Radio and Television Today (Jan-Nov 1941)

DIPOLE ANTENNAS i FUNDAMENTALS FOR THE SERVICEMAN The increased uses of high frequencies for broadcasting services are bringing new equipment and service methods to the serviceman. One particularly important field for the serviceman is dipole antennas which are being used in frequency modulation, television, and short-wave reception. This efficient type of antenna is practical at the high frequencies used for these services, while it would be impractical at broadcast band frequencies. Dipole antennas are balanced to ground electrically and depend upon the magnetic component of the radia 1 J i i i — i" ■ — i — ■144* -ELECTROSTATIC / FORCE / 'fiti? 4+ff /antenna / / : / w± / / -Mil 1 1 i i , /ELECTRO /----MAGNETIC / FORCE / OIRECTION OF I / TRANSMISSION. Fig. 1 — Approximate space relation of dipole antenna and the electrostatic and electromagnetic forces. Actual wave front is a curved surface. Vertical lines are electromagnetic. tion field for the voltage pick-up. The common single wire type of antenna, including the "hank" on the AC/DC sets, depends primarily on the electrostatic component of the radiation field for pick-up. That is, the single wire type of antenna acts as one plate of a capacitor, the other is the earth, and the electric charge stored in this capacitor by the electro-static radiation field, flows through the antenna input coil to introduce the signal into the receiver. RADIATION FIELDS The dipole in a horizontal position has theoretically equal capacities between each arm and ground, therefore, very little difference in charge exists between the two halves of the antenna due to the electro-static component. What are these electrostatic and electromagnetic components which have been mentioned? The current that is sent into the radiating antenna at the transmitter is alternating at the assigned frequency. The current may be amplitude modulated, or it may be constant in the case of frequency modulation. This current flowing in the antenna sets up an electromagnetic field in concentric circles with the antenna as an axis. This is similar to the magnetic field set-up about any conductor carrying a current. At low frequencies, that is below approximately 10,000 cycles, the energy stored in the magnetic field about a conductor is returned to the circuit. At frequencies above this however, less and less of the energy is returned from the magnetic field, and this energy is said to be radiated. About 90 per cent of the energy supplied to a good high frequency antenna is radiated. STATIC COMPONENT The electrostatic field about the antenna is representative of the electric force in the dielectric or air surrounding the antenna. This electrostatic force is similar to that which exists between two plates of a condenser which have a difference of potential between them. This electric field exists only in a dielectric or non-conductor and therefore can be made ineffective by shielding. This is done on some broadcast loops by surrounding the loop with a grounded copper screen. Only the magnetic component reaches the loop to produce a voltage. Both the electromagnetic and the electrostatic field travel together at approximately the speed of light, 300,000,000 meters per second, or about 186,000 miles per second. The direction of travel of the wave front is perpendicular to the electrostatic and electromagnetic lines of force which are in turn perpendicular to each other. This can be seen in Fig. 1. Reversal of either the static or magnetic lines reverses the direction of transmission, reversal of both does not alter the direction of travel. The polarization of the wave is that of the electrostatic field which is parallel to the antenna. Thus, horizontal antennas send horizontally polarized static lines and the wave is said to be horizontally polarized. The reverse is of course true for vertical antennas. PLACING THE ANTENNA Since the dipole antenna receives its voltage from the electromagnetic field, it must be placed so that its axis is perpendicular to this component in order that the advancing wave is "cut" by the receiving antenna. This process of "cutting" magnetic lines of force is similar to the action of a generator where the conductors are whirled through a magnetic field. Here, the magnetic lines move past the conductor, or antenna, with the same results. Referring to Fig. 1, it will be seen that the receiving antenna must be parallel with the transmitting antenna in order to be cut by the magnetic lines of force. If the transmitting antenna is vertical, the magnetic lines are in a horizontal plane, and the receiving antenna must be vertical also. This is one of the polarization prob lems which must be solved by standardization of broadcasters to one method. The FM stations have chosen horizontal polarization as the standard. The stations which are now operating vertically polarized are to be changed to conform with the standard. Polarization of the wave at the receiver is not determined solely by the position of the transmitting antenna with respect to the earth. It may change once or several times during its travel. Splitting and re-combining of a wave, reflection and refraction, all tend to change the polarization. The receiving dipole should in every case be adjusted to give the best possible signal from all stations in the receiving area. DIPOLE CHARACTERISTICS The design of the dipole receiving antenna will in many cases be no problem to the serviceman. The receiving range of nearly every set can be increased, however, with the use of a more elaborate antenna. In many localities where FM stations cannot be heard with ordinary dipole antennas, good reception can be obtained if reflectors are added to the installations and attention is paid to impedance matching, elevation, and orientation. An example of how effective this can be, is shown in the success of S. Harold Bent, the Gardner, Mass. dealer whose story was given in October Radio Today, page 34. Dipole antennas are designed to resonate within the band of frequencies that are to be covered. That is, the antenna is cut to approximately one Fig. 2 — Dimensions for FM antennas in 42-50 mc. band. A is plain dipole, and B is so-called extended zepp. B with a reflector is many times as efficient as A. Note matching stub. half the wave length of the center of the band. The formula for figuring half 468 wave antenna lengths Is L = • f(mc) For the center of the FM band, 46 mc, the length is 10.18 feet. Each quarter (.Continued on page 49) 44 RADIO TODAY