Aero ‘Lectrics

BY JIM WEIR

How to build temporary com, nav and transponder antennas for

composite and metal aircraft. For Composite Builders Only What we have here is nothing more than plain old 16 conductor computer ribbon cable joined to some plain old coax cable with the justly famous ferrite balun providing some balance and isolation for the antenna/radio interface. Here are some of the construction details and considerations for the “ribbontenna.” In general, we make our antennas a quarter wave long. Why we do this is explained in the second semester of a graduate school program in electronics engineering. But how we do this is quite

simple. A quarter wave is pretty close to 2953/f, where f is the frequency in MHz. If we take the aircraft com band of 118137 MHz and cut the antenna for the center of the band, this means that each “ear” of the dipole should be 2953/127.5, or about 23.2 inches long. Almost. That is the theoretical length for an infinitely small wire. For reasons that we will explain in a minute, we want a fat wire, and this foreshortens the antenna element. How much does it foreshorten the antenna? There are various ways of estimating the actual length using a length/width ratio, but the best way is with a professional antenna foreshortening tool (scissors)

The “ribbontenna” is shown mounted on a dry piece of wood with a pushpin.

n August 2004, we brought you Part 1 of the Ferry Box—the mechanical design of the beast. What we did not mention at the time is that if you are going to use this box for its intended purpose—ferrying an airplane with no radios and possibly no electrical system—those radios must each have their own antenna. In addition, the antennas we use must be made so we can easily cut and paste them onto the structure and just as easily remove them. I also said I would do something for you “bit-twiddlers” (computer geeks) to gently prod you into the wonderful world of analog design. The temporary com antenna for glass aircraft will do just that.

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ILLUSTRATIONS: JIM WEIR

This figure illustrates the “goodness” (VSWR) curves of the two com antennas described in the article. K I T P L A N E S

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Aero ‘Lectrics CONTINUED

Working With Metal

From left to right, (1) the Callendar transponder groundplane, (2) the Callendar GPS groundplane, and (3) the inspection plate with coax fitting mounting hole punched in the middle are shown.

Using our trusty aluminum pie pan, the GPS antenna is mounted on the Callendar groundplane. Here’s the transponder antenna mounted on the Callendar groundplane.

If you get two inspection hole covers, you can mount your com antenna on one and your transponder antenna on the other. In this case, the transponder antenna is mounted on the inspection plate.

and an antenna “goodness” meter. Make it half an inch too long, measure it a couple of MHz too low and start cutting a quarter of an inch at a time. When the goodness meter goes through its low point and starts back up, stop cutting. This particular antenna with this width of ribbon cable came out 21 inches long centered at exactly 127.5 MHz. Now, why should I care how wide the antenna is? Wouldn’t it be easier just to drape a piece of #14 Romex around? Yes, but with width comes bandwidth. That is, a hair-fine antenna is going to have a “good” bandwidth (a 96 K I T P L A N E S

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purely subjective term) of a couple of MHz. To cover the whole aircraft com band with a single antenna requires width. We can (and have for years) gotten good bandwidth out of 1⁄2-inch copper tape. However, as a temporary antenna, copper tape is a bit too gooey and sticky, not to mention somewhat difficult to stick up with tape or thumbtacks. Hence the wide ribbon cable. At the coax end of the ribbon cable, all 16 conductors are split apart, stripped, tinned and soldered together using the shortest possible lead length. At the tips of the antenna, the conductors are simply open-circuited. They are not soldered together at the ends of the wires. The antenna can be twisted around, made to run over curved surfaces and generally installed any way you want. Keep other metallic structures a foot or two away from the tips; the center of the antenna can be in the middle of ironmongery if you wish. You can use tape, thumbtacks, glue or another nonmetallic or small metallic fasteners. Com antennas should be installed vertically with respect to the normal flight attitude, and nav antennas should be installed horizontally. Make a second antenna with 23.5-inch ears, mount it horizontally, and there’s your plasticplane nav antenna. 2 0 0 4

Now for you folks with metal airplanes, here is an even easier way to make a temporary com antenna. You all cut inspection holes into your aircraft, right? And you can cut (or buy) an extra inspection plate, yes? Good. Drill a 1⁄2-inch hole into the center of that spare plate. Put a BNC bulkhead connector (that’s the kind with BNC-F connectors on both ends and threads in the middle) into that hole and fasten it down. Run a length of coax cable from the inside part of the bulkhead connector to the radio, and then bolt the plate onto the airframe. Make a rod antenna out of brazing wire, solder it to the center pin of a BNC-M connector, insert the pin into the connector shell, and then fill the shell with epoxy. Let the epoxy dry, and fasten this rod antenna to the outside of the bulkhead connector. Bingo—instant antenna! The same restrictions for the ribbon cable antenna also apply to this antenna—the fatter the rod, the broader the bandwidth. The tradeoff when making the rod fat is it’s not only difficult to solder a fat rod to that tiny pin, but the fat rod has far more wind load than a skinny wire. As for a nav antenna on a metal airplane, I’d use the plastic wingtips to conceal two metal rod antennas, use the last wingrib in each wingtip as my groundplane and run equal lengths of coax to a “T” BNC fitting behind the radio. This is not the kind of thing you want to do on a permanent mount, but for cut-and-paste it will get you marginally adequate performance.

Transponder Antennas While we are on the subject of inspection plate mounts, look at the little transponder antenna mounted exactly the same way. Fortunately, those little transponder rod antennas also mount in a 1⁄2-inch hole, so if you get two inspection hole covers, you can mount your com antenna on one of them and your transponder antenna on the other. The com antenna can be on the top or the bottom of the aircraft, but the transponW W W . K I T P L A N E S . C O M

der antenna must have the little ball tip pointed at the ground. For transponders in a glass airplane, you can build what we call a “Marie Callendar” groundplane. Simply get an aluminum pie pan, cut a 51⁄2-inch square out of the bottom, miter the corners to make an octagon, punch a 1⁄2inch hole in the middle and mount the transponder antenna to the groundplane. Mount the antenna-groundplane combination inside the skin wherever it will fit. (My personal favorite is the boysenberry cobbler pie pan, but use whatever you prefer.) Since we’re discussing Callendar groundplanes, cut yourself one 33⁄4-inch square, miter the corners, punch a 78⁄ -inch hole in the middle, and bingo, your new remote-mount GPS antenna groundplane is a done deal. Mount that rascal so the antenna has a good view of the sky, and you are done with that installation. I don’t think it is rocket science to simply say that an inspection plate on the top of the wing with a 7⁄8-inch hole cut in it will do the job for metal ships.

Next Year In the coming months, we’ll explore the electrical design of the Ferry Box. We’ll also do some more work on our VOR/ILS/MB test box and share some good words about newfangled cockpit lighting with cheap computer lights. Jim Weir is the chief avioniker at RST Engineering. He answers avionics questions in the Internet newsgroup rec.aviation.homebuilt. Check out his web site at www.rst-engr.com/kitplanes for previous articles and supplements.

The com wire whip is mounted on the inspection plate.

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