200W 136 kHz transmitter

Posted on Feb 6, 2014

Output transformer core: coming from flea market, light blue color, marked as A-438281-2-9H9-3, OD = 47 mm, ID = 24 mm, height = 13 mm. No information about µ Mosfet to be used: coming from flea market, with the following characteristics. It`s an obsolete (no data sheet could be retrieved) and cheap (1 euro) device and is similar to the well known IRFP450.

200W 136 kHz transmitter
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The ARRL handbook suggests that RF non resonant transformer must have an inductive reactance (XL) at least 4 times the impedance. I decided to use XL= 6 times the impedance. The secondary winding should match the antenna impedance, in my case about 50 ohms. Table 3 report the secondary winding turns, computed with the following formula: The transformer turn ratio (10 / 30) confirms the impedance matching from the 3. 3 ohm of the output stage to the 50 ohm of the antenna system (see formula 3). The input stage can handle a 5 to 12 V pp signal (TTL or CMOS, probably, but not tested, also sinusoidal signals). My Xtal oscillator uses an ex CB Xtal 27. xxx MHz divided by 100. The mosfet is mounted on a heat sink coming from an old Pentium 2 CPU, with the little fan running at 12 V. Another fan in the box keeps the temperature of the transmitter comfortable; the source pin go to the ground plane directly. The by-pass capacitors C2 e C3 must be of the best quality you can find, rated to 4 times Vdc, I use two 1 µF and one 2, 2 µF polyester 250 volt in parallel. Don`t use electrolytic capacitors and keep the terminals as short as possible.

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