The 137 kHz Low Power Transmitter

A Project for the Impatient

This is a very short description of my tiny low power cw transmitter for long wave. We have permit here in Germany since January 1999. Depending on the PI-Filter components and voltage for the final stage this transmitter would be able to reach the 100 Watts output level. Running two IRF630 in parallel I got more than 100 Watts of output "key-down" (16.5V/9A at full load). Here the 50 Watts version is shown (Fig. 2).

The construction is straight forward. Two crystal controlled oscillators, one is running as a VXO, with approx. 137 kHz difference are mixed (Fig. 1; slightly modified construction of the DJ1ZB-Exciter, CQ-DL 2/1998). The conversion product is filtered and amplified by an emitter follower, a square wave pre-driver and the switching stage to drive the power amplifier. This power amplifier does not make use of any output transformer since ferrite toroids for 100-200 kHz are not always available (Fig. 2). I built this transmitter from junk box in 2 - 3 hours. Only the two crystals and the Power MosFets are from local electronic shop. Here in Germany these crystals are available at OPPERMANN, Postfach: 1144, 31593 Steyerberg.

The components of the exciter and the two driver stages are soldered directly to the copper side of a simple pc-board (surface mounted). So the sizes of the components could be vary and it is much easier to modify it under experimental conditions (Fig. 3).

The final stage was built apart from exciter and drivers using "Ugly Construction". The 3 mH choke is wound on a ferrite toroid of unknown material (OD=30 mm, 1.18 inches). Running this toroid as output transformer failed. I'm using 1 mm diameter enamelled wire, 30 turns. Since the power amplifier runs in class "D" switching mode the components of the Pi-filter have to do a lot of work. The Pi-Filter coil is air wound and requires at least 2 mm of wire diameter (close wound 2.5 mm enam., n = 23.5, d = 45 mm / 1.77 inches). Use only high grade polypropylene caps for all output filtering (WIMA, FKP-1, 1000/400 Volts).

The latest experimental version of this concept runs with three IRF630 in parallel, each with its own 4.7 Ohms series gate resistor. The first capacitor of the Pi-filter had to be increased from 68nF to 82nF. Since it showed some "ringing" (not oscillation!) at approx. 1 MHz I increased the filter coil by adding 2 turns. The coil inductance then was tuned to minimum ringing (oscilloscope connected to the output coupling capacitor) by streching or squeezing the windings. Output:  >130 Watts at 160 Watts of input (10A/16V, efficiency > 80%).

The output from the PI-filter is not a clean sine wave, an additional two stage PI-Filter, 50 Ohms input/output,  is recommended. (Cin = 15 nF,  L1 =  53 uH, Cmid = 33 nF, L2 = 53 uH, Cout = 15 nF). T200-2 (AMIDON) are useful up to 500 Watt without cooling but I prefer air wound inductors (37 turns, 1mm enam. close wound over Polyacryl core, D = 50mm ). The tuned antenna will do the rest...

You can use the IRF630 for the final stage but the IRF640 is much better since its ON-Resistance is lower. If you wish to get more power from this small amplifier by using higher voltage, an IRF840 would be recommended.

The exciter is unconditionally stable since the two crystal oscillators are thermal coupled. If one drifts the other will do this at the same direction too but the difference remains at 137 kHz. I never observed any drift even in very slow CW mode. The frequency coverage of this design is about 2.5 kHz.

 Fig. 1: Exciter

Fig. 2: Driver and final stages

Fig. 3: PC-Board, exciter and driver stages

Many thanks to Ha-Jo, DJ1ZB and Rik, ON7YD !

© DF3LP, January, 1999