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I had previously seen the second of Wenzel’s circuits (Wenzel 2), shown in Neil’s link, in “The Low Power AM Broadcasters Handbook,” page 61. I had downloaded this handbook from the part15.us web site some time ago. Wenzel 2 is a very efficient circuit, primarily because of the autotransformer in the potcore that greatly increases the impedance of the transmitter output stage. This impedance transformation significantly improves the efficiency of the transmitter.
Wenzel 2 uses the antenna as a high-resistance load. The autotransformer inductance is in parallel with the antenna capacitance. The antenna parallel-equivalent antenna resistance, which is high, is the load. For best efficiency, the tuning capacitor connected to the antenna input should be as low as possible.
Wenzel 2, quite properly, drives the antenna from the collector of the final stage. A high resistance load should be driven from the collector.
The first Wenzel circuit in Neil’s link (Wenzel 1), which Neil said is similar to the SSTRAN, is quite different from Wenzel 2, and is a much less efficient circuit. It’s really unfortunate that the SSTRAN design is based on Wenzel 1 rather than Wenzel 2.
Wenzel 1 uses the antenna as a low resistance load. A loading coil in series with the antenna tunes out the series-equivalent antenna capacitance, which leaves the series-equivalent resistance, which is low. This low resistance is still driven by the collector, and it would be better if it were driven by the emitter. The Panaxis AM 100, in contrast, uses an emitter follower for driving the antenna. Wenzel 1 has a resistor connected to the collector of the final stage that absorbs some of the RF power, but that is not the primary cause of the inefficiency. The inefficiency is caused mostly by an improper driving impedance of the transmitter.
Wenzel knew exactly what he was doing. Wenzel 1 was designed for ease of construction and Wenzel 2 was designed for high efficiency.