Television and FM Broadcasting Antennas

Abstract

Abstract Television broadcast services are located within four bands: the lower very‐high‐frequency (VHF) bands of 54–72 and 76–88 MHz, the upper VHF band of 174–216 MHz, and the ultra‐high‐frequency (UHF) band of 470–890 MHz. The FM band is 88–108 MHz in the United States and the lower VHF band of 90–108 MHz, the upper VHF band of 170–222 MHz, and the UHF band of 470–770 MHz, the FM band of 76–90 MHz in Japan. The bandwidths of the TV and FM channels are 6 and 0.2 MHz, respectively. The antenna structure usually consists of a vertical array of radiating elements, such as dipoles, and slots, or of one or more bays of vertical traveling‐wave antennas such as helices. It is desirable to mount the antennas on top of the tower or support structure, but in many cases it is necessary to side‐mount them. Side‐mounted antennas present an interesting challenge to the antenna designer to control the azimuth pattern. The elements of a vertical array are commonly referred to as “bays”. The array‐feed techniques are conventional, such as a corporate feed or an end feed in a matched or resonant manner. All the characteristics of the batwing radiator, which is the heart of the superturnstile antenna system, are theoretically calculated by using the moment method. The matching condition is obtained on the basis of the characteristics, for the cases of changing the connection of the jumpers and of changing the distance between the support mast and the antenna elements. In this section, a comparative study of the results of theoretical analyses and actual measurements is conducted to determine whether the construction of the batwing antenna could be simplified by eliminating the element at its central part, without detracting from its favorable characteristics. A center‐fed full‐wave dipole antenna, located one quarter‐wavelength above a reflecting plate, has a very low potential relative to the plate, at the center of each half‐wave element. When metal support bars are attached between these centerpoints and the reflector plate, to support the antenna, the structure is called a “metal‐supported full‐wave dipole antenna.” An antenna made up of two such full‐wave dipole antennas has four half‐wavelength antenna elements, and has been named “Vierergruppe antenne.” This antenna and the Archterergruppe antenne were invented in Germany. Vierergruppe antenne (sometimes called “two‐dipole antennas”) can be rugged, because they are made entirely of metal and do not require insulator supports. They also have excellent electrical characteristics over a very wide frequency range, and are widely used by TV and FM broadcasting stations around the world. Since the full‐wave dipole antenna element is composed of a conductor having a diameter as large as th of the operating wavelength, it is unsuitable to use conventional thin‐conductor theory to analyze the antenna behavior. In particular, the current at the end of the thick, solid antenna element will be zero, as in the case of the thin one. Flat end‐face currents are determined by using a quasistatic approximation and the moment method of analysis. The excellent characteristics of this antenna have accordingly been found theoretically and fully clarified in the quarter‐century since its invention. The twin‐loop antenna, proposed by Dr. K. Endo, is used widely as a UHF‐band television‐transmitting antenna in Japan. This antenna has a wideband characteristic for horizontal polarization, and antennas with 2, 4, or 6 loops are used. A theoretical analysis was executed by Endo et al. for a sinusoidal current distribution, according to the EMF method, but no strict solution was obtained. For this section, theoretical calculations and experiments are executed with regard to the change in the radiation pattern with a change in the trap short‐circuit position at the top end, and with regard to the change in the impedance with a change in the antenna conductor diameter for 2L (twin‐loop) antennas, by use of the moment method. Consideration is given to wideband characteristics. Calculations according to the moment method are made for the case of an infinite reflector and for the case of a finite reflector, namely, the rear reflector of a 2L twin‐loop antenna consisting of a wire screen type above each other, and the results of comparative studies are given. The characteristics of the batwing radiator, which is the heart of the superturnstile antenna system for FM, VHF‐TV broadcasting, are broad bandwidth. We investigate the modified batwing antenna for UHF used as the digital terrestrial broadcasting. In this section, the characteristics of the 2,4‐element modified batwing antenna with the infinite reflector and with the wire screen for finite reflector are calculated by NEC‐Win Pro. We estimate these antennas from 470 to 770 MHz of the UHF‐TV broadcasting band in Japan.