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The Wenzel Part 15 AM transmitter circuit has been discussed extensively in this thread, and it seems to be a very popular circuit for homebrewing. Links to the Wenzel design are on a couple of the previous posts in this thread. I decided to construct the Wenzel as closely as I could to the original design, while using only parts I had on hand, and then report my observations to the Forum.
My overall impression of the Wenzel is that the circuit is very stable, and there is no problem in getting it to work. The modulation waveform looks very nice (no distortion). However, the design is a lot less efficient than I expected it to be. I expected the efficiency to be similar to what Neil reported for the SSTRAN, around 30%, but I got only 14%, using a 30 pf capacitor in series with a 46 ohm resistor as a dummy antenna at 1.5 MHz. The efficiency became lower as the resistor value in the dummy antenna was reduced. 30 pf is a typical capacitance for a 3 meter rod antenna above ground. 46 ohms is a typical ground resistance. The radiation resistance is orders of magnitude less than the ground resistance, so the efficiency of an actual antenna would be very low.
I built the second Wenzel circuit in the Wenzel link (Wenzel 2), but I at first used the output circuit for the first Wenzel circuit (Wenzel 1), just to see how well the circuit worked. I connected a few feet of wire to the output, not nearly 3 meters long, and I was able to hear the signal anywhere in my house. Of course, the radiated power had to be very low. Then, I increased the output power by connecting an autotransformer and tuning capacitor to the output, according to Wenzel’s design. I did not have an 1811 potcore, as specified by Wenzel, so I used an Indiana General F1707-1-Q2 3.5 ” OD ferrite toroid with u = 40 and AL = 57. I don’t think this toroid is available any more, but other similar toroids are made by other manufacturers. I used a 14/1 turns ratio, as is used in the potcore design. The secondary winding of the transformer is the loading coil, so no external loading coil is used.
The efficiency would have been even lower if the potcore specified for the design had been used. Potcore ferrite, is, in general lossy at RF frequencies. The low-permeability toroid I used has fairly low loss. I measured the Q of the secondary winding to be 265 at 1.5 MHz. Wenzel did not give enough technical details about his recommended potcore. The number, “1811” refers only to the potcocore dimensions, but not the potcore material. The highest frequency material available for the 1811 size is Ferroxcube 3F3, which is specified to 700 kHz max. This material will not give a high Q at the upper end of the AM broadcast band, and the efficiency with the potcore has to be even less than the low efficiency that I obtained. One advantage of the high loss potcore design is that harmonics are suppressed. In my test circuit, I observed what appeared to be a parasitic oscillation. This was surprising, because, a common-base amplifier is supposed to be relatively free of parasitics. This “oscillation” stopped when I removed the 1.5 MHz drive to the final amplifier. After further examination, I found that I did not have a true oscillation, but an enhanced 10th harmonic at exactly 15 MHz. If the lossy potcore specified by Wenzel were used, there would have been no harmonic at 15 MHz.
The principal cause of the low efficiency I observed in my Wenzel circuit is a mismatch between the final stage of the transmitter and the antenna resistance. The optimum load resistance of the final stage is about 1 k ohm. The function of the transformer is to match 270 k ohms, which is the equivalent parallel resistance of the 46 ohm resistor in series with the 30 pF capacitor at 1.5 MHz, to 1 k ohm. The input resistance of the transformer was about a third of what was required, and this was the cause of most of the inefficiency. There is really no way to adjust the impedance matching of the transformer, except by rewinding the transformer to a different turns ratio.
The final stage of the Wenzel circut has a 360 degree conduction angle, so it is definitely not a class C amlifier. It is class A. The voltage waveform at the output is sinusoidal, but the current waveform is roughly a square wave. The maximum theoretical efficiency is 63.7 %.
Since the Wenzel has a common base output amplifier, it receives a large amount of drive power. A small amount of this drive power goes to the output. The input drive power to my test circuit was about 16 mW. According to Part 15.219(a), this drive power should be included in the 100 mW input power allowed. Wenzel included only the DC input to the final stage, not the drive power.