1-2. The Model 415C Standing Wave Ratio Meter is a transistorized instrument designed for measuring standing wave ratios and attenuation. The instrument is essentially a high-gain amplifier, tuned to an audio frequency, with a calibrated meter output. Specifications for this instrument are given in table 1-1. The Model 415C is calibrated to accept inputs from square-law detectors, but because of the high sensitivity and tuned amplifier it is also, useful as a null detector for audio frequency bridges. 1-3. Normally the signal input to the Model 415C is taken from either crystal (200-ohm or 200K-ohm) or bolometer (4.3 ma or 8.7 ma) detectors. Bolometer bias is adjustable by ±10% and circuit protection prevents damage to a barretter-type bolometer if connected or disconnected while bias current is being supplied. SWR and attenuation are measured with high accuracy and resolution by expanding selected segments of the normal scales to a full scale presentation. SWR may also be measured accurately in terms of db and then converted to a swr. 1-4. This instrument is capable of portable operation when equipped with a battery. A battery for this use is available initially as Option 1 and later as a$ stock item. The Model 415C has a variable tuned amplifier and an adjustable bandwidth (15 cps to 100 cps) to meet a variety of measurement conditions. An ac Amplifier Output is provided on the rear panel for using the Model 415C as a high-gain (140 db) tuned amplifier. A Recorder jack on the rear panel permits operation with a dc recorder.