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Thread: Carbon Fibre ground plane

  1. #1

    Default Carbon Fibre ground plane

    I am trying to make a monopole antenna for 915Mhz that will be fitted in an aircraft which is made of Carbon fibre.
    Due to the conductivity of the CF I need to put the antenna outside.

    I assume the CF hull will act as a ground plane but do I need to connect it to the ground (shield of the coax) or is it just enough that it is a conducting area?

    I guess electrically connecting the CF/Epoxy to the ground may not be possible.


    Thanks

  2. #2

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    Connecting to the carbon fiber could be easily done through capacitive coupling if the electrodes are large enough. At 915MHz, I don't think it would take much of a metal patch. Antenna manufacturers use the same concept with magnetic mount antennas to capacitively couple to the vehicle roof through the paint and cell phone antennas that are fed directly through a glass window use the same concept. The aluminum foil tape used on HVAC ducts can be soldered to with a little practice, and although I do not know if it will adhere to the CF epoxy, my guess is that it would fairly well.

    If it did, I would drill a hole in the hull on the bottom side where a 1/4 wave element would protrude and, without touching that element, add a patch of foil tape to the outside of the hull around the element as a partial ground plane/coupling electrode and attach that to the coax shield through careful soldering.

    Another option to consider is a slot antenna (or several of them for pattern purposes) cut into the hull horizontally. This would give vertical polarization, but spreading out the inherent directivity of that type of antenna for 360 coverage would be a fun challenge.

  3. #3

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    Thanks for the info Brandon.
    Adding a foil ground plane to the outside of the hull is not practical.

    Would it be much worse if I add the foil inside so the capacitive coupling makes the hull itself being the grou

  4. #4
    Ots's Avatar
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    Is this a radio controlled FPV application (video) using 915 MHz? Or is this an actual full size aircraft?
    Last edited by Ots; Sat 27th Jul 2019 at 01:20. Reason: Clarification of my question.

  5. #5

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    Quote Originally Posted by JAx View Post
    Would it be much worse if I add the foil inside so the capacitive coupling makes the hull itself being the ground
    You do not want to make the capacitive coupling to the inside of the hull. My first reaction to this idea was that the skin effect (which keeps signals in a coax from going through the braid to the outer surface) would prevent an internal patch from acting as a connection to the outer surface of the hull. After thinking about how skin effect occurs (eddy currents), I began to wonder if that's an issue for carbon fiber. Although I did not find the answer, I did come across mention of CF being used to shield things from RF, but that the mechanism was one of absorption. Absorption is loss. The last thing you want is a resistor in series with the counterpoise/ground plane.

    If a patch of sticky foil tape on the outer surface (1.5" in diameter painted to match the CF) is not an option, you might be looking at a half wave element that is fed internally using a stub to match to the high impedance the transmitter would otherwise see looking into a 1/2 radiator. Problem with that is at 915MHz, its 5 to 6 inches of antenna sticking out of the hull.

  6. #6

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    Skin effect is not what causes RF current to stay inside coax shield. First, RF current passes thru woven coaxial shield because it leaks. It is not a perfect shield. Double shielded ... still leaks, just less. RF current flows on the inside of the shield because it is the return path (and opposite charge) to the current on the center conductor. Can write a lot more but there are volumes of well vetted material online easily availble. Some easy to read, some not so much.

    CF is a lossy conductor. So is copper, just that CF has greater loss. And, because CF is woven and similar to coax woven shield .. it also leaks. Depending on the weave density and number of CF layers will determine what energy gets thru. Resistive loss will have a substantial effect. And, if CF were not so lossy it would be the ground plane. CF is a conductor albeit a lossy one and will be a terrible ground plane.

    I agree that a patch inside won't be effective because CF is conductive and therefore shields.

    This boils down to "how well do you need the antenna to perform?" CF as a lossy ground plane *might* be adequate for you need? CF conductivity does vary quite a lot and YMMV.

    Effective ground planes do not need a DC connection. Yes, it is typical installation to drill hole and install a whip. That DC connection does affect the antenna tuning but is *not* a requirement. Capactive mounted antennas also use the metal surface as a ground plane via RF coupling. The metal surface still functions as a counterpoise. Plenty of vetted information available for this also. Personnally I bought the ARRL Antenna book and Handbook long ago as my first intros to all of this. Occasionally it is nice not to read a computer screen!

    Best is to try an external adjesive copper foil as a ground plane. If not acceptable, then just try the antenna in some temporary attachement to the plane on the ground (or elevated a few feet) and compare that range to the same antenna mounted on the roof of a sedan or suv. Given restrictions of mounting to the plane, testing is the only way to get some notion of the effect on range.

    Are you certain that you cannot just hang a wire out the tail? How to match the impedance will be an important topic.
    -Jeff NE1U

  7. #7

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    The application is for a real glider.
    The aerial is needed for a collision detection system that uses TX/RX and has a very low transmit power, it uses the ISM band.
    The fuselage being about 7m long and signal so weak, I suspect the cable losses would be too high for a tail antenna.
    The system can have one or two antennas, my previous glider had a single antenna inside the glider in top of the instrument panel, however it was glass fiber not carbon. The reception was acceptable but not great. The reception with the same equipment in the new CF glider is not as good and I suspect transmission too but I can't see that. The reception is particularly bad when I have gliders below and behind me, hence the idea of a belly antenna.

    The idea was to have a small 4mm hole in the belly, and use a 1/4 wave antenna made of a small diameter conductor reasonably rigid but also flexible lets say springy. A B or E guitar string
    That would be soldered in an MCX connector so it can be removed or easily replaced if damaged.

    I can put some adhesive copper foil on the outside and I am not worried about the aestetics but the connection is more difficult, it requires a second hole for the wire, hence the idea to use the CF hull as ground plane with the foil inside to act as a capacitive coupler to the hull.

    The CF is fairly thick, a few mm but being stratified with epoxy, I am not sure how well is the contact between the fibers.

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