Light Plane and Ultralight Antenna Guide
By Dan Grunloh, Editor – Light Plane World, EAA 173888
Many pilots on the light side of aviation may start out with a setup as simple as this typical trike pilot’s kneeboard. The radio, GPS, and intercom can be easily moved from one plane to the next, and it works well enough in the traffic pattern and near the airport. The desire to improve the reception may lead the pilot to attempt a variety of antenna solutions, some better than others. This guide is a layman’s survey of some of those options.
The handheld radio in the cockpit can work extremely well, largely because the antenna has been carefully matched to the radio by the manufacturer. Homemade antenna systems and older installations with poor connections sometimes won’t match the performance of a simple handheld radio with its rubber duck flexible antenna. The trike pilot above will have weak reception to the rear of his aircraft because his antenna is in the horizontal plane. An airplane pilot with the radio mounted vertically on the panel may do better, but any metal around the radio, the metal of the engine, and even the pilot’s body can block reception in certain directions. Regardless of how the radio is mounted, getting the antenna away from the cockpit with a clear “view” in all directions is a first step toward better radio reception.
The best antenna solution is the one recommended by the manufacturer or by other owners of your make and model of airplane. There’s a bewildering amount of technical information available about antenna construction and design, but nothing beats actual proven performance with your hardware. One would expect that most manufacturers of special light-sport aircraft would be able to provide that kind of assistance. The cost of such systems will likely match that of conventional general aviation antennas, and they would be expected to give good long-term service. See some examples of general aviation antennas from Aircraft Spruce. Most of these are intended to be mounted on airplanes with a metal skin or structure. Cumulus Soaring in Savage, Minnesota, has a helpful page of antenna information including an Antenna Selection Guide.
Can You Hear Me Now?
Why would we want to have better radio reception, anyway? You might want to hear more clearly those garbled radio calls sometimes heard while flying in the traffic pattern. Nothing is more disturbing than hearing partial position calls and not knowing who or what is out there. Would you like to listen to the automated weather observing system broadcast from nearby airports, or perhaps call a flight service station to make or inquire about a pilot weather report? If you start traveling more on cross-country flights, you might quickly discover that the distance you can transmit is much less than the distance from which you can receive radio communications. An external antenna is needed. If you end up receiving too many distant transmissions, try turning up the squelch control.
Not the recommended method for mounting a remote antenna
The first attempt at better antenna systems often starts with a simple extension cable to allow a rubber duck antenna to be mounted remotely. It’s easy and requires only a $15 RG58 BNC male to BNC female coaxial cable. One end goes on the radio, and the handheld antenna connects to the other.
Unfortunately these antennas were never intended to work this way. The rubber duck and most other aviation antennas are quarter wave antennas that require a ground plane. Think of the ground plane as a kind of reflector that helps your radio “see” the electromagnetic waves impinging on your antenna. It should be some type of metal radiating from the base of the antenna and connected to the outer shield of the cable. The simple extension type of installation lacks a suitable ground plane. The ground plane in this example is (at best) the metal case of your handheld radio mounted 15 feet away from the antenna. In spite of the fact that it shouldn’t work at all, the simple extension sometimes gives better results than having the antenna in the cockpit. It could be so much better if metal near the antenna is connected to the antenna ground.
The next two photos show two solutions that would be expected to give better results. The first photo shows the antenna mounted on an aluminum sheet on the center section of a Ridge Runner, and the second photo is a wingtip mount on a Quicksilver. Typical hardware would be an RG58 coaxial cable with BNC male connectors teamed with a BNC bulkhead connector mounted on a metal bracket or airframe member. The bulkhead connectors are commonly female to female BNC. A careful choice of the location and consideration of the optimum length of the feed line could be important. The next section on building your own quarter wave antenna contains a link to an article by Jim Hayward that explains why the length of the feed line is important.
Rubber ducky antenna mounted on a Ridge Runner
Mounting a rubber duck on the wingtip of a quicksilver
Make Your Own Quarter Wave Antenna
If you want to save on cost and get a stronger signal, consider building your own full-size quarter wave antenna. You could do no better than to begin with this article, “Build Your Own Comm Antenna” by Jim Hayward, among the many helpful articles for Challenger builders found on the website www.Challengers101.com. In the article, Jim explains where to obtain the materials, how to fabricate the antenna and mount it on the airplane, and also how to best match the antenna to the radio by properly selecting the optimum length for the coaxial cable connecting the radio to the antenna.
Homemade quarter wave antenna on a Challenger
Dipole Antenna Sometimes Your Best Solution
The dipole antenna is easy to understand. You don’t need a ground plane because it is provided by the antenna. The old-style “rabbit ears” television antenna is a dipole. If the feed line enters in the middle of the antenna instead of at the end, it surely is a dipole antenna. Top commercial examples of aviation dipole antennas feature internal electronic components and will be carefully tuned for optimum performance.
The Larsen Airband dipole shown here is the typical configuration for this type. Tim Mara, owner and operator of Wings and Wheels Soaring Supplies in Lakewood, New York, supplies this antenna and many others that have proven successful in a wide variety of aircraft from ultralights to sailplanes. His online catalog page of Communication, Transponder, and ELT Antennas is very informative and gives a good overview of the available products. Dipoles have been used on weight-shift trikes (especially overseas) for quite some time, but they’ve been hard to find.
The Wonder-Whip is a homemade dipole antenna that costs about $15 to make and requires no special skills beyond stripping insulation and winding some turns around a small spool. Cable diploles work by using part of the coaxial cable itself as the antenna. You get a flexible antenna that can be mounted almost anywhere, on anything, and still not need a separate ground plane. The lighter, thinner, and more flexible RG174 coaxial cable is used. The outer covering and woven shield is removed from the first approximately 23 inches of the cable to form the radiating element. It still has the white inner insulation covering the conductor so it doesn’t have to be insulated from metal aircraft structure. The next 23 inches are left intact, and its outer shield functions as the ground plane of the dipole. A choke winding defines the length of the ground plane so that the remainder of the line functions only as a feed line. The final product is a flexible end-feed dipole antenna.
The Wonder-Whip antenna was designed by Keith Pickersgill of Xplorer UltraFlight, Cape Town, South Africa, for paragliders where it can be taped up into the risers. Detailed instructions for building the Wonder-Whip can found here. You’ll construct it even easier than in his plans if you buy an RG174 cable with male BNC connectors already on each end. One such cable will make two antennas with no soldering involved. I used the cap of a felt-tip pen as the spool. The antenna works great. There’s also a very successful and popular commercial version of the cable dipole known as the Miracle Air Whip which is said to work well when routed inside airframe structures from Challenger fuselages to trike wings. Dipoles work best in the vertical orientation, so mine is taped up into the upper king-post wires of my trike wing.
The Inverted-Vee dipole antenna
The last antenna on our tour, the Inverted-Vee dipole is thought to be possibly the most powerful and sensitive homemade communications antenna available for sport aircraft. The antenna in the picture was built out of scrap plastic and coat hangar wire. It takes a bit more work than some others, but if you fly where airports are far apart it might be just what you need. The antenna shown was designed by Weedhopper builder Dean Scott and comes highly recommended by Light Plane World contributor Jackye Reynolds. Dean has put together detailed construction plans for this antenna with photos and enough text to qualify as a beginning lesson in antenna theory. Download the plans here. A commercial version for this design, called the Vee Rabbit Kingpost Antenna, was offered by Air Magic in Austraila and claimed to give a full 2.6-decibel gain factor over a conventional quarter wave whip antenna.
Try one of these antenna upgrades and you might find yourself using the radio phrase “say again” less often while you’re flying.