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Antennas for steel tube aircraft

Bob Archer

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ANTENNAS FOR STEEL TUBE AIRCRAFT
W/ Wood Wings or Glass or?
by Bob Archer of Sportcraft Antennas


The art of installing antennas internally that really work properly into aircraft with steel tube fuselages and wood wings has left many people confused and perplexed and all the worlds flooby dust doesn’t help so I have decided to try to give some tips and information on the subject. Some people have tried to install monopole antennas internally with a ground plane installed for the antenna to work against and this is just totally bad. One friend of mine informed me he had installed his COM antenna inside the aluminum box of his baggage compartment and it worked just fine. I beg to differ, but if he is happy with it who am I to knock it. Com antennas are vertically polarized and so need to be installed vertically. My number one preferred location for a COM antenna on any type aircraft is the top of the vertical stabilizer where it has a very nice field of view all the way around but that is not very practical on this type of aircraft. Lots of welding and approvals needed. At one time I was going to try a special feed device that I have developed to turn the entire vertical tail into an antenna but my friend with a spare fuselage moved away before I got a chance to attempt it. So there is no good way to install an internal COM antenna in a Bellanca that will work the way I believe an antenna should work. I also believe that it is better to use one really good antenna rather than two mediocre ( or poor ) ones. I have on my airplane a really good Transco broad band low drag antenna mounted to the belly pan next to the access panel and hooked up to both radios through a coaxial switch. It works very well and the VSWR checks out real well though I would prefer a larger ground plane. On my last trip to Phoenix I tuned in the Williams tower after crossing the river and could hear the aircraft in the pattern but not the tower. I didn't check the distance but I think it was well over 100 miles. I have had some problems in some locations talking to control towers while on the ground though. The landing gear legs will interfere with the gear down and I think the ground itself is lossy in some places and soaks up the RF energy. The last possible location for a COM antenna to work well is the top of the fuselage but there are problems. The steel tube structure can be used as a ground plane but to work properly the antenna ground must be tied electrically into the tubing. Remember, a ground plane at these frequencies should be about 48 inches in diameter at least to work properly. Larger is better. An antenna mounted to a smaller plate and the plate then attached to the tubing will be OK but the plate must be electrically attached to the tubing and I do not mean with a ground wire. Ideally the ground currents should go into the ground plane in a radial manner in several places but do the best you can. If the antenna ground is not done properly the VSWR of the antenna can be very high.


A word or two or several about VSWR. VSWR stands for Voltage Sanding Wave Ratio, Not Variable Standing Wave Ratio as was reported in a recent KITPLANES article. It is the ratio of the incident RF waves (those being transmitted) to the reflected RF waves (those being reflected by an antenna that is not matched properly to the impedance of the transmitter and the transmission line). Most Aerospace companies require a VSWR of less than 2:1 for transmit and 3:1 for receive. I require all my antennas to be less than 2:1. A VSWR of 2:1 reflects about 10% of the energy and 3:1 reflects about 25% and above that is way to much.


With wood or Glass wings you can install a VOR antenna in each wing out toward the tips .In my airplane I used two copper strips and a type two balun all of which were installed through the outermost inspection hole. I snaked the copper elements through the ribs and drilled holes through the plywood skins to install the strips and balun together.


Biplanes are particularly poor candidate for antennas in the wings with all the drag and anti drag wires and the compression struts and other stuff. But if the biplane wings have metal leading edges and wood or glass tip bows they could have VOR antennas installed in the tips using the metal leading edges as ground planes. I showed Dr. Dean Hall some years ago how to do this on the Charger he was building. The Idea was to put an aluminum false rib over the end rib under the leading edge skin connecting it all together and then installing a BNC panel connector in the false rib. Rout the outer edge of the wing tip bow to accept a 1/16<sup>" brazing rod about 20" long, drill a hole through the bow for the rod and solder it to the connector. The Dr. finished the airplane, flew it to OSHKOSH and reported to me that every tim he passed a VOR station he tuned the next one and always got a full flag. He was pleased.


 


Installing a Com antenna internally on biplanes is usually difficult. As I said the top of the vertcal is the preferred loxcation but that gets kind of messy. The top of the top wing would be good but most biplanes have fuel tanks there and it would still be outside anyway. Inside the steel tube structure is not recommended, the tubes will interfere in many unknown ways. Quite a few folks have had good results installing one of my SA-002 Com antennas under the turtle deck of the aft fuselage. That only works though if the turtle deck is fiberglass or wood. Stardusters come to mind. There must be a GOOD ground to the upper longeron. A friend of mine welded three or four tabs on the upper longeron to get a good ground and it worked fine.


 


The following are my recommendations for antenna locations on tube and rag type aircraft with wood wings for both optimum radio performance and minimum drag. Also best visual. I hate the "Stickle Back Trout" look.


1. Com antenna. One antenna on the bottom of the cowl next to the access panel if you have one. Use a coaxial switch to connect two radios to one antenna. I would rather use one really good antenna for two radios than two lousy ones. On the ground at some airports you may have some problems. I make a special switch that allows receiving on two radios and transmitting straight through to the single antenna when transmitting.


2. Vor antennas. One antenna in each wing for each receiver out toward the tip. Install through the outer inspection hole using flat head screws through the top surface of wing to secure balun. I have drawings of baluns for anyone that wants them or I can provide the baluns. One antenna into each VOR receiver will increase your VOR range about 25% over one antenna with a splitter.


3. Marker Beacon antenna. Standard boat type antenna may be installed in aft fuselage looking between the steel tubes. Or a 40 inch long conductor, metal tape or wire, could be installed inside on the bottom fabric alongside the bottom stringer with the center conductor of the coaxial cable connected to the conductor and the braid can be grounded to the aft edge of the bottom aluminum sheet behind the access panel but would not be required.


4. GPS. Inside the top fabric. I have mine installed between the fabric and the headliner over the cabin. My tests show no significant signal loss due to the aluminized fabric. Do not need a ground plane. On newer units the antennas all have built in amplifiers and seem to just plain WORK! The top of the instrument panel works well with a very nice short cable run.


5. Transponder / DME. A standard external monopole may be installed on the bottom metal near the bottom access or install one of my SA-005 dipole transponder antennas as I did inside the wing about half way out to the tip. It needs about six inches of wing depth for the installation. I installed mine through an inspection hole by trimming off enough of the excess fiber glass to allow it to fit through the hole. I then bonded it to a rib with the six inch dimension vertical and connector pointed toward the cabin. I used RG58/CU cable but if I had it to do over I would use a little better cable like RG55/U or RG 142/U cables perhaps.


6. Glide slope. Just couple the glide slope signal off the VOR antenna with a coupler box. Unless you are using a towel bar antenna or one of the small blade types. They don’t seem to support the glide slope frequencies.


7. Loran. Does anyone still use Loran? I do! Very sensitive to aircraft electrical noise. Need filters on every short mast just aft of the thing that could be noisy. Very low frequency with vertical polarization. A six foot ADF whip antenna would be great. Or a long wire antenna. Do the best you can. Get the maximum vertical dimension possible. I have a wire antenna going from an insulator at the base of the windshield to aupper edge of the windshield. I originally had a long wire antenna going all the way to the top of the vertical tail but it broke due to vibration. I wrapped the end of the broken wire around the mast and it worked just as well. So I left it that way. I did neaten it up a bit though.


One thing to remember though is that the Monopole and whip type antenna is just half of the antenna. The ground plane or more properly "counterpoise" is the second half and is important because it helps shape the radiation pattern. Ideally it should be about a half wave length, which is about 48" at VHF frequencies. Obviously we can’t have one that large on our small airplanes but we should do the best we can. The lack of a good ground can also cause a high VSWR.


Below are terms used in antenna parlance with short definitions of their meanings.


Active Elements: The part of the antenna that actually does the radiating or the receiving of the RF energy.


Aperture: The capture area of the antenna. On a dipole or monopole it is the overall dimension of the active elements, plus the airplane, on dish antennas it is the diameter of the dish.


Feed point: Generally the point at which the coaxial cable attaches to the antenna but could be where the feed device attaches to the active elements


VSWR: Voltage Standing Wave Ratio. The measurement of the ratio of incident to reflected RF energy. An indication of the quality of energy transference. The lower the number the better. 1:1 is perfect. 2:1 is good ,3:1 is OK, 4:1 and up is poor to terrible.


Radiation Pattern: A pattern showing the relative signal level around an antenna. Signal strength can be severely reduced in particular directions by other antennas, vertical stabilizers, landing gears etc.


Balun: A device that converts a balanced transmission line (such as TV lead in) to a coaxial line which is an unbalanced line. Provides balanced currents on dipole antennas while matching the 50 ohm transmission line to the nominally 150 ohm antenna and acts as a RF choke to prevent reflected energy from traveling down the outside of the coax back to the radio.


I would be happy to answer any antenna questions I can.
[formatted from Word by Scottly]</sup></sup>
 

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