A common method of measuring characteristics of waveguides for long-distance communication involves the use of millimicrosecond pulses in a multiple-reflection test; test waveguides of a few hundred feet in length are employed. The enormous bandwidth, typically 40 Gc/s, which must be covered constitutes a major difficulty because several thousand measurements are necessary if the guide is to be adequately tested. A programme of measurements would then take months to complete, so that not only daily, but seasonal, weather effects would have to be considered. In order to make complete tests possible, novel equipment capable of rapid graphical presentation has been developed.The construction of a swept-frequency receiver employing a backward-wave oscillator (b.w.o.) as both signal source and local oscillator is described. A square wave applied to the frequency-controlling electrode switches the b.w.o. into two states: signal frequency (100 ns duration) and local oscillator (1 ms duration). A graphical presentation is obtained by sweeping the b.w.o. slowly through its tuning range. Errors resulting from the inherent b.w.o. power variation with frequency are avoided by comparing the magnitude of the pulses, derived from multiple reflection in the guide under test, on a ratio basis. Pulses to be compared are gated into separate channels where they are converted into a suitable form for operating a commercial ratio meter. The output of the latter feeds a pen recorder. Facilities for providing frequency markers are incorporated. The apparatus provides a complete record of the frequency characteristics of the guide in about half-an-hour.