Bandwidth of a Radiator

Author: Edmund A. Laport

In modern design, cognizance is taken of the bandwidth of emission characteristic of the type of emission to be radiated from the antenna. Both resistance and reactance of an antenna vary with frequency, and it is desirable that the system have the least practicable variation in impedance between the center frequency and the maximum and minimum side frequencies. In general, the bandwidth capabilities of a radiator are increased (for a given amount of impedance variation) as the cross section of the radiator is increased. Thin wire antennas have much greater selectivity than steel tower radiators of substantial cross section. When the desired cross section is greater than is structurally or economically desirable for a given bandwidth to be transmitted, an outrigger at the top of the tower can be used to support vertical wires or cables some distance away from the supporting center tower, as in Fig. 2.14.

Experience has shown that not enough attention was given to bandwidth in many existing systems. Bandwidth should be one of the primary considerations in planning any type of radiating system.48 Whenever the bandwidth exceeds 1 percent of the carrier frequency, special design considerations are certainly involved. In choosing the final design to accommodate a specified bandwidth, there are no absolute criteria, other than the designer's good judgment, to determine when a satisfactory set of parameters has been found.

In any applications involving extreme bandwidths, care must be taken to avoid selectivity in the feeder coupling network. The smaller the amount of stored energy in all the reactive elements of a coupling network, the smaller its intrinsic selectivity.

Minimum selectivity is afforded by using a feeder having a characteristic impedance equal to the actual working resistance of the antenna system at the center frequency of the transmission band and then using a simple series reactance to cancel the antenna reactance at this center frequency. Occasionally it may be found impractical to provide transmission lines of the correct characteristic impedance for direct resistance match to an antenna. With coaxial feeders it is practical to obtain characteristic impedances of 15 to 75 ohms, using paralleled lines if necessary. With open-wire lines of the single-end (unbalanced) type, characteristic impedances from 150 to 350 ohms are easily obtained, and by special measures, including paralleled lines, the gap between 75 and 150 ohms can be closed, using open-wire lines. A total resistance range of 15 to 350 ohms can therefore be accommodated for direct resistance match between feeder and antenna if required.

When coupling networks are used between feeder and antenna and between transmitter and feeder, but where precautions are desirable to avoid unnecessary selectivity, conservative networks of minimum total stored energy should be chosen and the changes in impedance levels at coupling points minimized as much as practicable. Standing waves on a transmission line also represent stored energy and add to the selectivity of the system. Feeders of tapered characteristic impedance may be used instead of networks to effect moderate impedance changes. These matters are treated in Chap. 4.

The bandwidth of a series-resonant circuit is usually defined as the frequency band between the limits where the input impedance has equal resistance and reactance components. At these two limits, above and below the resonant frequency, the phase angle of the impedance is 45 degrees, and the circuit response with a zero-impedance generator is 3 decibels below that at resonance. An antenna circuit may be viewed as a series-resonant circuit in the same way, even though the resistance as well as the reactance changes with frequency.

In broadcasting, it is customary to allow only about 1 decibel attenuation for the side frequencies produced by the highest modulating frequency. The phase angle of the antenna impedance for these side frequencies to maintain 1 decibel response in the antenna must not exceed 27.5 degrees. For any other response limit in decibels, the maximum phase angle of the impedance φ is found from