Measuring methods for r.f. cables in the u.h.f. range

The paper deals with methods of measurement employed to ascertain the special characteristics of r.f. cables in the frequency range 470¿960Mc/s. In the first part, general methods are described for determining the mean characteristic impedance, attenuation and power-handling capacity, and dielectric strength and screening efficiency are also discussed. The second part of the paper deals with methods for equalising the electrical lengths of the two feeder cables in split-aerial systems. The choice of the measuring methods has been determined primarily by the consideration that nonuniformity of the feeders must not interfere with the accuracy of the results. Next, a method is described for the continuous measurement of the v.s.w.r. in the operating frequency range. A sufficiently low value of v.s.w.r. must be ensured, as otherwise inadmissible distortion would occur in the filter curve of the diplexer installed between transmitter and cable. In addition, a measuring method is discussed which operates with a pulse modulated on a carrier frequency. The halfpulse duration is approximately 0.1 ¿, equal to the duration of the image point. The carrier frequency is adjustable within the operating frequency range. Local reflections in the cable, which might distort the video signal, become apparent by this method. Finally, a method is presented for measuring the characteristics of cable connectors, and for compensating for any possible mismatches at the joints between cable and connectors. Results are given of the measurements obtained for feeder cables installed in the United Kingdom, using the methods referred to. These give some indication of the quality of the feeder cables obtainable at the present time for use in the u.h.f. range.