Transmission Bandwidth

Author: Edmund A. Laport

Antenna selectivity is usually the limiting selectivity factor in a low-frequency transmission system. Transmitter, receiver, and other selective circuits associated with the system should be examined, however, in all applications where special attention to bandwidth is required. Selectivity limits transmission speed and contributes to telegraph distortion because, by filtering the side frequencies inherent in the modulated signal and shifting their relative phases, the signal loses definition. In the case of telegraphic on-off keying (an example of 100 percent amplitude modulation with a square wave) a long series of symmetrical side frequencies, all in correct phase and amplitude relationships, is necessary for distortionless transmission. As the dot length is decreased and the dot frequency increased, the spectrum of the symmetrical side frequencies must be greater. If the spectrum width exceeds the bandwidth of the system, the higher side frequencies are lost or so modified in phase and amplitude that the carrier envelope is seriously distorted. Beyond a certain point, it may be impossible to regenerate the signal envelope at the receiver.

In the case of teleprinter operation, there are strict limits on telegraph distortion for correct printing. In speech transmission, inadequate bandwidth capability will seriously restrict the frequency range that can be transmitted. In pulse transmission, antenna selectivity may produce serious pulse distortion, making it difficult or impossible to match two separate pulses at the receiver because of lack of definition at the leading edges where two separate pulses are compared. Therefore, in all but the most elementary applications, special attention to the bandwidth capabilities of the antenna (both transmitting and receiving) must be examined critically during design.

A corollary condition of high selectivity is accuracy of tuning. Any mistuning causes asymmetry of side-frequency transmission with resultant distortion. Unless the bandwidth of the system is considerably greater than required, there is little or no tolerance in the tuning.

An electrically short antenna has a reactance very large with respect to its total resistance, which permits us to use the ratio X/R as the Q of the antenna. Such an antenna, when tuned for operation by means of series inductance, forms a series-resonant circuit. The bandwidth of the antenna can therefore be defined in the usual way as where f is the frequency of resonance and BW is the total bandwidth, in cycles, between the upper and lower points of 3-decibel attenuation (45-degree phase angle.)

It is seen that the bandwidth is inversely proportional to antenna (or total circuit) Q. To decrease Q, the same design considerations are required as for the reduction of antenna potential. Therefore the same measures may be required in the design of a low-power system for large bandwidths as for a high-power system for narrow bandwidths, with the exception of the insulation.

To transmit the fundamental and third-harmonic sidefrequencies of a square pulse of dot frequency F, the bandwidth required is 7.4F. Therefore, the maximum permissible Q of the antenna is

If it is desired to operate a standard five-unit start-stop teleprinter (60-word speed of 23 dot cycles per second with equal mark and space intervals) at 40 kilocycles, the Q of the antenna must not exceed 235. Furthermore, at this value, the tuning must be exact at all times, or excessive telegraph distortion would result and impair the accuracy of operation. To be more precise, the entire selectivity in the transmitter and receiver systems together should not exceed an equivalent total Q of this value. The transmitting antenna should therefore have a Q less than this value in considering system performance and allowing for tuning tolerance to accommodate inevitable variations in antenna capacitance with weather.

One method of increasing bandwidth is to use an antenna of very large cross section. A cage or a tower is more effective in this respect than a single thin wire. A number of towers or vertical conductors with large separation and operated in parallel, either by individual feed or by multiple tuning, is a further step in this direction and one which is quite practical. In addition to the advantages of improving ground-current distribution and transforming the feed-point impedance to more convenient values, multiple tuning is a desirable method for increasing the bandwidth of the system. The disposition and the number of vertical leads in the system can simulate flat or circular current sheets having intrinsically much larger bandwidth characteristics than conventional single-tuned antennas.