Total posts : 45366
I rewired the output of my Wenzel circuit to incorporate the SSTRAN test circuit I proposed in my last post. I did not mention in my post that the test circuit requires a DC blocking capacitor. I used a .01 uF ceramic. The SSTRAN connected to an actual antenna does not require such a capacitor because the antenna is a capacitor.
I first used a 46 ohm load resistor in series with a 22 uH inductor.
As was stated in my previous post, the 22 uH inductor represents antenna loading coil inductance in excess of what is required to tune the antenna to resonance. This excess inductance forms the inductor of a capacitor-input L network for transforming the load resistance (46 ohms, in this case) to the optimum load resistance at the collector of the final transmitter stage. My efficiency measurement was only 27%. This is nearly twice as good as the efficiency I obtained with my version of the original Wenzel circuit, but this is considerably less than 63.7%, which is the theoretical maximum efficiency.
(Neil pointed out that, since the Wenzel is a class A amplifier, its theoretical maximum efficiency should be only 50%. The Wenzel final stage, however, is overdriven, causing the current waveform to be a square wave, and this causes the theoretical maximum efficiency to increase.)
The amplitude of the current waveform is constant with loading (nominally 20 mA p-p.) This is because the final stage of the Wenzel transmitter is approximately a current source. However, the amplitude of the voltage waveform varies directly with the load resistance seen by the collector. When I obtained only 27% efficency, the amplitude of the voltage waveform at the collector was only about 60% of the optimum value, which is nominally 20 V p-p, since the nominal DC voltage across the output stage is 10 VDC. I found that increasing the test inductance increased efficiency. The best efficiency I was able to get was 41%, which I obtained by using a 30 uH inductor. I was not able to get the full 20 V p-p at the collector output because clipping of the collector waveform started at around 18 V p-p. If the full 20 V p-p range were available, the efficiency would have been higher.
In my test of the original version of the Wenzel, I observed something that looked like parasitic oscillation, but was actually an enhancement of the 10th harmonic of the transmitter at exactly 15 MHz. In this circuit, I observed something similar: an enhancement of the 34th harmonic at exactly 51 MHz. If such enhanced harmonics occur, they have to be suppressed, because the short Part 15 AM antenna probably works a lot better at the harmonic frequency than at the fundamental.
My efficiency calculations are based only on the DC input, and do not include the appreciable drive power to the emitter of the common-base final stage. In a common-grid tube amplifier, about 10% of the drive power feeds through to the output. In the Wenzel, it is much less than that. This is because transistors have a lot more transconductance than tubes. Nevertheless, please be aware that the way Section 15.219(a) reads, drive power is included in the 100 mW input power that is allowed. The rule excludes filament power, but nothing else. There has been a lot of discussion about Section 15.219(b) lately, but Section 15.219(a) has to be considered, also.