LF/MF dual-band transmit amplifier update

🔲The original version of my LF/MF MOSFET Class-E 🗼transmitter power amplifier design was described on 📅6 December 2015, with follow-ups on 📅9 January 2016 & 📅21 September 2017. It has been in regular use since then. However, several modifications have been made recently.

The ➰inductive reactance of the bias choke has been increased to the approximate optimal value of 30x MOSFET drain resistance, ( see Note below ), to improve the performance on the LF 📏2190m/∿136KHz band. So a second ⊚T200-26 core and a ⊚T157-52 core with 30 & 18 turn windings respectively have been added in series with the original ⊚T200-26 core ( 29 turn winding ); all are iron powder toroidal cores.

The ➿coil in the original LF band module could become quite hot. Two replacement modules have been made; one is completely new and uses Litz wire for the ➿coil, while the other is a redesign of the original still using 16 gauge enamelled copper wire (ecw), but changing capacitor values by "select-on-test". The coil-former for the 'ecw' version was 3-D printed from PETG filament, and is partially ribbed along its length. The DC blocking capacitor has to be a low-loss type  as the RF current through it can be considerable. Here, I have used several capacitors connected in parallel to share the current. The spacing between the ➿coil and ground-plane has been increased to reduce losses. Both designs for the LF band module will be tested in turn for comparison.

Internal view cover removed - note toroids & LF band module with ecw coil

Currently the output power is 172* watts on MF (📻📏630m/∿472KHz band ), 282 watts on LF (📻 📏2190m/∿136KHz band Litz wire coil in band module ) and 302 watts on LF ( 📻📏2190m/∿136KHz band  16-gauge ecw coil in band module ) - see image below.🔳

Oscilloscope display - (yellow) input voltage Vgs, (blue) output voltage Vo

Note: for Class E, Drain Load Resistance = (Supply Volts)🠉2 / 1.2 x Output Power

* Previously 210 watts with 2 x T200-26 bias choke ( 29 turn + 30 turn windings )

Dual band transmitter power amplifier for the LF ( 2190m ) and MF ( 630m ) bands

home made enclosure 25 x 24.5 x 12cm

band module for the 2190m band is shown installed

I've recently finished building another amplifier for my transmitting setup for the 2190m/136KHz and 630m/475KHz bands. It is a switching amplifier design based on Class 'E' topology, using four IRF640N  MOSFETs in parallel. To allow operation on both bands I constructed the output tuning and matching circuit for each band as a removable module; changing the frequency band of operation just requires installing the appropriate module. I have also fitted a RF voltage sensed automatic antenna changeover circuit.
In use the amplifier runs only slightly warm. Each MOSFET is mounted on a separate 4.4degC/W heat-sink, two cooling fans are running and there is ample ventilation. So my work on the thermal aspects of the design was worth the effort. By using 4 MOSFETs in parallel there is very little heat to be dissipated anyway as their combined 'on-resistance' is extremely low. Amplifier efficiency is about 83%.
I was very pleased to get a reception report of my signal on the 630m band from Bantry, south-west Ireland, ( distance 2085 kms ), as well as reports from Greece and Spain.
My future plans are to paint the front panel of the enclosure, and make a hinged top cover.
I can provide the circuit diagram on request by email.

Improvements to the 475KHz amplifier

Note the new, larger heat-sink and its position

Automatic antenna changeover circuit

I've made a couple of changes to the amplifier since I first reported on this project on 22 September 2014.
The mosfet has now been fitted to a larger ( 4.5degC/W ) heat-sink for better cooling, and repositioned to make device replacement much quicker and easier than before, should it again be necessary !
I've also added a circuit for 'rf voltage sensed' automatic antenna changeover switching which is a much more convenient arrangement than a manually operated  coax switch which I was using.

By the way, my 'countries worked' total on the 475KHz / 630m band has now risen to 14.

A compact transmitter power amplifier for 475KHz

Internal view: Right - input circuit, Left - output circuit and fan

Ex-pc power supply cases are ideal for small projects

With the previous amplifier, ( see 14 February ), I contacted a couple of countries on the 475KHz/630m band. I have now replaced that amplifier with a more efficient Class 'E' design. It uses a type IRF640N mosfet and 13.8Vdc supply to produce an output power of 50 watts with an efficiency of about 81%.
I built the amplifier in an old pc power supply case, measuring just 165 x 90 x 85mm, retaining only the original fan.
During the evenings I have been transmitting a beacon signal on 478.5KHz using my DDS frequency synthesiser as the drive signal source, and been getting reception reports from across Europe. So far Essex in England at 1380Kms is the farthest that reception of my signal has been confirmed. If the amplifier remains healthy during these tests I shall attempt to increase the output power to about 140 watts, and start making some more two-way contacts with other radio amateurs on the band.

19.10.2014. Till now my 50 watt beacon transmission has been received the farthest in NW England at 1602kms, and my 'countries-worked' total has risen to 4; Poland, Germany, Finland and France. So as planned, I have increased the output power of the amplifier to 140 watts by connecting it to a 24Vdc supply. The efficiency has improved to 86%.