Schematic
The 1S5 tube functioned as a regenerative detector. Audio detected by the 1S5 was resistance-coupled to the 3V4, where it was amplified and supplied to the earphone.
The detector was provided with a cathode feedback level well into the oscillation range by the 330 pF capacitor. The received carrier blocked the oscillations, allowing strong local stations to be received clearly. In addition, the loop antenna was part of the resonant tuning circuit, resulting in near-unity coupling between the antenna and the detector, which helped provide a high enough level of carrier for the blocking function. A regenerative detector operated in this mode is sometimes called a superregenerative detector, but it should be noted that in this circuit there was no separate quenching oscillator and the blocking signal was ideally at the same frequency as the oscillation, as opposed to the usually lower frequency employed in a true supperregenerative detector.
The regenerative detector in the radio hat had adequate sensitivity to receive stations much more distant than the stipulated twenty-mile range, but distant stations would not have had a strong enough carrier to block the oscillations and so would be received with an objectionable heterodyne, audible as an astable squealing noise. Furthermore, the loop antenna was somewhat directional. This was a limitation for a portable radio; the signal level could vary when the listener turned their head. If the target station was accidentally nulled, the carrier signal could fall below blocking level, resulting in an annoying squealing heterodyne similar to that present on stations outside the normal range of the radio.
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