregnated rubber gasket used in EMI applications??? Comments . . .

Bare metal

Mount your antenna on bare metal (after it has been alodined), and then proseal around the base of the antenna to keep water out.

When I worked at a small GA avionics company we were given two options depending on skin curvature, fit, ect.

1. Dispense with the cork, install the antenna, and RTV the seam to seal it.

2. Install washers between the antenna base plate and skin, which were just smaller than the gasket, allowing the gasket to compress but making full electric contact. Preferred.

Just remember to thoroughly clean and burnish both surfaces, then Alodine the aluminum before installing.

Screws and washers...

MS19087 said:

I am installing a bent whip on belly of my RV-9A. Comant supplies a cork gasket for mounting between antenna & skin. In the next breathe, they state that the antenna is best mounted with antenna base in contact with skin - if this is the case and one uses the gasket, is grounding achieved through mounting screws? I installed nut plates on my doubler . . . is grounding achieved through the rivets I used to secure my doubler??? Maybe I replace the cork with a metal impregnated rubber gasket used in EMI applications??? Comments . . .

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On one of my antennae, can't remember which brand though, the grounding is performed through the mounting screws as you mention.

The screws were 82 degree countersunk head ones (check your mounting screws and the antenna holes, aircraft ones are 100 degrees and will be a poor fit) and came with a countersunk star locking washer. I would presume that the "teeth" in the star washer would give a good contact between the antenna base and the screws. Similar star washers should be used on the inside, against a washer - AC 43-13 gives the correct materials and stack up to use for this electrical grounding situation. Follow the AC, and you will have no long term problems - the RTV after installation is a good idea though, and is probably in the antenna instructions...

gil A

On my whip antenna, the whip is insulated from the airframe and is connected to the center conductor of the coax. The shield from the coax is atached to the airframe.

Ground Plane

Bubblehead said:

On my whip antenna, the whip is insulated from the airframe and is connected to the center conductor of the coax. The shield from the coax is attached to the airframe.

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What do you mean, "The shield from the coax is attached to the airframe."? The center conductor is isolated. The ground from the shield will not provide the ground plane.

You need a "Ground Plane". Lets say you held the antenna it in your hand out on the ramp, the performance would be terrible, even with the center conductor to the whip. The shield needs to ground to a ground plane.

The antenna (the whip part) actually "reflects" off the airframe and requires the antenna be locally grounded to make that "matched pair". We have a quarter-wave dipole antenna. With out a ground plane its a "monopole". For our antennas to properly work, LOCAL ground plane is need, right there. To add any resistance or isolation locally is bad for SWR (standing wave ratio). Also from the WWW:

If a quarter-wave antenna is used without a sufficient ground plane, two things will happen:

Radiated power will tend to go out equally in all directions. Up and down as well as towards the horizon, so energy is wasted.

An improperly installed quarter-wave antenna does not establish a good reference plane for the radiating field. When this happens, the antenna is said to be mismatched and the transmitter will see a high SWR (Standing Wave Ratio). Much of the power that the transmitter generates will then bounce back into the transmitter and be dissipated as heat. This reduces the signal.​

The above is the worse case with no ground plane. The fact that you mount on metal and the whole plane is metal is good. You can't count on the shield to provide that ground. If it works with out the local ground plane connection, its not going to be as efficient. It's not designed to work isolated. Some antennas do work isolate but not a 1/4 wave dipole. Isolated it becomes a Monopole.

If it works fine, you may be grounding the antenna through the mounting screws, even with a gasket. The antenna should not be totally isolate electrically from local airframe ground. Its basically half the antenna. On a composite or fabric plane a metal backing plate is needed. There are ways to make antennas which do need a ground plane but not ours.


Handheld transceivers and rubber ducky antennas - how do they work whill being so short, and where is the ground plane?

I believe they use a small coil to make the antenna electrically seem longer. In other words of you stretched the coil out it would equal a longer antenna to match the wave length. As far as ground plane, I believe sometimes the metal case or some internal metal shielding or battery pack of the transceiver acts as the ground plane. Basically they're monopoles and not very efficient.

Some guys put a stub (rubber ducky) antenna on their ELT v. the 22" antenna that came with their ELT. Not a good idea IMHO. Some stub antennas you buy are not optimized for our air-band Freq range or transmitting on our particular radio. Also they have a narrow frequency range. My guess is the short stubs don't perform well, even on their best day. When an ELT puts out 100 milliwatts you don't want a stub antenna on it to save your life. That's also why its a good idea to have a way to connect your handheld VHF radio to your planes external antenna, especially if you plan on using it for emergency communications in flight.

Mount your antenna on bare metal (after it has been alodined), and then proseal around the base of the antenna to keep water out.

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I think this is the best RF solution. A good DC ground is NOT the same as a good RF ground at 120 MHz. If you just ground it by the teeth of a star washer, it's a good DC ground, but a terrible RF ground. A good RF ground is obtained by joining big pieces of metal, like the base plate of the antenna, and the fuse skin.

One reason for this is that as frequency goes up, the current tends to flow along the surface of the conductors, and away from the core. The ultimate conclusion of this is a wave guide (used at microwave frequencies), where the conductor is actually a hollow rectangular tube of metal.

So was that a 10 on the pedant-o-meter, or what?

Clear skies,

What the manual says....

tomcostanza said:

I think this is the best RF solution. A good DC ground is NOT the same as a good RF ground at 120 MHz. If you just ground it by the teeth of a star washer, it's a good DC ground, but a terrible RF ground. A good RF ground is obtained by joining big pieces of metal, like the base plate of the antenna, and the fuse skin.
.....
Clear skies,

Click to expand...

However, it's one of the options stated by the manufacturer for his TSO'd product.

The manufacturer must think it meets his specifications at the relatively low RF frequencies involved. It can't be that "terrible".

In general, "follow the instructions" is an OK approach....

Commant does recommend SS srews though and this would be applicable to any mounting method.

Either option previously presented will work.... I would just use the easiest one...

This is what Comant says....

How do I ground an antenna ?

Method 1. With the exception of certain ELT blade antennas, all Comant antennas can be grounded through the mounting screws. To obtain the proper electrical bond (grounding), the area inside the aircraft, where the antenna is to be mounted, must be free of paint and debris. A backing or doubler plate is placed in the aircraft interior with the antenna mounting screws affixed to the necessary nuts and lock washers. The mounting hardware will make contact to the backing plate, and the backing plate contacts the aircraft skin (interior), with this, an electrical bond is achieved. This method accommodates the use of a non-conductive gasket.

Method 2. Determine the area where the antenna will mount to the outside of the aircraft. Carefully remove paint where the antenna base plate makes contact to the aircraft skin. Follow the remaining instructions as described in Method 1. With this method, a conductive gasket may be required. Be advised that many conductive gaskets contain metallic materials that are subject to rust or corrosion. Therefore, be sure to run a bead of RTV around the antenna perimeter to seal the gap between the bottom of the antenna and the aircraft skin.
Important notes: With either method, apply alodine to any bare aircraft skin. With both types of installations, run a bead of RTV silicone sealant along the edge of the antenna where it meets the exterior aircraft skin. After completing the installation, check electrical bonding with an ohmmeter. It should read no greater than .003 Ohms between a mounting screw and the aircraft skin.

Part of their FAQs here....

http://www.comant.com/faqs.html

PDF guidelines here...

http://www.comant.com/pdfs/AntennaInstall.pdf


Gil A - read another manual....

What do you mean, what do I mean?

gmcjetpilot asked "What do you mean, 'The shield from the coax is attached to the airframe.'? The center conductor is isolated. The ground from the shield will not provide the ground plane."

Here's the applicable part of the instructions from the antenna, which came from Meriden Electronics Corp via Wag Aero. And the antenna works fine. I airchecked it last weekend after I finished installing it.

"4. Attach the dielectric lead of the co-axial cable to a solderless connector on the mast and ground the shielding of the co-axial cable to the skin with an aircraft screw and an eleastic stop nut."

Here's a picture of the old antenna with the threaded portion broken off and the new antenna with all the hardware. The big black part is actually two parts that insulate the antenna from the airframe. Notice that there is just a threaded rod to attach the center conductor of the coax to.

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It seems to me (Advanced Class ham KC9PR) that if the antenna was electrically connected directly to the airframe, then the entire airframe would be the antenna, and then there is no ground plane. If you want to do that, why not just eliminate the antenna and connect the coax directly to the airframe?

I think the confusion lies in we're not being very specific about what antenna we're talking about. My antenna is a simple whip antenna with a single threaded post to attach the center lead of the coax. I think it needs to be insulated from the airframe and the shield of the coax connected to the airframe. It's no different than a bunch of CB and 2m Ham antennas I've attached to cars and trucks for years.

Antennas that incorporate a coax connector on them still need to be grounded to the airframe to establish a "ground plane." In that case metal on metal with the skin of the aircraft may be necessary, either by eliminating the insulating washer or through the mounting screws.

Also, my ELT came with a whip antenna and a rubber duckie. The whip is mounted on the airframe and the duckie is used once the ELT is out of the plane to make it portable. The ELT also has voice transmission capability.

Update on RF grounding option

Comant gave me a lead to procure a conductive antenna gasket with the footprint of the CI-122. I have contacted PIC Wire & Cable - they produce Avionic Antenna Gaskets - not cheap but I like the idea of a true mechanical RF ground between the antenna base and the a/c skin. The cost is worth not having to drill out the rivets of my installed doubler, remanufacture a new one, etc. Thanks for all of your feedback . . . gotta love this forum!

Here's a thought for youse guys with the al-u-min-i-um airframes. One of the class of antennas is the slot antenna. This can take the form of a slot in a metal sheet that is about one-half wavelength long and has a width very much less than a WL. A good match can be had by feeding it with 50 ohm coax about 1/20 WL in from one. A horizontal slot is vertically polarized, as required for VHF Comm. That means you could integrate one of these into the LE of the wing and have a very good wide-band, wide pattern, low drag antenna. A slot on the Vstab LE would make a good horizontally-polarized VOR antenna. 'Food for thought for you "Experimental" types! ref: JD Kraus, "Antennas", 1988, Ch. 13.

Yes I was confused

Bubblehead said:

It seems to me (Advanced Class ham KC9PR) that if the antenna was electrically connected directly to the airframe, then the entire airframe would be the antenna, and then there is no ground plane.

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CQ CQ CQ KC9PR - No offense, I see your antenna shield was locally grounded, via the tab. Yes I was confused by the type of antenna you had. They are all simple whips, but the antennas with a sealed base and BNC connector are grounded through the mounting base, yours is made through the tab. Sorry KC9PR. Got it. Bottom line the shielding braid gets ground to the "ground plane".

Also, my ELT came with a whip antenna and a rubber duckie. The whip is mounted on the airframe and the duckie is used once the ELT is out of the plane to make it portable. The ELT also has voice transmission capability.

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What brand of ELT came with a rubber duckie? The ACK and Amerking have portable antennas, but "old fashion" car style telescoping chrome metal deals, 1/4, may be 1/2 wave length (22" or 44") antenna?


Anyone is interested in all variations of ground plane/non-ground plane antennas, like a 5/8 gamma match/tuning coil antenna, which does not need a ground plane, here is a good link. The 5/8 antenna of course is not good for for airplane use at 60" long, but it works well OK even with out a ground plane. Below are some 1/4 wave dipole antennas with clever ways around the no ground plane.



Keep in mind the caveat, ELT's are not required for sailplanes and therefore certified TSO installation is not required. For our RV's I would argue the FAR's do require us to use an ELT, and that ELT must be TSO'ed, meaning installed per the manual with approved antenna. If your ELT came with a rubber duckie than its fine.

Clearly you can make the rubber duckie work with some ELT's, just look at all the PLB's, all have short little antennas. However the word work does not necessarily mean performance, at least on the 121.5 Mhz. That is why PLB's are not substitutes for ELT's, they don't have the performance despite the same rated transmitter power. The antenna is key. On the other hand if your approved TSO'ed antenna with ground plane is smashed under the plane, its not going to work well either. Air-band handheld VHF transceivers use short rubber duckies, but the radio was made to work with that particular antenna. Still they would perform with a full 1/4 wave ground plane antenna.


Artex has the ME406 ELT that does not allow portable antenna use or odd ball monopole antennas, at least efficiently, but they just came out with the ME406/P which does have a portable antenna option. They had to make the case out of metal to act as a ground plane. The antenna as you can see is more than a rubber duckie. It cost a little more than the base line ME406. Regardless the base line ME406 CAN NOT be used with a portable antenna of any kind as I'm told. Bottom line they had to change the whole ELT to work well with a portable antenna.

gmcjetpilot said:

Keep in mind the caveat, ELT's are not required for sailplanes and therefore certified TSO installation is not required. For our RV's I would argue the FAR's do require us to use an ELT,

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Only partially true.....

Single place aircraft, including the RV-3, are exempt from the requirements of an ELT.

Additionally, multplace aircraft used for training within 50 miles of the local origin are also exempted.

It's true that most sailplanes do not have ELT's, and can slip in under the exemptions. But, I suspect there are a few that are used for X-country that shouldn't!

Thanks good points

ddurakovich said:

Only partially true.....

Single place aircraft, including the RV-3, are exempt from the requirements of an ELT. Additionally, multplace aircraft used for training within 50 miles of the local origin are also exempted. It's true that most sailplanes do not have ELT's, and can slip in under the exemptions. But, I suspect there are a few that are used for X-country that shouldn't!

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Good points. I ignored those exceptions intentionally since they don't apply to most RV's. Also airline part 121 / air transports do not need an ELT!

No offense taken

GMCJetPilot - no offense taken. Like I said, it had to do with us not defining the antenna and the type of connection. Also, you're right, my ELT doesn't have a duckie, it's a telescoping antenna like you show in your post. I have an AmeriKing ELT.

I've enjoyed reading your technical postings on antennas. It's been a long time since I read up on antennas and it was refreshing to have my brain stretched!

gmcjetpilot said:

Keep in mind the caveat, ELT's are not required for sailplanes and therefore certified TSO installation is not required

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I am not aware of any sailplane exemption for ELTs. Do you have a reference? Of course many sailplanes do not require ELTs because they are single place or are used for local training.

...just look at all the PLB's, all have short little antennas.

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I happen to have my PLB right here. The antenna is approximately 12" long. That is pretty long at 406 Mhz, around 1/2 wavelength. Of course I don't know what king of feed and loading the antenna is using. I suspect it is an electrically 5/8 wave antenna, which should be real effective.