A transmission enhancement system is used for strengthening low frequency cable locating signals on the metallic shields of long underground cables. The system includes equalization circuits on the cable armour or shield at regular intervals, for example at splice locations. The equalization circuit includes an inductor that significantly reduces the rate of attenuation of the low frequency tone signal and greatly enhances the strength of the signal at distances where the signal would otherwise be very weak or unusable. The equalization circuit may be a resonant circuit having a capacitor connected in parallel with the inductor. The capacitor compensates for the inductor's attenuation of high frequency tones for local location purposes. The preferred embodiments of the equalization circuit include a surge arrestor coupled in parallel with the inductor and capacitor.