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 )

My LF radio transmissions in February 2026

🔘I was particularly active transmitting on  the LF ∿136KHz/2190m long-wave 📻radio band during 📅February.  My radio transmissions on that band have now paused until I resume them later in the year when the long nights return.

My transmitter ⚡is home made to my own design. The output power is 150 watts. My🗼 antenna consists of vertical sections of aluminium tube totalling 📏14m long  to the top of which is connected a wire which extends horizontally for 📏47m. Two loading coils➿➰are also connected; one ➿at the base of the vertical section and the other ➰to the far end of the wire. Both the 🗼antenna & transmitter have been featured in several previous posts.

The modes which I predominantly used were the 32 minute ⏳beacon mode called "Opera32", and the slow morse modes known as "DFCW10" ( Dual Frequency CW with 🕑10 second dashes ) and "QRSS4" ( 🕑4 second dots ).

My transmissions were reliably received 🎧by the 📻receiving station ( 'grabber' ) of DL0AO near Amburg in Germany at a distance of 700 kilometres. The signals being received can be viewed 🔗online in near real-time, from which I made the screen-shots below. Additionally my Opera-32 signal was at various times also received in 🗺 Greece, Croatia, Norway, Russia & Sweden.

List of Opera32 detections by DL0AO

 

Opera32

DFCW10

QRSS4

I have had 2-way contacts with radio amateurs in 11 other countries on the 2190m band. Unfortunately during February 2026 my transmissions were not answered !