Airband VHF antenna challenge

Question

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.

Answer 1

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.

Answer 2

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.