Firstly why are you putting the antenna on the underside of your DR400 ?
There is an approved position inside the rear fuselage and a bit of wooden structure to fit both the antenna and the metal ground plane to the underside of.
I would use a bit of 2024 about as wide as the fuselage and about as long as the wooden structure and fold the edges to give it a bit of rigidity. As the ground plane has no structural function it can be made out of the thinnest sheet that is available.
The advantages of putting the antenna inside are less drag, no corrosion issues with the antenna and no chance of it getting hit by a bit of FOD or sticking in your back when you clean the inderside of the aircraft.
The #2 VHF on my DR400 uses an antenna so mounted and performs exactly the same as the top external mounted #1 VHF.
Thanks, are there any details available of the approved position ?
Yes photo of the old UK CAA approved antenna position data is attached, if you can wait until next week when I return from holiday I can send you a photo of the installation on my aircraft that will be far more informative.
Thanks very much that would be great ! Ours is fitted with an underbelly top loaded ( no ground plane) aerial which seems to be original equipment.
Paul.
The top loaded antenna is almost defenatly an original Robin item, they did also fit an antenna inside the fin and it was my intention to do this and have no external antenna’s.
However due to the lack of space on the aircraft I could not get the separation of GPS and VHF antennas without using a combined VHF/GPS anntena mounted on the rear upper fuselage for #1 VHF/GPS.
The CAA document above is only previously approved positions and you can use other locations but a flight test would be needed to check the function of the system.
I simply stuck copper tape to the inside of the fuselage to act as a ground plane.It works fine but then i’m LAA so it’s much easier for me to do these pragmatic things.
I also tried fitting the aerial inside but got feedback issues.
I have seen both copper and aluminium tape used on certified aircraft so this is a solution but I have also seen the adhesive deteriorate and the tape come adrift, Below the top Anntena mount Robin installed aluminium mesh held in with a single layer of doped fabric.
The labour content of installing the two best practice ground planes is about the same as is the cost in raw materials but this one time cost has to be balanced aganst the probable cost renewal of copper tape in the future.
As an LAA inspector my engineering standards don’t differ between LAA and certified aircraft, the certified system allows for pragmatic thinking in most aspects of new system installation but a lot of people who are too lazy to think use the certification aspect to avoid anything requiring a bit of effort or lateral thinking.
I’ve merged two threads on the same topic.
Some amazing ideas above by @celtico …
In my TCAS installation the installer could not be bothered to remove the composite to reach the metal mesh inside so they stuck an adhesive foil underneath the ceiling liner, and connected a wire from it to the rest of the airframe… One day I might strip that stuff out. But this leads to the Q of how you protect any composite aircraft from not just lightning but routine static (generated by the shedding of water droplets) unless there is a metal mesh in the composite. I guess the answer is “you don’t” but that means a lot of people aren’t going to end up with properly working radios.
Peter
As you said some composite aircraft have a conductor woven into the fabric, Boeing at one time used a very thin layer of cold metal spray on composite panels. Quite what the score is on the B787 I don’t know.
As for your TB20 composite issues I would have to take a look at the structure to see what has been done .
Spraying (or painting) a ground plane is another method.
The cheapest sort of paint is zinc based; the best is silver based and tends to come in a tin, for brush application. I have used zinc in production, and have used silver for attaching wires to ITO glass panels for ice protection of cameras.
Here are some examples. However you have to make sure the antenna base is firmly screwed to the coated surface. For internal antenna mounting it should be easier to form the ground plane with a piece of aluminium as described earlier.
@wigglyamp knows a lot about this but I am not sure if he is still here. @garryiae does avionics installations and might know about the specific case of a Robin.
