Measurement of radio frequency cable

Measurement of attenuation at 400 MC is required in the JAN Specification for R. F. Cables. The Naval Research Laboratories developed´ a test set for this purpose which is undoubtedly familiar to many companies manufacturing cable. A schematic diagram of this test set is shown. The theory and operation of the set is as follows: The generator and its coupling loop can be replaced by the series resonant circuit shown. It is well known that the maximum power that can be delivered by this series resonant circuit to any external circuit is E²/4R. The detector is first coupled directly to the generator with the test cable out, pf the way. The coupling is critically adjusted and all circuits are tuned so that the detector receives the maximum possible power from the generator. The detector reading is noted. Coupling loops tuned to 400 MC are now soldered to the ends of the test cable and these are coupled to the generator and detector, as shown. The generator and detector are of course separated by some substantial distance at this point, so that there is no direct coupling. If all circuits are properly tuned, and if all couplings are properly adjusted, the detector will receive the maximum possible, power from the generator minus the power absorbed by the test cable. Also, the cable will be terminated at each end and in its characteristic impedance Zo. so that no standing waves exist in the cable.: Hence, the decrease in the riding of, the detector meter is a measure, of the attenuation in the test cable, and if the detector meter is calibrated, the attenuation of the cable can be readily computed.