U.H.F. Receivers

Author: J.B. Hoag

Superheterodyne receivers are used extensively for frequencies up to 60 MHz (λ = 5 m.) and occasionally for frequencies up to 100 MHz (3 m). The customary procedure at 60 MHz is to use two intermediate frequencies, as in Fig. 37 A.

Fig. 37 A. Block diagram of a 5-meter, double-superheterodyne receiver

A converter is employed to amplify the incoming signal and replace its carrier frequency with an " intermediate " value of say 10 MHz (30 m). This i.f.

is then fed into a regular short-wave receiver tuned to the 10 MHz. Of course, a special i.f. amplifier tuned at 10 MHz (or other suitable fixed value) may be used instead of the usual r.f. stages of a regular receiver. Since the f.m. and a.m. receivers differ only in the last i.f. and in the detector stage, the addition of a converter to the regular set is all that is required. The circuit of an excellent 56-MHz converter with an r.f. amplifier is shown in Fig. 37 B.

Fig. 37 B. Circuit of an u.h.f. amplifier and converter

Super-regenerative circuits are often used for the reception of frequencies from 100 MHz to 300 MHz (3 m to 1 m). A circuit using a self-quenched detector and two a.f. stages is shown in Fig. 37 C.

Fig. 37 C. Circuit of an u.h.f. super-regenerative receiver

At still higher frequencies, the circuits must be of the linear type and the tubes must be of special design. Figure 37 D shows a quarter-wave concentric line used as the grid-tuning circuit of a simple " acorn " tube detector.¹

Fig. 37 D. An ultra-high frequency receiver using a quarter-wave concentric line in the tuning circuit

1	Circuit constants and constructional details for receivers and transmitters like those of Figs. 37 B, C, D and E will be found in The Radio Amateur's Handbook and in The " Radio " Handbook.