Antennas for Aircraft by Bob Archer This document is a compilation of Bob’s articles published on aeroelectric.com by permission TENNA TIP # 1 COM ANTENNAS FOR COMPOSITE AIRCRAFT by Bob Archer of Sportcraft Antennas The art of installing antennas internally into composite aircraft has left many people confused and perplexed 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. To work properly a ground plane should be at least a half wavelength in diameter and at VHF frequencies this is about four feet. Needless to say there is no place this large to put a ground plane on a small composite aircraft. First of all I believe that only dipole type antennas should be used in composite (non-conductive) aircraft and that the best location for COM antennas is in the vertical stabilizer while the second best is in the fuselage tailcone. The reason being that COM antennas are vertically polarized and therefore require a vertical space of about 46" for a half wave dipole. The larger aircraft have enough height for an aperture of a full half wavelength but the smaller types need an antenna that is shortened a bit. Think of aperture as the capture area of an antenna. Mounted high in the vertical tail the antenna is high enough to see all the way around while the fuselage mounted antenna may suffer some signal loss due to blockage from engine, passengers, and miscellaneous "stuff". The SA-006 COMM antenna was designed to replace some very bad antennas that were sold in the early days to two place composite builders. It is constructed of .016 inch 2024 Alclad aluminum so that it can conform to the inside curvature of the fuselage and is 26 inches high by 12 inches fore and aft. After the tail is sealed up of course it is very difficult to replace the tail antenna so this antenna was made as a replacement. This antenna was test flown mounted in the fuselage of the LANCAIR 320 prototype aircraft and a transmission was received from an aircraft at a distance of about 170 miles. The SA-008 COMM antenna is designed for installation in aircraft with larger vertical tails such as the Lancair IV, Glasair, Express and the many versions of Ex’s. This dipole antenna is a full half wavelength which provides the maximum aperture at this frequency. This antenna is shipped with the elements folded up so the elements must be unfolded and adjusted in sweep to fit into the available space in the Vert. stab., riveted in this configuration and then bonded in. Remember that graphite composite material is conductive so it must be treated as if it were metal. This means that on the Lancair IV, the only place to install internal antennas is inside the fiber glass wingtips, the dorsal / vertical stabilizer and perhaps the Plexiglas windows. Graphite structures do complicate matters for antenna installations. If spar caps are constructed of graphite materials horizontally polarized antennas should not be mounted nearby because practically no signal will be able to get past the long conductors that will act as reflectors. The same goes for any long conductors in the same alignment as the antenna such as horizontal and vertical tail spars, control cables, push rods, wires etc. Each aircraft needs to be analyzed individually for the best antenna locations. I do not recommend any antenna on the market that has a little black box in the center of the antenna. This device is a ferrite transformer which provides a very good VSWR and a very good bandwidth but at the cost of being a very lossy ( absorbs energy )device. The very best specification that I have seen on ferrite transformers is a loss of 2.5 dB and the worst goes up to 12 dB. As a reference, a 3-dB loss gives an output of 50% and 10 dB gives just 10% out. So if you have a 5 watt transmitter into an antenna like this, you get just .5 watt out, and it works the same on receiving. Not a bargain. An antenna you can easily make yourself would be to just solder quarter wave elements to the inner and outer conductors of the coaxial cable and go with it. Also if you were planning to go with Jim Weir of RST's designs don't bother with the ferrite beads. At these frequencies the beads don't do anything that I could detect in the RF lab. A good balun would work better as a dipole feed because it balances the currents on the elements and matches the impedance at the same time and it doesn't absorb RF energy. My antenna designs do not need a balun because I use a modified version of a feed called a Gamma match that feeds the antenna at the fifty-ohm point and automatically balances the currents on the elements. When installing any antenna remember that antennas do interfere with each other when installed too close together and that close metallic (or conductive : remember graphite) objects that are as long or longer in the same polarization/plane will reflect RF energy away from them. Close meaning one wavelength or closer and closer being worse. One wavelength is about eight feet at VHF frequencies. Less than a quarter wavelength is really bad from both a VSWR and the radiation pattern standpoint. The formula for wavelength is: 11803/ FREQ. in Mhz. = wavelength in inches. So there being only one good location for an internal COMM antenna (unless you have twin verticals) and antennas don't work well close together, it follows that for aircraft with dual transceivers and no good location for a second antenna there is a need for a two set switching device of some type. I recommend a RF switch, single pole double throw type. A switch like this will allow you to receive and transmit on a single frequency and when switching from one set to the other, the switch will connect the proper set to the antenna. This type of switch is available from ham radio outlets. Another type of switch that allows receiving on two sets at the same time while transmitting straight to the antenna is now available due to popular demand from Sportcraft Antennas. After you airplane is flying it would be a good idea to check the radiation pattern for holes or nulls. A good way of doing this would be to climb to about 5000 feet at fifty miles or so from an airport with an atis and fly flat circles while listening to the signal strength. After a few turns you should be aware of the directions of the weak or non existent signal reception. This works better than field strength meter on the ground. These suggestions and recommendations come from 35 years of spacecraft and aircraft antenna experience and are meant to help in obtaining optimum antenna performance and do not mean that anything else will not "work". I have seen a lot of really bad antenna installations whose owners are perfectly happy with their antenna performance. I will address some other antenna installation problem areas in the future. Bob Archer Sportcraft Antennas 21818 Ocean Ave. Torrance, Cal.,90503 Tel. 310-316-8796 “E” mail <bobsantennas@earthlink,net> Below are frequently used antenna terms and with short definitions . • 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, 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 twin lead ) to a coaxial line which is an unbalanced line. Provides balanced currents on dipole antennas while matching the 50-ohm line to the nominally 150-ohm antenna. TENNA TIP #2 VOR ANTENNAS FOR COMPOSITE AIRCRAFT by Bob Archer of Sportcraft Antennas In the first installment I talked mainly about COM antennas so in this one we will discuss internally mounted VOR antennas and their installation problems in composite aircraft. The use of VOR receivers is decreasing with the advent of LORAN and GPS but they are still useful and I believe that, as with fuel pumps, you can't have too many backups. Also if you fly IFR you still need the Localizer and glide slope. Navigation antennas being horizontally polarized require that they be mounted in an area that has a large enough horizontal area that a half wave dipole can be installed. If the elements are swept the dipole can be reduced to about 40 inches. The logical locations are the wings, horizontal stabilizer and the top and bottom of the fuselage. There are always gotchas though and wings with large conductive horizontal structures such as graphite wing spar caps, push rods, cables etc. are not ideal locations. If the antennas in such wings can be positioned so that they are about 0.2 wave lengths (about 20 inches ) ahead of the spars or such they would have more gain from the front but would reduce signals from the rear. If mounted behind the spar or other conductive device the reverse would be true and the signal would be stronger from the rear. If the horizontal tail is constructed of graphite of course it is not suitable for antenna installations. Other than that it is a pretty good location except in the case where the signal is coming from directly in front and has to pass through the engine and cabin area to arrive at the antenna. If the aircraft is a canard of course the reverse is true. In this condition the signal will be weakened and the CDI needle could possibly wander. This also happens in the case of a metal aircraft with the VOR antenna on the top of the vertical stabilizer. When the airframe gets between the transmit and receive antennas as you approach the VOR station there is usually a bit of needle wagging going on. In the case of aircraft with almost all graphite construction and graphite being conductive the only location for internally mounted VOR antennas would be in the fiber glass wing tips. Because of the wing tip lights, wires and strobes normal types of antennas work very poorly in this location. The light wires have a very low impedance to ground and being so close to the antenna tend to short the RF energy to ground. To solve this problem I have developed a series of wing tip VOR antennas that are shorted to ground DC wise and are very good antennas RF wise. These antennas are grounded monopoles fed with a modified gamma match and the parameters are such that the VSRW matches well across the VOR frequency band. The wing tip lights and wires on these antennas become part of the antenna and therefore have no effect on the antenna performance. Antennas of these types have been flown for years on various types of aircraft with excellent results. If your installation has two VOR antennas do not connect them together using any means unless you really, really know what you are doing. Being as far apart as the wing tips are will cause a phasing problem between the antennas that causes multiple lobes to be formed, the number of lobes being determined by the distance apart. Using one antenna for each receiver will result in the increase in range of about 25% over one antenna with a two set coupler into two receivers. Try to keep at least a 2" radius on all RG58/U cable installations. The same things apply to VOR antennas that I mentioned last time such as little black ferrite transformer boxes and ferrite beads. I don't believe that ferrite devices have any place in antennas because they do absorb RF energy. Just a few words on dipole antennas for clarification. There are two types of dipoles, the two element type that is fed in the center, and is the type that is common and everyone is familiar with. There is also a continuous conductor type that is a continuous conductor from tip to tip for a full half wavelength. Each of these types require a different type of feed device. A center fed dipole dipole really should have a balun installed for proper operation. Without a balun on a center fed dipole fed with coaxial cable the antenna has uneven currents on the elements because half the currents on the braid side travel down the outside of the cable, the impedance is poor because the nominal impedance of this type of dipole is 150 ohms and is being fed with 50 ohm cable and the radiation pattern is bad because the radiation off the outside of the outer conductor interferes with the radiation off the elements. So a good balun does three things: 1. Balances the currents on the elements. 2. Matches the impedance of the cable to the elements. 3. Chokes off the currents that would otherwise travel on the outside of the cable. For those that would rather do it themselves I have enclosed a rather rough sketch of a modified type two balun, I believe it is, that very neatly solves these problems. I recommend using RG142/U cable if available for its resistance to melting during soldering. By the way those ferrite beads that so many composite builders are using are totally useless. The beads do absolutely nothing at these frequencies that I could determine. And the ones with the ferrite transformers in the center are very lossy. Up to 12 dB depending on the type they are using. I strongly recommend using a real balun. The continuous conductor dipole is interesting in that the center point has an impedance of zero ohms and the tips have an impedance of infinity, or there abouts, so somewhere between zero and infinity there is bound to be an impedance point that we could use. If we were to split the conductors apart on a 300 ohm twin lead cable and attach a conductor to the 150 ohm point on each side of the neutral center we would have a very good 300 ohm antenna. To feed with a 50 ohm cable though is a bit more of a problem. What we need to do here is to connect the braid of the cable to the neutral center point and the center conductor to the 50 ohm point on one side. A problem arises in that in connecting the center conductor to the antenna element we introduce an inductive loop which needs to be balanced out with a series capacitance. When all the parameters are correct this also becomes a very good antenna. This antenna design allows control of the length, feed point impedance and capacitance/inductance relationships so can be tuned to a high level of performance. Specialized antenna measuring equipment must be used for tuning this type of antenna. Most of my antennas are designed in this manner. Bob Archer Sportcraft Antennas 21818 Ocean Ave. Torrance, Cal., 90503 Tel. 310-316-8796 “E” mail.<[email protected]> Below are frequently asked questions about antennas with their answers. 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, 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 line to the nominally 150 ohm antenna. Polarization? The plane in which RF energy is radiated. Normally either vertical, such as COM, or horizontal, such as VOR, or any angle in between, such as bent COM antennas. There is also circular polarization, which we will discuss along with GPS antennas. TENNA TIP #3 GPS Antennas for All Types of Aircraft by Bob Archer of Sportcraft Antennas Because there seems to be at present more interest in GPS than any other aircraft system I think we should talk about it first. The theory of how GPS operates is very similar to Loran in that it uses the time difference between three or four transmitted pulse strings from three or four sources to determine Hyperbolic equal time difference lines and calculates position from this data. Of course space based transmitters transmit the GPS signals and Loran transmitters are land based. And the frequencies of operation are very different, the Loran systems operating at 100 Kilohertz and GPS at 1.575 Gigahertz, with a secondary frequency at 1.227 Ghz. A wavelength at 100 Khz. is 9836 ft. and at 1.575 Ghz. is about 7.5 inches. These wavelengths give an indication of the size of the required antennas; a large antenna for Loran, a small one for GPS. About 3 3/4 inches for a dipole. There is a galaxy of 24 GPS satellites now in earth orbit at an altitude of about 10,600 miles which gives them an orbit period of about 12 hours. Their calibrations are updated as to time and position every orbit to maintain their accuracy. Most satellite systems, including GPS, use circular polarization for their RF systems which means that the electric field rotates at the rate of the frequency being used. You might think of it as 360 degrees of polarization rotation in one wavelength of travel. Circular polarization is used because linear polarization when passing through the heavyside layer of the upper atmosphere rotates the polarization depending on the angle of passage and it is possible for the signal to completely cancel out. When using circular polarization on both transmitter and receiver antennas these effects are minimized and maximum signal may be utilized. Of course both ends must use the same rotation, either RH or LH circular polarization, or the signals could cancel out. There are several different types of receiving antennas that can be used by GPS receivers and I'll try to touch on each of them. All GPS receiving antennas should have a radiation pattern that optimally would look like a flattened hemisphere so that there is somewhat more gain at the low elevation angles because that is where the satellites are the farthest away. The first and most common at this time is the patch antenna. It is small and flat and can be installed easily on the outside of metal or graphite aircraft skins and inside of non- conductive composite aircraft with some minimum type of mounting bracket. Also there is what is called a turnstile antenna which consists of four small pieces of flat metal in a square pattern and each piece fed with 0,90,180 and 270 degrees of sequentially phased signal that provide the circular polarization. This type antenna is usually somewhat short on gain because of being loaded with dielectric material to make it small and having a quadrature hybrid of some type to provide the phased signals which will absorb some signal. A lot of these have built in amplifiers and they are then called "active" antennas. Another type is the normal mode helix antenna. Garmin used these on their early models and still do on their handhelds and are about .75 inch in diameter and three inches long. A regular helix antenna radiates most energy off the end but a normal mode helix is smaller in diameter and radiates more energy off the sides. With the correct ratio of length to diameter the radiation pattern can be optimized. I have one of the Garmin handhelds and I wanted to put an antenna between the headliner and the fabric over the cabin of my Bellanca so I built a foam conical form two inches high by two inches in diameter and wound on extra long dipole type elements that wind down from the apex to the base in a spiral manner. Works very well. Then there are Archimedean spirals, log conical spirals and bifilar and quadrifilar helices and numerous turnstile types. All of which are circularly polarized with various positive and negative attributes. The installation of all these types of antennas in a non-conductive composite aircraft, or even fabric, is simple in that none of them require ground planes and they are so small and light that they do not require much room or structure to support them. All that is required is that they be mounted in the top part of the aircraft looking up in such a manner that they can see most of the sky through the non-conductive skin when installed. In metal or conductive aircraft of course they must be installed on outside of the conductive skin of the aircraft or installed inside a fiber-glass fairing of some type. On tandem type aircraft with canopies the top of the roll over structure is a great location. We will just leave the details to the builders. To test the reception of signal through glass or other types of skins just fire up the old hand held GPS out side the airplane and then take it inside and check the signal strength difference. There should be very little to no difference for glass, wood or fabric but graphite will cut the signal off. In my meanderings I have come across some primers that are heavily loadede with zinc chromate and such for fiber glass that are very lossy. This is probably more information than anyone ever wanted to know but maybe it will help someone. I am always open to questions if there are any out there. BOB ARCHER SPORTCRAFT ANTENNAS 21818 Ocean Avenue Torrance, California 90503 Tel. 310-316-8796 “E” mail <[email protected]> TENNA TIP #2 VOR ANTENNAS FOR COMPOSITE AIRCRAFT by Bob Archer of Sportcraft Antennas In the first installment I talked mainly about COM antennas so in this one we will discuss internally mounted VOR antennas and their installation problems in composite aircraft. The use of VOR receivers is decreasing with the advent of LORAN and GPS but they are still useful and I believe that, as with fuel pumps, you can't have too many backups. Also if you fly IFR you still need the Localizer and glide slope. Navigation antennas being horizontally polarized require that they be mounted in an area that has a large enough horizontal area that a half wave dipole can be installed. If the elements are swept the dipole can be reduced to about 40 inches. The logical locations are the wings, horizontal stabilizer and the top and bottom of the fuselage. There are always gotchas though and wings with large conductive horizontal structures such as graphite wing spar caps, push rods, cables etc. are not ideal locations. If the antennas in such wings can be positioned so that they are about 0.2 wave lengths (about 20 inches ) ahead of the spars or such they would have more gain from the front but would reduce signals from the rear. If mounted behind the spar or other conductive device the reverse would be true and the signal would be stronger from the rear. If the horizontal tail is constructed of graphite of course it is not suitable for antenna installations. Other than that it is a pretty good location except in the case where the signal is coming from directly in front and has to pass through the engine and cabin area to arrive at the antenna. If the aircraft is a canard of course the reverse is true. In this condition the signal will be weakened and the CDI needle could possibly wander. This also happens in the case of a metal aircraft with the VOR antenna on the top of the vertical stabilizer. When the airframe gets between the transmit and receive antennas as you approach the VOR station there is usually a bit of needle wagging going on. In the case of aircraft with almost all graphite construction and graphite being conductive the only location for internally mounted VOR antennas would be in the fiber glass wing tips. Because of the wing tip lights, wires and strobes normal types of antennas work very poorly in this location. The light wires have a very low impedance to ground and being so close to the antenna tend to short the RF energy to ground. To solve this problem I have developed a series of wing tip VOR antennas that are shorted to ground DC wise and are very good antennas RF wise. These antennas are grounded monopoles fed with a modified gamma match and the parameters are such that the VSRW matches well across the VOR frequency band. The wing tip lights and wires on these antennas become part of the antenna and therefore have no effect on the antenna performance. Antennas of these types have been flown for years on various types of aircraft with excellent results. If your installation has two VOR antennas do not connect them together using any means unless you really, really know what you are doing. Being as far apart as the wing tips are will cause a phasing problem between the antennas that causes multiple lobes to be formed, the number of lobes being determined by the distance apart. Using one antenna for each receiver will result in the increase in range of about 25% over one antenna with a two set coupler into two receivers. Try to keep at least a 2" radius on all RG58/U cable installations. The same things apply to VOR antennas that I mentioned last time such as little black ferrite transformer boxes and ferrite beads. I don't believe that ferrite devices have any place in antennas because they do absorb RF energy. Just a few words on dipole antennas for clarification. There are two types of dipoles, the two element type that is fed in the center, and is the type that is common and everyone is familiar with. There is also a continuous conductor type that is a continuous conductor from tip to tip for a full half wavelength. Each