Jim Weir <jim@rst-engr.com> wrote to rec.aviation.homebuilt:
If you haven't read Dr. Dean Hall and Bob Archer's excellent article on antennas in the wingtips of metal airplanes, you need to get a hold of the December 1977 SPORT AVIATION.
I intend to hide my com and nav antennas in the wingtips of my RV. Com in the left, so as not to blanked off in an airport pattern, and nav in the right one.
Nice idea, and one that I hadn't thought about. With this configuration, the left wing (in a standard left turn pattern) will always be pointed at the airport and have slightly better performance in that direction. Slightly.
Now the questions: Is it better to tape or glue the copper foil or rod to the
fibreglass wingtips or to use the last aluminium rib and maybe even plug the lightening holes in it to have a better ground surface?
No, that last aluminium rib and the sheet surfaces of the wing are going to be your ground plane. You want the rod connected perpendicularly to the CENTRE of that ground plane as much as possible. That is, you want it centred vertically and horizontally on that rib to the extent possible. My best idea would be to mount a female BNC connector (UG-1094) at your desired location on the rib and then "somehow" (I ain't the best mechanical engineer on the planet) get that wide tape to connect to a male BNC plug. The tape is going to have to be "supported" somehow, and how you do that is why we call them "experimentals". You *could* use solid brass or copper rod for the element, but to get the bandwidth that you get with wide tape it is going to be hell for heavy.
You do *not* have to plug the lightening holes as the ground plane provided by the wing surfaces will be more than sufficient. OTOH, strobes on the wingtips WILL affect the antenna and I'd keep the antenna physically as far away as possible.
I know the nav will have the polarization quite horizontal, but what about the com side. Is it possible to use the height of the wingtip to some advantage and run the antenna on a zigzag line?
Yes, let the antenna "droop" to the maximum vertical extent. As a matter of fact, on the COM antenna I'd probably cheat and put that connector 1/3 of the camber down from the top so that I'd have more verticality AND more ground plane in the direction I want to usually talk (down).
RV-List message posted by: bjnash@connectnet.com (BJ Nash)
Want to know how to make your own transponder antenna? Here's how: Take an ordinary panel mount BNC "through" connector that is mounted with a nut. You want the kind with a BNC receptacle on one side and the other for soldering a coax cable to it. Go accurately measure a commercial transponder antenna for length (within a 1/16th inch). Measure from the skin of the aircraft to the tip. Pick up some1/16th inch music wire from the hobby shop and solder that measured length to the panel mount BNC paying attention to the overall length. Put some epoxy on the wire where it goes into the connector to give it some strength and "WALLA!" a $60 antenna! I've made dozens of these and never had a problem. I did it because they get wiped off so easy when washing a plane, it's good to have a spare or two, they can be changed in minutes...
If you want to be real fancy and fool your AI at license time, (he'll squawk the antenna as "defective" without a ball tip, even though it works perfectly), pick up some small beads at the craft store and epoxy one onto the tip.
RV-List message posted by: "Robert L. Nuckolls, III" <nuckolls@aeroelectric.com>:
Brass wire or small-bore tubing is a little easier to solder and electrically more efficient than steel. 2.60 inches from aircraft skin surface to tip is real close.
The instructions for all the antennas I got from COMANT (VOR, COM, MKR) all say: "As a general rule, this antenna should be at least one antenna length from any other (the greater of the two lengths)."
In case anyone is interested in more detail, here is the mounting locations" portion of the instructions for my COM antenna.
"Communications antennas may be mounted on top or to the underside of the aircraft. Normally only blade or bent whip styles are mounted on the underside since ground clearance must be considered under worst case conditions. Remember to include the flex of the landing gear in a hard landing at stall angle. The antenna should not be mounted between the landing gear unless the gear is retractable and the antenna will not be used for ground communication.
"For maximum signal strength, the length of antenna coax to the receiver should be minimized. A ground plane is required and the antenna base must contact the ground plane."
In addition to the above, you should use doublers when mounting, per FAA AC43.13-1A, "Acceptable Methods, Techniques and Practices - Aircraft Inspection, Repair and Alteration".