Antenna Fundamentals By George V. Copland Rt. 2, Box 12 Duncan, OK 73533

AIRPLANE design and construction techniques in small airplanes has shown remarkable change in the last 10 years. Likewise new and better avionics are on the market covering new and improved services for aviation. Miniaturization of electronic equipment allows installation of more complex units in light aircraft or a variety of antennas are required due to the different radio frequencies used. Frequency is inversely proportional to wave length of the radio wave. The higher the frequency, the shorter the wave length. A metal conductor whose length is 1/4 of the wave length involved is the basic element used in antennas. Table 1 shows the relationship of aviation services parameters. The equation length (inches) = 2952/Frequency (MHz) defines the 1/4 length relationship. Due to reciprocity, an antenna performs the same in either transmitting or receiving mode. The pattern, impedance and efficiency are the same. Radio waves are polarized depending on the structure that radiates the energy. A vertical antenna will transmit a vertically polarized wave and a vertical antenna is used to receive the radio frequency wave. The key words are aperture and polarization. Since space is a limitation in small aircraft, antenna of 1 /4 wave length and shorter are used where applicable. The 1/4 wave conductor is generally mounted perpendicularly on a ground plane by use of an insulator at one end. A coaxial cable is used to electrically connect to the antenna. The outer shield is connected to the ground plane and the center conductor connected to the end of the antenna at the insulator. This is referred to as an unbalanced feed. Note from Table 1 that it is impractical to use a 1/4 wave vertical element for the Loran C of LF/ADF frequencies. More on this later. For reception of horizontal polarized waves, such as VOR, marker beacon and glideslope, dipole antennas are used. A dipole antenna is a 1/2 wave length conductor, cut in the middle, and the electrical connection made at the center. This antenna is a balanced antenna and may not be connected di-

rectly to a single conductor coaxial cable. A matching transformer or a balun is used. Two practical balun designs for single conductor coaxial cable will be discussed later. The radiation pattern for a 1/4 wave vertical antenna is different than the pattern obtained from a horizontal dipole antenna. Since we are interested in antennas mounted on an aircraft, a distinction must be made. We desire reception for communication and navigation from all directions. This is called an omni-directional pattern. In the case of homing, such as glideslope, we are interested in forward directivity. Vertical antennas by nature are omni-directional. Figure 1a shows the circular nature of the radiation pattern of a 1/4 wave vertical, operating over a ground plane, as viewed from above. Figure 1 b shows a side view of the configuration. Notice that if the ground plane is infinite, the outside curve applies. The maximum energy is almost maximum in a horizontal direction. As we reduce the diameter of the ground plane, the pattern is altered as shown by the inner curve. Now the directivity is more in an upwards direction. A vertical antenna mounted on the lower half of the airplane would be more efficient location for communication with a ground station for this reason. Notice how inefficient the antenna is in the vertical direction. Since we are trying to capture energy as an aperture by the conductor it appears to reason that a vertical antenna pointed to the source would be a highly inefficient method of receiving a signal. Some vertical antennas are swept back for streamlining and appearance. This modifies the forward portion of the pattern to a higher radiation angle. However, this small loss is acceptable. Any obstruction, such as landing gears, other antennas, tail fins and metal propellers distort the pattern. It is good practice to keep antennas 1/4 wave length or more away from metal conductors. This also includes control cables, electrical wiring, drag and antidrag bracing and streamline flying wires. The shape of the aircraft highly modifies the radiation pattern of an an-

tenna. The radiation pattern of a horizontal dipole antenna is shown in Figure 2. Maximum directivity to a received signal is broadside to the antenna. The pattern is in the shape of a torus or doughnut and, therefore, has a good down looking pattern. As before, very little signal is expected to be received oft the ends of the antenna. Using this style of an antenna for VOR reception one should expect good forward and rearward reception and marginal reception from the side. This type of antenna is excellent for glideslope reception as sidelobe interference is minimized while forward directivity is at a maximum. This type of antenna is also used for marker beacon reception where downward gain is important. The marker beacon signal is transmitted upward and is not intended as a homing signal. There is enough signal so as the null end effect of the receiving antenna is not as apparent. Today, most marker beacon receiving antennas are miniaturized and not of the 1/2 wave dipole type. VOR antennas are generally a Vee shape, that is, two 1/4 wave length elements separated by approximately 90 degrees as shown in Figure 3. The gain is not as high due to the aperture reduction, but the pattern is almost omni-directional. The relative signal strength minimum to maximum varies no more than 85%. Again, the aircraft modifies the radiation pattern. As before, it is important to keep conductors away from the elements. This is the main reason VOR antennas are mounted high on the vertical fin of a metal airplane with the Vee looking forward. Reduced ice formation and lower static electrical discharge from the element tips is a plus in this mounting. This type of antenna should not be mounted close to the fuselage or wing of a metal airplane as the conducting surface of the aircraft and modifies the gain and pattern of the antenna. Commercial Vee antennas generally have a mast mounting built in and have been optimized in design. A U-shape or rams horn design is also used which approximates the pattern of Vee design. Transponder and DME antennas SPORT AVIATION 55

both operate around 1,000 MHz as shown in Table 1. The transponder operates at 1,030 and 1,090 MHz while

the DME operation is over a band of frequencies. Their physical dimensions

are essentially the same, but the element shapes are quite different. Practical 1 /4 wave lengths of transponder and DME antennas are generally 2.25 inches above the Teflon insulator. The transponder antenna is a small conductor, 1/8 inch in diameter, with a small ball on the end to minimize static electrical discharge. In contrast DME antennas are generally 1/2 inch in diameter and no ball on top. The larger diameter is one way to increase the band width of an antenna at the expense of small loss in gain. The transponder antenna would be sharper in resonance at 1/4 wave and a little better match to the transmitter in the transponder. Blade antennas are used for both services to reduce air drag. Transponder antennas are mounted forward on the bottom of the metal fuselage with a clear line of sight. It is a good idea to have a ground plane at least 4 wave lengths in diameter. This is not so bad as we are only talking

Table 1 Service

Frequency

LORAN C LF/ADF

100KHz 200KHz -400KHz 75MHz

MARKER BEACON NAV/VOR/LOC COMM/ELT GLIDE SLOPE DME TRANSPONDER

108MHz -118MHz 118MHz -137MHz 329MHz - 335MHz 962MHz -1213MHz 1030MHz, 1090MHz

Aircraft that use composite materials for construction, such as plastic foam, epoxy-fiberglass, carbon fiber and

wood, must use special mounting

techniques for antennas. In the case of 1 /4 wave antennas with their associated ground planes, the ground plane needs to be built into the structure. The ground plane should be a minimum of 40 inches in diameter for VHP frequencies. Two copper foil strips, 1/2 to 1 inch

wide, crossed at 90 degrees in form of an X and bonded to the coaxial cable sheath at the center would be a practical substitution. Two copper wires, 1/16

about 36 inches in diameter. One other inch diameter (16 gal.) used in a similar note of importance is to keep the in- pattern could be used. It may not be sulator clean. At the 1,000 MHz fre- practical to install the conductors in a quency, contamination will greatly af- flat plane, but it is the ideal. For a transfect signal strength. A top mounted ponder antenna a solid disk is to be pretransponder antenna is not a good ferred. A 10 inch diameter, .025 inch choice as the radiation pattern is up- thick aluminum disk would be a ward and the closer you get to an ARSA minimum for use. The transponder anor TCA the aircraft will shield the an- tenna element needs to point downtenna's operation. ward. The radio frequency cable used to Loran C and LF/ADF antennas fall into a similar category. Both radio carry signals to and from the antennas is in practice determined by the connecwaves are long, 1/4 wave lengths in hundreds of feet, and do not depend tors on the equipment and the routing of the cable. A 50 Ohm impedance coupon line of sight for reception as VHF axial cable is used. Two common verand above radio frequencies do. Both sions of this cable are the RG8/U and services are vertically polarized but due the RG58/AU. 50 Ohm impedance is an to ground conduction and skywave electrical term which relates the size of modification mainly due to the ionosthe center inner wire to the inside phere, almost any conductor will work diameter of the outer conductor and for reception. The longer the antenna based on the dielectric medium the better is the rule. Easier said than separating the inner and outer conducdone on aircraft. Trailing wire antennas have long gone out of style due to mod- tor. The RG8/U is approximately .4 inch ern electronic circuit design. Since Loran C operates on a single frequency of 100 KHz high gain preamplifiers may be used at the antenna. Most Loran C systems are of this type. Couplers are used that utilize the vertical VHF antennas for receiving the Loran C signal while effectively blocking out the VHF transmitted energy when in Comm use. This cuts down on the number of antennas involved. LF/ADF system antennas are of a self-contained loop design matched to the receiver. A long wire on the aircraft may be required for the reference signal

depending on manufacturer. 56 AUGUST 1990

outside diameter and the RG58/AU is approximately .2 inch outside diameter. The RG8/U is much more rigid and does not bend easily. RG58/AU is quite flexible but is more lossy. The length of cable on the Comm side of the transceiver is usually short, so electrically this is no problem. These cables have a stranded center conductor and are to be preferred. Do not use RG58/U (notice the "A" is left out) because it has a solid center conductor and is difficult to use. Extreme care must be taken not to scratch or nick the center conductor when assembling connectors. The wire with the smallest scratch under vibra-

1

/4 Wave Length 2460 ft. 900ft.

39.36 in. 26.1 in. 23.1 in. 8.9 in. 2.7 in. 2.7 in.

Polarization Vert. Vert. Horz. Horz. Vert. Horz. Vert. Vert

tion in time will break and cause an intermittent connection. These breaks are hard to find, so don't consider building

in this inherent problem. RG58/AU and

RG58/CU both have stranded center

connectors and are acceptable. The difference between the AU and CU is the

cable covering. The CU is noncontaminating and slightly more expensive. A more preferred version of the RG58/ AU is the RG58/AU Foam. This is much

preferred in the installation to the transponder antenna. Where as at 100 MHz

both RG58/AU (std.) and RG58/AU

Foam have about the same loss, 4.9 db vs. 4.5 db/100 ft., at 1000 MHz the

losses are 21.5 vs. 14.5 db/100 ft. Remembering 3db is half power, this is significant. The RG58/AU Foam is Belden Part No. 8219.

Many different types of connectors are used with coaxial cable. Two of the

most common found are the UHF83 series and the BNC type. The UHF con-

nectors are a little larger and easier to install on the coaxial cable. Usually the UHF style is used with RG8/U and the BNC is used with the RG58/AU. Am-

phenol Part No. 83-1SP is used with

RG8/U. An 83-185 adapter may be used with this connector when RG58/ AU is desired. Amphenol Part No. 31301 is a BNC style used with RG58/AU.

This connector is an improved captivated contact style and is much pre-

ferred over the 31 -002 model. Generally

one of these two connectors is used on the Comm side of the transceiver. On some equipment the Nav receiver has a receptacle for Motorola 13B coaxial

connector. This is the common style used on automotive antenna systems. Using this style, antenna connections are not interchangeable on receivers of Nav/Com equipment which require separate antennas. This connector is not used much any more. The VOR antenna is made up of two (2) - 1/4 wave length rods separately insulated and mounted horizontally in a Vee configuration. The angle between the rods is 90 degrees for an omni-directional pattern. Length of the rods for 113 MHz is 26 inches. This antenna is

considered a balanced design since

both elements are insulated from ground. In order to connect a balanced dipole antenna to an unbalanced coaxial line, a balun is generally used. The 1/2 wave length balun of coaxial cable between the two elements in Figure 4a is equal to 5904/MHz times the Velocity of Propagation of the electrical wave in the coax. A value of 65% for V.P. is used for RG58/AU. Therefore, for a mid-range frequency of 113 MHz, the length would be 34 inches. The connection is shown in Figure 4a. A 1/4 wave matching stub balun is shown in Figure 4b. The length of the short section is 18 inches long. Leave approximately 1/4 to 1/2 inchs of center polyethylene insulation with center conductor inside sticking out of soldered area to assure no shorting. A separate short piece of shrink tubing or insulating tape can be used in addition. Care must be taken when soldering the lower end to the coaxial cable so as not to damage the inner insulation in the coax. Do not connect the top shields together. No connection is made to the inside conductor of the shorting stub at the bottom. This second method is not as an efficient system above 50 MHz, but is sometimes preferred because of ease of construction, durability and improvement in reducing static discharge noise since both elements are effectively at ground potential. The current trend in composite materials aircraft seems to be to conceal antennas wherever possible. Nav antennas for horizontal wave reception lend themselves to be placed in the wings. For omni-directional receiving the Vee style should be used. Mounting outboard in the wing gets the engine out of the way in the fore and aft pattern along centerline. Consideration must be given not to have a conductor across the open end of the Vee. The Vee may be pointed either forward or rearward depending on wing design construction. The VHF antenna with its associated ground plane offers a challenge for concealment. Height of the vertical element can be accommodated by bending it over after the first 6 to 8 inches. A logical location is aft of the baggage compartment. The vertical element can be located in the vertical fin with the ground plane elements built into the stabilizers. A difficult design and construction choice. It appears that a vertical or swept back element on the underside of the aircraft, even though not concealed, is the best from construction and performance. The ground plane could then easily be in the wing or wing root area. The antenna does not have to be on centerline. One consideration which has seemed to be ignored is that when

Fig. 1b Fig. 1a

Fig. 2

Elements 26" Insulator

L = 34" for - RG58/AU

Fig. 4a

using foil or imbedded elements in epoxy-glass or foam structures, modification of element length must be made. The 1 /4 wave element length is a free space determination based on the speed of light. Electromagnetic waves travel slower through dielectrics. Note the effect given to the V.P. (velocity of propagation) constant in coaxial cable. For cellular polyethylene V. P. is 78%, that is, the elements of an antenna enclosed in that foam medium would be shortened by 78%. It would be expected similar consideration should be taken into account for foams used in composite structures. The same reasoning holds for the attachment of foils directly on epoxy-glass surfaces. No direct figures are available but it is a factor. Antenna location in small aircraft is a real challenge to the designer. Perfor-

mance patterns are greatly modified by the aircraft itself. What works for one aircraft may not be directly applicable for a similar design. This article should prove a guide for considerations in antenna applications of small aircraft.

References Antennas for Small Aircraft, George Copland, EAA Forum Oshkosh 76. Antennas, John Kraus, McGraw Hill 1950. Antenna Engineering Handbook, Henry Jasik, McGraw Hill 1961. Radio Amateurs, VHF Manual, ARRL, 1972. SPORT AVIATION 57