I III Antennas 1ml Their Installation By Neurman B. Martindale, EAA 10349 1928 Montmarte Dr. Jacksonville, Florida 32210

T volumes have been written, and can become very complicated. Fortunately, aircraft UHF antennas fall into

HE SUBJECT OF ANTENNAS is one on which many

two general categories that can be explained in relatively simple terms. Fig. 1 is a sketch showing the two types of antennas and their typical location on an airplane. First, let us examine the transmitting or communication antenna, sometimes referred to as a whip antenna, this antenna is of the type known as a ground plane. The term (ground plane) has nothing to do with the fact that it is mounted on an airplane. This term is derived from the fact that the base of the antenna is mounted on a metal surface which serves as a ground, i.e., ground plane. The second antenna known as the receiving or Omni antenna is a modified half wave dipole, and is a free space antenna, that is, a ground is not required for its operation although this antenna does not require a ground, one may be required for the balancing network used with it. The balancing network known as a Balun is usually installed at the center of the antenna and is considered as part of the antenna.

t

Omni Antenna Whip Antenna

FIG.

1

signals that are in the same plane, i.e., will not receive a station that one of the ends is pointing at. Obviously, a directional antenna would not be very desirable for aircraft use, so in order to overcome this characteristic, the antenna is made in the shape of a V, enabling it to receive signals from all directions. There are two major differences between the whip and Omni antennas. The first difference is known as polarization. Polarization is too involved to discuss in a work of this nature, but for our purposes all we need to know is that the whip antenna is vertically polarized,

s __

\f

transceiver

To

Transceiver B

B FIG. 2 Dipole Antennas (A) is a conventional antenna and will not receive stations in the directions of the dashed arrows, but will receive stations in the directions of the solid arrows. (B) is the same antenna bent in the shape of a V. It will receive signals from all directions.

Fig. 2 is a sketch of a half wave dipole antenna, the "A" part shows a conventional antenna, the "B" part shows one modified for aircraft use by bending it into a V shape. The conventional dipole is a directional antenna. That is it will receive signals that are coming from a direction that is at right angles to its plane, and will not receive 40

APRIL 1966

To Transceiver Ground

FIG.

3

therefore, must be mounted vertically (a light sweepback is O.K.). The Omni antenna is horizontally polarized, therefore must be mounted horizontally. The second major difference between the whip and Omni antennas has lo do with the method used to connocl them to the transceiver. To understand this consider Fig. 3. "A" is a dipole of the type used for Omni antennas, "B" is a whip antenna. Observe that "A" has its center connected to the transceiver and that both halves are identical, therefore "A"' is considered to be a balanced antenna. On the other hand, the whip antenna "B" has one end connected to the transceiver, and as a result is considered to be an unbalanced antenna. A simple analogy would be to

consider both antennas suspended at their connection points, the dipole "A" would balance, the whip "B" would not balance. This same reasoning is used throughout electronics. Anything with two equal parts is considered to be balanced, anything with only one part or two parts that are not equal is considered unbalanced. Furthermore, you cannot connect something that is balanced to something else that is unbalanced or vice versa. Now the cable used to connect the antennas to the transceiver is a type known as a coaxial cable. Fig. 4 is a cross sectional view of a co-

center conductor

insulation

insulation

outer conductor FIG. 4 Enlarged cross-sectional view of coaxial cable.

axial cable, as you will notice it has two conductors, an inner conductor and an outer conductor, but they are not identical, therefore this is an unbalanced cable. We previously determined that a whip antenna is unbalanced, therefore, a coaxial cable can be connected directly to it. On the other hand the Omni antenna is balanced so we cannot connect it directly to a coaxial cable. The solution to this problem is a device known as a Balun. The word Balun is a contraction of the expression "Balanced to unbalanced", and this expression tells us exactly what it does — it allows a balanced antenna to be connected to an unbalanced cable.

Whip Antenna Foil Strips

Metal Plate

Installation of whip antenna on fabric or wooden aircraft.

attached to the under side of the skin. The foil strips should radiate outward from the antenna base about 22 in., or as far as possible, the metal plate should be large enough to give good mechanical support. If desired the foil strips may be replaced with pieces of wire, or the metal plate made larger, and the foil strips eliminated. The Omni antenna should be mounted so that when the aircraft is in level flight, the antenna rods are level and pointing either directly forward or directly aft. It should be mounted as far from the engine compartment and propeller as possible. This is to minimize engine noise and prop-chopping. A good location is on the vertical fin as high as possible. The connecting cables for all antennas should be cut to appropriate length and the proper antenna plug attached. Do not coil antenna cable to eliminate excess. Either trim to the required length or locate the antenna in such a way as to use all of the cable supplied. The length

of the cable is not critical, but when it is coiled the cable

takes on unwanted inductive properties which seriously limit its ability to carry Radio Frequency energy. All transmitters must meet the requirement established by the Federal Communication Commission, and have a license issued by them. For more details on this see "So You Want To Communicate" in the December 1965 issue of SPORT AVIATION. ®

ANTENNA INSTALLATION

The whip or communication antenna should be mounted well clear of metal structures. A sufficient ground plane should be provided under the antenna. On metal covered aircraft the whip antenna should be installed on a large flat surface to enable the aircraft skin to serve as the ground plane. Of course, on wooden or fabric covered aircraft it is necessary to create a metal surface to furnish the ground. This is a relatively simple matter and is illustrated in Fig. 5. The metal plate and foil strips should be

Dick Bach, editor of AAA's Antiquer magazine, paid EAA Headquarters a visit along with AAA's Airpower Museum's Stinson SM8A. We certainly enjoyed his visit which was followed by AAA President Bob Taylor, who also spent

some time with us.

SPORT AVIATION

41