Radio and Television Today (Jan-Nov 1941)

INSTALLING FM SETS Insuring perfect performance with the correct antenna system Any radio receiving set will give better performance if it is installed properly with a good antenna, and ground. This statement means any radio, FM, AM, or television. Although most of the AM and many of the FM sets are now equipped with loop antennas which are satisfactory for many installations, it is a fact that you can demonstrate any time, that a good outdoor antenna will give improved reception for more stations. FM receivers and the FM system are outstanding for several reasons, one of which is ability to discriminate against noise. While this ability is remarkable, there is no reason to handicap it with a poor installation. For all conditions the outdoor di-pole antenna will give the best results on FM. This, of course is due to the greater signal strength which can be developed, and usually there is less noise voltage surrounding a high di-pole. The noise is, of course, wiped off on strong signals but it can come through on weak, distant stations. DI-POLE ANTENNAS Di-poles may be unfamiliar to servicemen who have not encountered them in "ham" activities or FM and Television work. Fundamentally, the di-pole aerial is two quarter-wave conductors, in-line, with the inner ends separated a short distance and the two conductor feed-line attached to the inner ends for the transfer of energy. The two-conductor feed-line is usually a twisted pair of specially insulated wires designed for low losses at medium high frequencies. Typical losses for transmission cable of the type used for FM antennas is about 6 DB per 100 feet at 40 mc. This 6 db corresponds to a loss of 50 per cent of the voltage present at the di-pole. Usually the runs are less than 100 feet, and not as much voltage is lost. The surge impedance of the transmission line is usually 72 ohms. This matches the impedance at the center of the di-pole and must be terminated at the receiver with a like value of impedance. This problem of matching the impedances will not be troublesome to servicemen who use complete antenna kits. If the transmission line is bought separately, its surge impedance should be known since some of the lines are as high as 100 ohms. A few ohms mis-match is not important, but 100-ohm lines should not be connected to a 72-ohm di-pole. DJRECT/OAML POINTS Di-pole antennas are broadly directional on the "side" and have sharp minimum reception points on the ends. Theoretically, the pick-up of the antenna will be maximum when the line of the di-poles is at right angles to the line between the receiver and the station. Reflections and intervening objects occasionally change this, and the di-pole should be rotated for maximum signal from the weakest Home installation engineered by S. H. Bent, Gardner, Mass. Mast is iron pipe. Horizontal member is strut for guy wire. See page 34. station if this does not interfere with the reception of other FM stations. One important consideration of installing an FM antenna is to get it high. The extra length of transmission line and possible loss will be more than compensated for by the greater signal. Noise-free FM reception is dependent upon sufficient signal level to insure complete action by the limiter. COMBINATION A-FM ANTENNAS Since most of the FM sets are also AM sets, it is desirable to combine the FM and broadcast band antennas. This can be accomplished in several ways. Some antenna manufacturers are selling units which consist of a long wire section connected to the open end of one di-pole through a wave trap resonated at 46 mc. This trap isolates the long antenna section from the ordinary FM di-pole for the 40 to 50 mc. range but permits the full length of the wire to operate on the BC band. This type of A-FM antenna is shown in the diagram. The tuned circuit connecting the end of the quarter wave dipole with the long wire BC antenna is usually a single layer solenoid wound on an insulator and tuned by its distributed capacity. (Continued on page 68) Typical FM antennas with dimensions for di-pole types. Multiples of half-wave lengths are less directional. 5'4"-HXh 5'4"— i FEED LINE . /l DIPOLE AND DIPOLE-J/fcA. REFLECTOR. ,. 46 MC TRAP. V4 — H / ( ANY LENGTH COMBINATION F-AM 42 FOLDED ANTENNA