Total posts : 45366
That article makes a Class E transmitter sound like a complicated and hazardous undertaking. He’s talking about a 5 W transmitter for the 7 MHz and 14 MHz ham bands using ultra-cheap transistors. Ham transmitters are designed to drive a 50 ohm load connected to a self-resonant antenna or an external antenna tuner that presents a 50 ohm load while resonating the antenna. This transmitter has a low Q output network consisting of a very low-Q resonant circuit to satisfy the Class E requirement and additional low pass filtering.
The configuration for a MW 100mW transmitter feeding a 3 meter antenna is a LOT simpler, primarily due to the much lower power and the much higher Q of the output network. All of his points under the heading “Problems with Class E QRP Amps” quite simply do not apply at all to a Class E Part 15 MW transmitter.
A Part 15 Class E amplifier only requires a few components for excellent performance, stability and reliability. Take a look at this article discussing a 1 W Class E “lowFER” beacon transmitter: http://www.k3pgp.org/Notebook/Wd5cvg/Classetx/classetx.htm This article is much more appropriate for understanding a Part 15 Class E transmitter.
One thing that caught my eye is the graph labeled “Figure 5 – Output Power and Efficiency vs. Lcoil Adjustment”. You can click on the graph to bring up a more readable view. I have done similar plots for the AMT5000 and my results are consistent with Figure 5. In my plots, curve B corresponds to the meter readings at the “RF INPUT CURRENT” test points. Note that the current peaks to the left (higher frequency side) of the high efficiency region. For the AMT5000, the transistor efficiency is about 82%. at the peak current point. The AMT5000 tuning procedure calls for first adjusting the trimmer to peak input current and then adjusting the trimmer one full turn clockwise (about .7 pF additional) to reduce the tuned frequency slightly to move into the high efficiency region where the transistor efficiency is 96% – 98%. Note that the high efficiency region is rather broad, so the required slight decrease in frequency is not a critical adjustment. Overall transmitter output efficiency is lower due to coil loss, but the change in overall efficiency exactly tracks the change in transistor efficiency.
I keep seeing people whispering down the lane that tuning the AMT5000 is difficult. What’s so hard about adjusting a trimmer to get a peak meter reading and then turning the screw one turn clockwise?