"richfair" over at the antique radio forums had been working on a Class E Part15AM transmitter for about a year and in 2021 came back with some improvements to the design. A lot of comments of praise on his build which is PDFs are provided freely of the schematics and build notes for download.
He also has a bit to say about how to properly tune a Class E transmitter. It's an interesting thread, below are some key excerpts:
https://antiqueradios.com/forums/viewtopic.php?t=354147
[Excerpts]
richfair's am transmitter
richfair#1 · OP · 02/11/2019 3:22 pm
UPDATED 01/2021 ...
PREVIOUS UPDATE 01/2020
I revisited this project with a goal to improve it without added cost. Rather than start a new thread I have uploaded and replaced all schematics and pdf documents to reflect revisions. ..... ..
It is capable of full 100mw input power and full modulation, and can be tuned to match practically any FCC-legal wire or whip antenna. A mosfet modulator operates in class E mode to generate an rf carrier. An audio driver provides an audio-modulated DC drive current for the mosfet. RF output power is set by a potentiometer or trimmer. Antenna length should be 3 meters or less freely suspended in air. The transmitter is designed to accept DC power from a battery or regulated 9 to 12 supply. Average current draw is about 60ma but the supply should be capable of peak output current of at least 250ma.
Inductor L2 is a very important component that affects antenna tuning, radiated strength, audio linearity, and audio distortion. My build includes a range of mini inductors that can be jumpered in or out in any combination to cover the full frequency range.
For higher audio frequencies the available modulation headroom decreases while distortion increases at those frequencies. Since the distortion increase is mostly in the frequency range above 5kHz, it is generally not audible.
The cause of this phenomenon is buried deep in the peculiarities of class E operation and how inductors alter harmonics phase shifts and upper/lower am sidebands. .. My earliest version of this design incorporated an audio feedback loop from the onboard envelope detector back to the audio driver. It caused as many problems as it solved and was reluctantly removed.
Tuning of the antenna matching network is most easily done while monitoring the strength of the radiated signal. I use a ‘scope with a short piece of wire near the antenna. An alternative method is to use a test point (refer to schematic) at which a DC voltage in proportion to antenna tuning can be observed. An optional further adjustment for lowest distortion is described in the build notes.