200W 136 kHz transmitter

  
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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.

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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