Transmission-line Design for Wide-frequency Band

Author: Edmund A. Laport

A device such as a transmission line with standing waves, like a lumped reactive element, stores electrical energy. As the amount of energy stored in a system becomes large with respect to the amount of energy transmitted per second, the selectivity of the system is increased. The selectivity of a system limits the bandwidth that can be transmitted without distortion.

There are many common applications where special attention is required to design a system for the band of radio frequencies that must be accommodated. Where the total spectrum of an emission (including upper and lower sidebands) is less than 1 percent of the carrier frequency, usually no special attention to bandwidth is necessary in ordinary antenna systems and for ordinary services. When the bandwidth surpasses 1 percent. the designer should always examine the selectivity situation and be prepared to undertake special provisions for circuit design for the bandwidth required.

At all frequencies a transmission line is an aperiodic system when correctly terminated in its characteristic impedance. When the load impedance is frequency-selective, the line may be perfectly terminated at one frequency and improperly terminated at other frequencies.

An antenna system, when properly designed for adequate bandwidth, will provide a wide-band termination for the line.51 In this case, the terminated line itself does not add to the selectivity of the system.

The most important first step in system design is to design the antenna for adequate bandwidth whenever possible. When this cannot be fully achieved, it is sometimes possible to employ wide-band coupling circuits between the antenna and the line, which function as impedance equalizers.

A system involving antenna and feeder has its maximum intrinsic bandwidth when their respective impedances are equal and therefore self-matching (see Fig. 4.29). When this is possible, there is no need for

FIG. 4.29. Self-matching antenna and feeder, in which the bandwidth is limited only by the antenna.

reactive coupling elements and the stored energy in the system is minimized. If an antenna has an impedance that is resistive at the operating frequency (or at the middle of the emission band in asymmetric sideband emissions), the feeder should be designed to have a characteristic impedance equal to the input resistance of the antenna.

When the antenna impedance at the feed point is complex, the line impedance can be made equal to the resistive component of the antenna impedance. Then a simple series reactance can be used to neutralize the reactive component and so correctly terminate the line. When the antenna input resistance is lower than realizable characteristic impedances for reasonable transmission lines, the antenna input impedance can be transformed upward by using a single parallel reactance of proper sign to tune the load impedance to parallel resonance. The resistance of the load circuit may then be brought to a value that will match the line.

In certain cases, it may be possible to design the antenna for an impedance that will fit a particular feeder impedance. In other cases, the antenna impedance may not be controllable, and then it is necessary to design the feeder for a particular characteristic impedance to fit the antenna conditions.

In wide-band systems, therefore, the feeder should always be correctly

matched in impedance over the emission band. It is always desirable to use a value of characteristic impedance equal to that of the mid-frequency load resistance after applying the simplest possible power-factor correction. For wide-band receiving systems, there is the additional requirement to provide a wide-band termination for the feeder on the end opposite to the antenna. The receiver input impedance must therefore have a value over the band equal to the characteristic impedance of the feeder.

Bandwidth becomes a problem in high-speed signaling and broadcasting on low and medium frequencies, low-frequency loran, and certain navigational transmissions. Occasionally a bandwidth problem arises in the high-frequency band, such as the transmission and reception of very-highspeed facsimile or multiplexed telephony.