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I have a very difficult antenna design/installation challenge. I need to install an VHF antenna (118 to 140 MHz) into a nearly all carbon fiber glider. The radio provides 5 watts and 50 ohm, and most signals of interest are vertically polarized.

Since this is a glider, and aerodynamic efficiency is the highest priority, mounting an antenna external to the hull is not considered an acceptable option. So the options seem to be:

  1. Mount a rubber ducky style antenna in the lower front of the aircraft, below the cockpit center-line, which is the only place on the aircraft that is fiberglass. But the maximum vertical height in this region is only about 5 inches and it there is a carbon fiber bulkhead above and behind this small zone.

  2. Mount a rubber ducky style antenna in the upper half of the cockpit, where the antenna would at least have some line of sight forward, to the sides, and upward. This would at least allow the antenna to be mounted fully vertically.

  3. I've considered a full wave loop around the outside of the fuselage. But my research into this says that even if the antenna is isolated from carbon fiber skin with some dielectric (like Kapton tape), I'll end up with RF coupling and reflections that cancel most of the signal.

So, with all of this said, I'm wondering if there is an approach I'm missing, or if this is just a bad situation and an electrically short monopole antenna is about the best that could be done.

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  • $\begingroup$ Interesting question. Wild speculation: might there be some benefit by combining options 1 and 2, making a shortened “dipole that happens to have some stuff in the middle”? $\endgroup$ – Kevin Reid AG6YO Feb 8 '16 at 20:38
  • $\begingroup$ could you mount it vertically inside the vertical stabiliser? $\endgroup$ – captcha Feb 9 '16 at 5:25
  • $\begingroup$ Unfortunately not because the vertical stabilizer and rudder are carbon fiber. $\endgroup$ – Chris Young Feb 9 '16 at 6:44
  • $\begingroup$ When airborne would polarization of signal be an issue as it would be constantly changing during flight ? Is carbon fiber transparent to RF or would it shield the signal? $\endgroup$ – Old_Fossil Sep 1 '17 at 5:51
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I wouldn't worry too much about polarization. If you are 45 degrees off vertical, that's only a -1.5 dB loss, in a theoretical free-space situation. In practice, reflections and absorption from the carbon airframe will alter the polarization anyhow.

I'd consider a slot antenna. This way, the conductivity of the carbon fiber works as part of the antenna, instead of a conductive plane that interferes with the antenna. I'd think the hole in the fuselage where the cockpit attaches would make a dandy slot. You'll probably want to add some copper or aluminium wire or foil around the slot to increase conductivity and reduce losses, since carbon is a relatively poor conductor.

Also keep in mind that you aren't limited to quarter wave whips, half wave dipoles, full-wave loops, or other kinds of self-resonant antennas. Just about any wire or loop can be made into an antenna, so I'd focus on making an antenna within the geometry you have, then you can add a matching network to get the right impedance at the feedpoint.

These are just a few ideas. Building a mock-up and then testing performance is usually easier than trying to predict what an antenna will do in complex situations like this. So I'd grab a roll of copper foil and wire and do a couple tests rather than fretting over every detail.

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  • $\begingroup$ Thanks for this, I hadn't considered a slot antenna. So if I understand your suggesting, you're proposing that the polycarbonate canopy could act as the slot and the interior of the carbon fuselage (possibly coated with some conductive foil could act a cavity behind the slot? $\endgroup$ – Chris Young Feb 10 '16 at 3:20
  • $\begingroup$ @ChrisYoung I was thinking the rim of the carbon fuselage where the canopy attaches is the slot, and the polycarbonate canopy is mostly irrelevant to the antenna (though it might alter the tuning, but you can adjust for that). But it may not matter, you might also try putting a loop antenna around the canopy. I'd mock up a couple designs and see what works best. $\endgroup$ – Phil Frost - W8II Feb 10 '16 at 13:24
  • $\begingroup$ @ChrisYoung other possible slots might where the control surfaces attach, though I imaging routing the feedline to those locations will be more difficult than the canopy. $\endgroup$ – Phil Frost - W8II Feb 10 '16 at 13:36
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A duck, with a guide wire to prevent bending, mounted forward of the vertical fin and attached to a counterpoise might do the trick. Depending on the height of fin you may be able to get a full 1/4 wave with minimal drag. I suggest forward of the fin for two reasons. First, The air is going to drag at that point anyway, maybe this will provide a pre-break. I don't know aeronautics any better than I do brain surgery and you do not want me doing that. Second, the fin will offer the least signal blockage, only a few degree from the aft area. The counterpoise should overcome the effects of being so close to the carbon fiber of the fin with a minimal increase in weight.

As an added thought, a proper 1/2 wave whip antenna can provide up to an 11dB gain and should be about 43.5 inches or 1.1 meters. The dB gain could also serve to counter the carbon fiber concerns.

I am very interested in hearing your thoughts, and any others, on these ideas. Please provide any calculations that may allow me to better visualize the problem and adjust any further thoughts on the matter. I am intrigued by the idea and would love to work the issue as a good mental exercise, at a minimum. I know there is an optimal solution.

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  • $\begingroup$ So the dilemma with mounting even a relatively small antenna in front of the fin is that the drag penalty (aerodynamically speaking) is actually quite significant for a glider. Compared to your typical powered aircraft, we're incredibly slippery in the air. Even the accumulation of bugs on the leading edges of our airfoils are enough to reduce our glider performance by a couple of percentage points. The slot antenna idea above is interesting, although I'm going to need to read up on it quite a bit to figure out how that might be implemented since RF engineering is not my day job. $\endgroup$ – Chris Young Feb 10 '16 at 3:25
  • $\begingroup$ I have looked at photos of gliders for a spell and have seen more than a few with a narrow wire running from the forward top of the tail to just about a 45deg. angle. Is this an antenna wire or some other purpose? I agree with the above post that the carbon shell will offer little in the way of RF, positive or negative. An inverted v antenna picks up both vert & horz. transmissions equally. Just with a bit of loss on the signal, a small bit. $\endgroup$ – KC3FWL Feb 11 '16 at 5:17

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