Page 112 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Depending on the turns ratio of the antenna coil, the value of VR1 can affect Q.

Thus the last regenerative radio design shown will not affect the Q of the antenna
coil by not loading the secondary winding with resistance while allowing a range of
number of turns on the secondary winding for regeneration. When all the previous
designs (Figures 7-4 and 7-5) were analyzed and assessed for performance, there
seemed to be something missing. In Figure 7-4, one had to experiment in winding
a number of turns to hit the optimal spot in terms of RF gain before oscillation

broke out. In Figure 7-5, there was the potential problem of loading down the Q of
the antenna coil.
Certainly a higher-performance design could be made, but how? It occurred to me
that maybe I could separate the regeneration from the RF amplifier and have a
separate second-stage amplifier to increase the RF level. The block diagram for this

idea is shown in Figure 7-6.
By using a dedicated transistor Q3 for the positive-feedback circuit, the
regeneration can be controlled for a range of turns on the secondary of the
antenna coil, whereas a separate gain stage in Q2 can further amplify the signal for
even better sensitivity. This scheme worked pretty well considering that only a
2-inch-long ferrite antenna coil was used and the radio in the San Francisco Bay

Area picked up radio station KNX (1,070 kHz) from Los Angeles.
Parts List

• Cl, C3, C9, Cl0: 1 IJF, 35 volts
• C2: 7 pF or 6.8 pF
• C4, CS, C6, ell, C12: 0.01 IJF
• C7: 0.15 IJF

• cs: 0.0015 IJF
• R1: 56 k

• R2: 1 M

• R3, R7, R14: 1 k

• R4, R8: 4,700

• RS, R9, R12: 100 k

• R6: 2,200

• R10, R13: 10 k

• R11: 20 k