300 W  136kHz Transmitter


Single Printed Circuit Board 178 x 128mm   including:

Variable Crystal Oscillator
Low Pass Filter
Tx / Rx switching relays
'Optically coupled' over-current protection
VSWR Protection

The circuit is based around a high efficiency class D amplifier which uses two STW34NB20 power MOSFETS. Rated at 200V and 34 Amps these FETs combined with protection circuits that can respond to fault conditions in a few tens of microseconds make for an extremely robust design.

If you would like to download a copy of the article from the January / February editions of the RSGB RadCom magazine please click on the link below.  
Please note that this article was published by the Radio Society of Great Britain. The RSGB hold the copyright and it has been reproduced here with their permission. Reprinting copying etc is not permitted.

  - Acrobat reader will be needed to open this file.  (apx 600 kb + 300kb)


   RadCom Jan 2003                      RadCom Feb 2003                      M0BMU modifications

The efficiency of the transformer design has been improved by Jim Moritz. This new design reduces leakage inductance and increases power and improves the drain waveforms as a result. Jim has removed the decoupling capacitor on the transformer DC feed point and has added a Zobel network to each drain. Although not tried here as yet, I suspect Jim's new transformer can be wound on the 3C90 toroid supplied with the kit.

Pictures          Click to enlarge                                   

    Front View 28kb        Rear View 33kb       Internal View 40kb       Side View 38kb

  PCB Overlay

The prototype transmitter pictured above uses an ETD44 core and is designed to produce 400W output from a 45V supply. Although the ETD44 transformer works well, it is difficult to reproduce. Consequently, a  42mm toroid is recommended as it's easier to construct and can still handle over 300 Watts

Note from 2017:  A kit of parts was available from 2003-2007. Now this circuit can be constructed on the 630m PCB                                                                 HOME