RADIATION AND PROPAGATION

General Antenna Properties

All antennas have certain general properties which apply both to receiving and transmitting modes. Thus, the more efficient the antenna is for transmitting, the more effective it is for receiving. Directive properties will be the same for transmission as for reception and, in the , case of directive antennas, the gain will be the same on high-frequency ommunications,gt shouldlbe noted, the  `. ·i often observed odd behavior and seeming perversity of antennas which often occurs, is due to the fact that the I waves may not take exactly the same paths through the ionosphere when going in opposite directions, the two waves utilizing different portions of the directive pattern i :. of the antenna. Even so, the concept of reciprocity be- tween transmission and reception still stands correct.

Antenna Resonance The strength of the radio wave radiated by an antenna

I .. depends on antenna size and the amount of current flowing in it. It is reasonable to expect the largest amount of current that can be achieved from the power available will provide the best radiation from a given antenna. The greatest amount of current flows when ii the reactance of the antenna is cancelled and the an- tenna made resonant at the operating frequency. The i shortest conductor that will be self-resonant at a given

frequency is one that is about half as long as the size of the radio wave. The half-wavelength antenna is used as a basis for all antenna theory and is a fundamental building block in antenna design (Figure 7). The electrical length of a half-wave of electromagnetic energy is related to the speed of travel of the wave (the same velocity as the speed of light) and also to the fre- quency of the wave by an equation that is similar to equations dealing with other waves (such as waves in the ocean, or the vibrations of a piano string). In the case of a radio wave in free space, the metric formula is:

Half wavelength (meters) = 150,000,000

----------------
Frequency in Hz

_ 150
------- Frequency in MHz

Figure 6. Reflection of the Electromagnetic Wave from a Conducting Surface
When an electromagnetic wave is re· flected from a conducting surface the electric field is reflected with reversed sign (A) so that the eledric field at the refled- ing surface is zero. The magnetic field is reflected with unchanging sign and is so doubled at the reflecting surface (B). The resulting wave in each case is the sum of the two traveling waves and oscillates in magnitude, but is fixed in space. It is termed a "standing wave."

The formula in the English system is: Half wavelength (ft) = 492
------- Frequency in MHz
The physical length of an antenna element varies slightly from this fundamental electrical length because the element has thickness and is affected by nearby objects. Information will be presented in a later section defining this relationship in practical terms.

Radiation Resistance and Reactance
When rf power is applied to an antenna, it is radiated into space, the antenna acting as a load, or sink, for the transmitter. In order to establish a frame of reference, the power dissipated in a dummy load (such as a resis- tor) may be compared in terms of voltage and current with the power radiated by a real antenna. This refer- ence frame is defined in terms of the radiation resis- tance of the antenna. Simply stated, the radiation resistance of an antenna is that imaginary resistance exhibited which seems to dissipate the power the an- tenna actually radiates into space. Radiation resistance is expressed in ohms and is normally measured at a point in the antenna which has the maximum value of current flowing in it. A more general term used in this connection is antenna impedance which, in addition to implying radiation resistance, also implies the presence of reactance in the antenna circuit. In addition to radiation resistance, practical antennas also exhibit loss resistance which is energy dissipated in heat loss in the antenna element and nearby dielectrics. The total resistance of the antenna, which is the sum of these two figures, is often referred to as feedpoint resis- tance, although in popular usage the term "radiation resistance" usually encompasses the two separate entities. The radiation resistance and resonant frequency of an antenna depend on the antenna size with respect to the radio wave and the proximity of the antenna to nearby objects which either absorb or reradiate power, such as the ground, or other antennas or conductors. The length-to-diameter ratio of the antenna also affects the radiation resistance; as the antenna becomes thicker with respect to the length, the radiation resistance de- creases (Figure 8).

REFLECTING SURFACE

SHOWING HOW STANDING WAVES Dl IST ON A HOR IlONTAL ANTENNA CURRENTIS MAXIMUM ATCENTER VOLTAGE _ VOLTACE IS MAX IMUM AT ENDS Figure 7. The Resonant Antenna The greatest amount of current flows in the antenna when it is resonant. The shortest condudor that is self- resonant at a given frequency is one that is about a half- wavelength long. The refledion pattern on the antenna creates a standing wave of both voltage and current. The half-wave, center-fed antenna is often called a "doublet."

Figure 8. Length-to-Diameter Ratio of Antenna Affects Radiation Resistance As the antenna becomes thicker with resped to length, the radiation resistance decreases and the antenna must be shortened to re-establish resonance. This chart illus- trates the amount of shortening required with a reso- nant h wavelength antenna in the frequency range of 2 MHz to 30 MHz.

The feedpoint resistance of a resonant antenna is the load for the transmitter and its value is important in detennining the method used to couple the two to- BettLer,

Antenna Impedance
HeCause the power at any point in an antenna is the ame at any other point, the impedance at any point vong the antenna expresses the ratio between voltage nd current at that point (Figure 7). Thus, the lowest impedance occurs where the current is highest and the impedance rises uniformly toward the ends of the an- tenna, where it can reach a value as high as 10,000 ohms for a thin dipole remote from ground (Figure 9). Like a tank circuit, an antenna may exhibit reactance at the feedpoint. Since the antenna, by definition, is nonreactive at resonance, antenna reactance implies a ttate of nonresonance. Antenna reactance rises rapidly Figure 9. Impedance of Antenna Varies Along the Length and Expresses the Ratio Between Voltage and Current at any Point on the Antenna

The feedpoint resistance of a center-fed antenna is a fundion of the physical length. For example, a half-wave antenna has a center feedpoint resistance of about 73 ohms, while an antenna one wavelength long has a center feedpoint resistance of 1000 ohms to 9500 ohms (depending on the diameter of the element). As the length of the radiator increases, the impedance excur- sions become less drastic, especially for "fat" radiators.

off-resonance and the manner in which the reactive component varies is illustrated in Figure 10. The rate- of-change of the reactance increases as the antenna tength departs from resonance and also increases as the length-to-diameter ratio decreases. The reactive compo- nent of an antenna is zero when the overall antenna length is slightly less than a multiple of quarter- wavelengths long. Near resonance, the resistance and reactance terms of an antenna vary much in the manner shown in Figure 11. Both feedpoint resistance and reactance change more slowly with frequency for a fixed radiator length with "fat" elements than with "thin" elements, indicating that the effective antenna Q is lowered as element diam- eter increases. Lower Q is desirable, because it permits the use of a radiator over a wide frequency range with- out resorting to means of eliminating the reactive com- ponent. If the antenna Q is low enough, the radiator is termed a broadband antenna. The curves in Figure 12 indicate the theoretical feed- point resistanee of a dipole antenna for various heights above a perfect ground plane. In free space, the feed- point resistance of a thin dipole is approximately 73 ohms. The modifying effects of the ground change this nominal value as shown, with the value approaching 73

HEIGHT IN WAVELENGTHS OF CENTER OF VERTIGL HALF-WAVE ANTENNA ABOVE PERFECT GROUND

Figure 12. Feedpoint Resistance of Dipole Suspended Above a Perfect Cround

Figure 10. Reactive Component at Feedpoint of Center-fed Antenna Feedpoint reactance rises rapidly when antenna is in nonresonant condition and also increases as the length- to-diameter ratio of the antenna decreases. "Fat" anten- nas exhibit less reactance than "thin" ones. Reactance varies rapidly for center-fed antenna one wavelength long.

Figure 11. Feedpoint Resistance and Reactance as Function of Antenna Length Near resonance, the resistance and reactan: e of a dipole antenna vary in this typica) manner. Reac'ance is zero when the antenna is slightly less than  wavelength long. The reactance changes more rapid'y for "thin" antennas than for "fat" ones.

ohms as tlTe dipole is removed from iie ground by more than a wavefength.

In free space the feedpoint resistance of a half-wave dipole is about 73 ohms. The modifying effeds of the ground change this, as shown above, with the value approaching 73 ohms as the dipole is far removed from the ground. The ground has less effect on the feedpoint impedance of a vertical antenna.

Antenna Directivity Because of the manner in which current ffows in an antenna, radiation from practical antennas is not uni- form, but is directive to a certain degree. The amount of directivity can be altered or enhanced through the use of extra radiating elements, reflecting planes or curved surfaces; or, in the microwave portion of the radio spectrum, by the use of electromagnetic horns, lenses, and slotted devices. The directive pattern of an antenna may also be mod- ified by wave reflection from the ground or from nearby objects. Structures which lie within a few wavelengths of the antenna have the greatest influence on the directivity of the antenna. The change in direc- tivity is caused by the ability of the nearby conducting structure to reradiate energy emitted by the antenna. This reradiation may either reinforce or cancel the di- rect radiation of energy from the antenna, thus produc- ing a distortion of the f-ree-space pattern of the antenna (Figure 13). By using properly adjusted conducting ob- jects (called driven elenenls, refleclors, or direc'tors) the antenna radiation pattern may be deliberately dis- torted to produce an enhanced signaf in a desired direc- tion (Figure 14). 'The signaf gain varies with the adjustment and spacing of' the various elenents and the radiation resistance of tlte parent antenna, as welf as its tuning, is affected as welf.

The Isotropic Radiator Directivity of an antenna is tlte ability ol tlTe antcnna to concentrate radiation in a particullr direction. Afl prac- tical antennas exhibit somc degree ol' direrlivity. A completely nondirectional antenll;l (one wltich radiates

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