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 !

Low Pass Filter for the 2190m Long-Wave band

🔘It is only a small accessory. However, the purpose of the filter described here is to improve the spectral purity of the output signal from my low power RF amplifier ( 📅22.02.2013 ), for the LF 135.7-137.8KHz 2190m longwave 🗼amateur radio band, when being driven from the phasing-exciter (📅 23.01.2024 ).

The filter family is the Chebychev low-pass type, having 50 Ohm input/output impedance, and a theoretical response of 5dB passband ripple, bandwidth 160KHz, and insertion loss better than 0.3dB between 135.8KHz and 140.9KHz; essentially a single section, 3-pole, low-pass, 𝞹-filter.

Schematic diagram of the filter

The input is connected to the low power RF amplifier, and the output to the antenna 🗼via any swr/power meter. Input and output are interchangeable as the filter is symmetrical and bi-directional.

The completed filter - cover removed

I designed the ribbed ( grooved ) coil ➿former ( just visible, lower centre in the above image ) for the precise coil length, coil diameter and wire thickness required, and made it from dark-grey PETG filament on a 3D printer. Two capacitors connected in parallel are required at both the input and output terminations to obtain the correct overall value. The enclosure chosen is a two piece U-section aluminium box. I didn't remove it's outer blue protective film.

The filter was connected to the amplifier and tested at ∿ 136.130KHz, modulated with an audio 👂tone of 1400Hz ∿ by interfacing with my DdsModTerm software (📅 31.12.2024 & 27.03.2025 ). A spectral plot, ( purple: vertical - amplitude dBVrms, horizontal - frequency Hz ), of the output signal ( see image below ) was displayed on an oscilloscope.

Spectral plot (purple) of the signal at the filter output

The line 'cursor A' was placed across the top of the signal with the largest amplitude of +17.2dBV, i.e., the main carrier signal of 136.13KHz located at the centre of the display. The line 'cursor B' was placed across the top of the next largest signal of -30.4dBV; an unwanted  spurious distortion signal that has been generated at approximately 7x the carrier frequency i.e., 950KHz. The difference in amplitude of these two signals is 47.6dBV. All the other spurious signals are more than 47.6dB down on the wanted carrier. Without the filter the unwanted distortion products were much higher in level. So the filter has made a considerable improvement. I am very pleased with the result !

The yellow waveform ∿ is the output signal versus 🕠time. The amplitude is 23.4V peak to peak. This equates to an output power of just 1.4 watts. It will be interesting to see what can be achieved when transmitting at this power level regarding📻 reception distance🌐. To find out, I shall have to wait until the ❄winter when propagation conditions on the 2190m band are most favourable. 🔘

My low power LF radio signal is received in Germany

🔘 Almost 11 years 🗓 have passed since I last used my low-power transmitter power amplifier ( see 08.05.2013 ) based on the TDA2030 class AB audio 🔉 amplifier i.c. ( see 22.02.2013 ). Since then several data 💾 transmission modes, e.g., FST4W,  have become popular among radio amateurs who are active transmitting on the LF 2190m/136KHz 〰 ( longwave ) 📻 band. I also have high power transmitting equipment for that frequency band. However I wanted to conduct a simple test by transmitting a very low power beacon signal using FST4W to determine at what distance it might be received.

My setup for the test was the phasing exciter ( see 02.11.2017 ) as the signal source driving the low power amplifier. The antenna 🗼 was my usual one for the 2190m band; a 47m  long x 13.5m tall base and end-loaded inverted 'L' ( Ꞁ ) ;  see 19.02.2010 et al.  The transmit frequency ∿ was 136.13KHz, transmitter output power only 3.5 watts, ( similar to the power consumption of a small LED lamp 💡 ), and beacon transmission, consisting of my callsign, location and power level, sent at 5 minute intervals.

Equipment used for the low power test on 2190m band

I began sending beacon transmissions during the evening of  21.01.2024. Previously, during the tests on 8 May 2013, ( albeit using a different mode ), the reception distance had been only 17 kms. I was doubtful if anyone beyond that range would receive my signal. So I was very surprised, when, at 2120 utc 🕤, a reception report was posted 📮 on wspr rocks  ☁ that my beacon signal had been received 📶  near Chemnitz in Germany, at a distance of 582 kms. Incredible and amazing 😀 !

 

LF = Low Frequency.

135.7-137.8KHz ( 2190m band ) is the lowest frequency band allocated to radio amateurs.   

Schmidt trigger input for the dual band amplifier

50% +DUT drive signal on the gate of one of the four MOSFETs

Schmidt trigger circuit installed

I have improved the amplifier efficiency by increasing the duty cycle ( +DUT ) of the drive signal on the MOSFETs' gates from 43% to 50% with the addition of a Schmidt trigger first stage. After experimenting with several CMOS NAND and inverter logic chips, ( i.e., CD4093BE, CD74HCT132E, SN74HC14N, SN74HCT14N ), which already have Schmidt trigger inputs, I found I could only obtain the 50% +DUT I wanted by making my own Schmidt trigger circuit using a quad 2-input NOR gate chip ( CD4001BCN or HEF4001BP ). I eventually decided to retain the older CD4001BCN in the circuit as the low pulse rate doesn't really justify using the newer HEF4001BP for this application. I built the circuit on a tiny piece of pad board and fitted it above the main driver circuit board. ( See also post dated 6 December 2015 ).

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.

More details of my low power transmitting setup for longwave

Since the posts on 22nd February and 8th May, I have received requests to post more information on the setup I used for my low power test transmissions on the longwave 2190m band.

The circuit schematic and pcb artwork for the AF amplifier are shown above; click on the images to expand them. The original size of the artwork is 70 x 100mm. The pcb is single-sided; top component layer, bottom copper layer. Anyone wishing to copy my pcb design might need to modify the tracks connecting T1, depending on the actual transformer which is available and the windings used.

A +18V dc power supply can be used for greater output power. I didn't try this only because I don't have a convenient way of providing that voltage, and also the fan is a 12V unit.

My very low power transmissions on longwave

Last night I made successful radio test transmissions on 137.7KHz, 2190m band, using only 3.5W transmitter power. My signal was received, ( screen capture below ), at a distance of 17Km. The signal strength suggests that 2-way communication at this power level would be possible over a much longer distance. The vertical streaks are probably static crashes as a thunder storm was active in the vicinity.

My setup was my own-design PIC controlled DDS and the TDA2030 AF amplifier featured on 22 February. 

It is unfortunate that amateur radio activity on the 2190m band is so low, as it is possible to enjoy communicating on this band with a minimal setup, as I have just shown.  

Experimental low power amplifier for 2190m longwave

I salvaged some potentially useful parts from a faulty pc power supply, e.g. bridge rectifier, schottky diodes, heatsink, fan, chokes, transformers. The 12V-0-12V, 5V-0-5V output transformer typically operates near 40KHz. I thought of using it for the output matching transformer in a low power transmit amplifier for the 136KHz, 2190m longwave band.
My design is based on the very cheap, ( half a $ ), TDA2030 class AB audio amplifier ic, which has a bandwidth of 140KHz.
The circuit is experimental. I was curious to find out if such an amplifier would be useful for 136KHz, despite using some untypical, possibly 'unsuitable', components.

I built the amplifier on a home-made printed circuit board, 70 x 100mm. The ex-pc transformer, ( yellow & black ), is on the left. The TDA2030 is mounted on the ex-pc heatsink. ( Pcb artwork and the circuit schematic are available from me on request ).

Fitting the circuit board inside the old pc power supply box, ( cover not shown ), with its original 12V fan, and adding a LED, rf and dc connectors, completed the construction.

For testing, I powered the amplifier from a +13.6Vdc power supply and connected the input to my frequency synthesiser tuned to 137.8KHz. With the input attenuation set to minimum, and the output terminated in a 50 Ohm load, the measured voltage gain was 41.75dB. Output power was 3.5W.
I could now either connect the amplifier directly to my longwave antenna and make some very low power test transmissions, or use it as an intermediate amplifier stage in a much more powerful transmitter, yet to be built.

Straight up

Today I finished installing the vertical section of a new 'trapped inverted-L' antenna for the 160 metre and 2190 metre bands to the final height of 17.8m. It is constructed from 4m lengths of aluminium tubes of various diameters giving it a taper from 45mm diameter at the base to 26mm at the top.
I have been making ground-mounted vertical antenna elements for low frequencies for many years and in my experience 18 metres is about the maximum height for this form of construction using light material, as well as being at the limit of what one person can erect. Higher than this and the construction can quickly become uncontrollable during lifting, resulting in disaster. However, I still have about 2m in reserve should I feel bold enough one day to try to increase the height still further.
Prior to erecting the vertical part, I had already connected a 15.8m top wire. I now have to fit the trap and an additional 15m of wire to an anchor point on a 12m pole about 30m away.
The storage box just visible at the antenna base will contain the L-type 'L-C' matching network to 50 Ohm coax cable feedline for operation on the 160m band.
Thunderstorms are forecast here tomorrow; an early survival test !

Two generations

The early MKI ( lower ) and very recent MKII ( upper ) embedded control frequency synthesisers are both resting on top of my longwave transmitter. I will interface the MKII with the transmitter as that was always one of my intended applications. The MKI will now be used as an item of test equipment on the workbench to provide an lf signal source.

Weather bad, longwave good

Tonight it is snowing again. The large coil is outside and connected to my antenna, but has little protection from precipitation. Although the weather is poor, radio propagation conditions on longwave tonight have been good. I have just completed a contact with Gerhard in Austria. The screen shot shows him calling me at the start. The other horizontal lines are sidebands from the LORAN-C navigational system on 100KHz. There are LORAN-C sites in coastal regions of northern Europe. We have to put up with its interference until the system is supersceded by GPS. The sound it makes is like a steam locomotive travelling at 100mph. To our advantage, however, these 'lines' give a useful indication of propagation conditions on 136KHz, and were strong and clear tonight; hence the possibility for my contact with Gerhard.
My signal was also picked up 745 kms away in Nuernberg, Germany !

Friday night activity night

Yesterday evening the first activity period on longwave took place, to encourage Polish radio amateurs with longwave receiving and/or transmitting equipment to meet up "on the air". I think that in Poland only myself and one other guy can transmit on 2190m wavelength. Amateurs' antennas for this wavelength are relatively short and inefficient. So we have to transmit information slowly in a very narrow bandwidth to have any chance of being picked up. As an example, the screen capture shows how my slow morse signal, ( 4 sec dot , 12 sec dash ), was received by another Polish receiving station during this activity period.

Extreme loading coil appears in ham press

A description and picture featuring the massive loading coil I use with my Marconi antenna for 136KHz, have recently appeared in RadCom, the monthly magazine of the Radio Society of Great Britain, RSGB, December 2009, page 29. ( click on the post title to visit the RSGB web site ).
The main winding consists of about 400 turns on a 15cm diameter, 108cm long, sewer pipe and has multiple tapping points. The rotatable variometer winding inside is made from 37 turns on a 11cm diameter plastic water-pipe. The upturned plastic food container on top keeps water out.
The impedance matching network to the 50 Ohm coaxial cable feeder from the transmitter uses an ETD44 transformer core with a 9 turn primary winding and a 20 turn secondary winding. I can obtain a perfect match with this method, though occasionally there is slight detuning of the antenna system owing to environmental effects, requiring readjustment of the variometer.

Longwave station

This is the operating position for the 136KHz longwave transmitter. The VLF SWR meter sits on the transmitter case, and the 25V 20A power supply is behind.

Longwave transmitter

This transmitter for the 136KHz, ( 2190m ), band is another of my creations. It is capable of producing 350 watts from four IRF640 power mosfets with a 50V DC power supply. With this transmitter, I was able to complete the first longwave two-way communication ever accomplished in the history of the universe between Poland and Belarus, and Poland and the Czech Republic. Many thanks to my contact partners, Andrei and Ruda, for being at the other end of these historic contacts.