Considerations in making low-loss measurements around one megacycle

One of the primary prerequisites of a good insulating material is that it have a low power factor. The best insulators in this respect are frequently difficult to evaluate accurately because, equipment conveniently available does not lend itself readily to low-loss measurements. The smallest power factors determined by most commercial bridges is hardly better than 10^-4, whereas materials such as polystyrene and teflon have power factors only two to three times this value. When resonance methods are used the smallest detectable power factor is inversely proportional to the Q. For a. Q of about 200 and the usual voltmeters and generators the sensitivity is also of the order of 10^-4. The precision can be improved by increasing the Q which in turn can be, accomplished by introducing negative resistance in the resonant circuit to compensate for the energy dissipated in the circuit. This idea is hot new. In fact radio engineers have for many years used regenerative circuits to improve the selectivity and gain of tuning circuits. Apparently Towsley* was the first to apply regeneration to dielectric measurements. The approach arid the circuit described in this paper are different and the apparatus is simpler.