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. 🔘

Programming AD9850 & AD9851 DDS

🔘I previously posted about using the AD9850 & AD9851 DDS chip evaluation modules in 2018 and again in 2023. To recap, before these devices can be used as a signal source they require programming with 5 bytes of data related to frequency and phase, which form a 'tuning word'.

A µ-controller and a smart phone🖁 App could be used to upload 🠝 the tuning word ( see 16 July 2023 ), or a USB dongle and pc💻 interface software, ( see 4 January 2018 ).  As I have recently updated that software it would now be a good time to give a description.

My dedicated pc 💻 software, called "DdsModTerm", is the user interface which I started developing in about 2015. Since then I have updated it 15 times; the latest revision appearing this month.

DdsModTerm user window

The clock 🕰 frequency and the required output frequency ∿ & phase are entered either manually or by recall from memory. By clicking 'Confirm' the software generates the 5 configuration bytes required from the user input data. In the example in the image above the output frequency is 137700Hz* & bytes hex 00C88AC604. The pc 💻 is connected to the serial data interface of the DDS module via a COM port and a USB-SPI protocol converter dongle.

(L) USB-SPI dongle (R) AD9851 DDS module on adaptor

Clicking 'Update DDS' then uploads the bytes to the registers of the DDS chip using SPI and a voltage having an amplitude 1V peak to peak at the programmed frequency ∿ is then present on the output.

DDS output signal, 1Vp-p, 137.7KHz

Other features of the software include up/down step 🪜tuning, slider tuning control, view of 255 byte eeprom addresses E0-FF, 3 memories for storing frequency, saving custom clock🕓, alias frequencies calculated, and general purpose output ( GPO ) toggling on/off.

The dongle and software are available from me. Post a comment to receive more information. Note that both AD9850 & AD9851 DDS devices are supported.🔘

( Click on images to enlarge detail. )

* 137.7KHz is a calling frequency on the radio amateur 2190m long-wave band, 135.7-137.8KHz.

SPI = Serial Peripheral Interface, 3-wire bus.

AD9850, AD9851 : 🔗Analog Devices Inc. parts, 32-bit CMOS Direct Digital Synthesiser (DDS) chips.

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.   

New mosfets for the dual band amplifier

Four IRFP360 mosfets mounted on heatsinks inside the dual band amplifier

In March I bought new mosfets for the dual band amplifier ( see post 6 December 2015), but only recently had the time to fit them. I always knew that the original IRF640 types were underrated when I started running the amplifier from a 54V power supply, and there were reliability issues with several of them failing with a loud bang. The new type I've now fitted is the IRFP360 which is a 400V mosfet. During the last couple of evenings I've been transmitting with the amplifier for long periods without any further mishaps occurring. I had to fit these mosfets with a different orientation from the IRF640 ( see previous amplifier images ) as the mounting hole is insulated ( no insulating collar required for the bolt ) and the drain connection was made to the centre pin, not the case. A mica insulator, however, was still necessary under each mosfet between it and the heatsink. The insulator required is slightly larger than the standard TO-220 size. To begin with I didn't have any suitable until I found that Farnell stock them, ( item code 520-214 ). The mosfet and data sheet can also be found at Farnell, ( item code 864-9359 ). 

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.

Chirp-Hell on longwave

On 14th November 2012 I reported success with the tests on the work-bench of the improved ssb phasing exciter for my 2190m longwave transmitter. Soon after that I installed it inside the transmitter enclosure. Since then I had been waiting for an opportunity to test it using full transmitter power into my antenna in a real 'on-air' situation, with a more distant receiver. 

So early this morning at about 1.00am I carried out transmission tests with Jacek, SQ5BPF, in Warsaw. My signal was quite readable on his grabber; screen captures above. The noisy band conditions at his location are also very evident.

I was transmitting on 136.9KHz upper sideband, modulating with 800-810Hz chirp-hellschreiber audio tones. Transmission speed was either 5 or 10 secs/character; the vertical markers are 1min apart. We then completed a chirp-hell to qrss1 cross-mode contact; quite obscure, so it's probably the first time ever it has been done !

Improved phasing exciter

I have completed some improvements to the single-sideband phasing exciter, ( post 25 January 2010 ), for my longwave transmitter, just in time for use during the good propagation conditions on the 2190m band over the winter.
The passive phase shift network was not producing accurate 0 and 90 degree phase shifted AF. In fact it was well outside specification; perhaps not surprising as I bought it in 1978. So I replaced it with an active circuit based on a dual op-amp. I optimised the unwanted ( lower ) sideband suppression at 800Hz. There is little point anyway in achieving exact quadrature audio channels over the entire speech band as the modes in use on longwave are extremely narrow-band.
I added a tuned class-A post-mixer transistor amplifier stage.
Upper sideband is now selected automatically as I have disabled the sideband switching facility; until such time when I see a lower sideband signal on the band.
The top picture is my signal, received on 137.7KHz, from just the exciter sitting on the workbench, ( lower pic ), when transmitting using chirped multi-tone Hellschreiber mode. As can be seen, the signal occupies only about 5Hz of band !

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.

Wire in the sky

I have erected a new antenna in readiness for the restart of my radio activity on the 136KHz 2190m longwave band during the coming autumn and winter season; simply a 45.9m long wire, and a number of bare copper conductors buried in the ground directly below. The wire is supported at the mid point by a fibreglass pole, and a spring counter-weight ( see inset ) running through a pulley tied at the highest point I could reach to a tree at the far end.
The (in)famous loading coil, ( posts dated Feb 19th & Sept 13th 2010 ), will still have to be pressed into service; but fortunately it will be installed inside my radio room. What a huge relief that I will no longer have to use the coil outdoors or carry out frequent maintenance on it inspite of the snow and ice, as with the previous antenna !
My last attempt to put up a different antenna for long wave was in 2009. It was a much more complex design, both electrically and mechanically, requiring 'miles' of wire. It would have been an excellent performer. Sadly it never came to fruition, being blown down / away four times during summer thunder storms before I had even finished building it. There was an important lesson to be learnt from that ! It's better to have a simple antenna than no antenna.
While the performance of the new antenna will be inferior to the one I didn't complete, it should out-perform the last operational one.

More "firsts" on longwave

Yesterday evening, Szigy, callsign YO2IS, in Timisoara, Romania, 710 kms away, and I had a contact on the 2190m longwave band, which we will claim as the first ever Romania - Poland contact on that band, thereby adding Romania to my two prior "firsts" with the Czech Republic and Belarus.
I copy part of Szigy's email which he sent to me soon after our meeting 'on the air', particularly because it emphasises how challenging amateur radio communication is on longwave; as I know only too well myself, even over distances of just a few hundred kilometres. Szigy wrote:

"Dear Steve pleased to run a fine QSO with you, the very first SP-YO on 2.2Km. Signal was nice but with a deep QSB on the midle of the QSO. At the beginning had some problem with a flashover in the teflon feedtrough my window, it take one hour to change the isolator.Will send you a direct QSL in the next days. Once more thanks for the new one ! Have fun on VLF it's always a big chalenge, gl."

One of the challenges is typically the use of short inefficient antennas, ( because of the nearly 2.2km wavelength ), causing high voltages of several KV to appear at various places in the antenna system. Unluckily for Szigy during our contact, he had to take time out to tackle a problem of insulator flashover !
Earlier this week, I cleared ice and snow from my antenna in order to make it useable once more; but even then I had a flashover problem which I was able to prevent happening again, and fortunately nothing went wrong at my end during the contact with Szigy.

Stop press: In the last few minutes I have achieved another "first", Estonia. Incredible conditions on longwave this weekend, and some stations active making the most of them. What a start to 2011 on 2190 !

Four on longwave

Most weekends, except in summer, I am transmitting my radio signal on longwave, ( 2190m / 137.7KHz ), and hoping someone will reply. Usually my signal goes unanswered as no one else is about; but last night Marek, a Polish radio amateur, callsign SP2OVY, from near Gdansk, called me. I had never received his signal before. So I assume that he has only recently begun to transmit on longwave. To my knowledge there are now 4 Polish stations, including myself, who have transmitters for the 2190m band; though I am aware of a few others who monitor on receive only. Two years ago I was the only one active on longwave in Poland. So the numbers are gradually increasing.
The distance between Marek and me is only 277kms; but I was happy my signal made it that far as snow on my antenna had significantly detuned it. We had a successful 2-way contact, ( report "O" bothways ), ending in the early hours of this morning.
The screen capture shows his slow morse ( QRSS ) signal to which I have added the letters; of course 'E' and 'N' were the following two letters on the next screen capture, ( not shown ).
I am still using the same home-made transmitter and antenna which I featured on the blog on 25th January and 19th February.

Matchbox

During the summer I don't operate my amateur radio station on the low frequency bands, ( 80m, 160m and 2190m wavelengths ); the atmospheric noise, mostly from thunderstorms within a radius of about 4000kms from me, just doesn't give a pleasant listening experience, as well as blotting out the weak signals which I need to hear. Conditions in winter for radio communication on these bands are much better, particularly for making very long distance contacts. My favourite time for concentrating on these bands is around the time of the autumn and spring equinoxes. I am starting to get excited as such conditions will soon be arriving again.
I use the same antenna on each band, namely a vertical, aluminium pole which rests on an insulator at its base. Over the years I have collected many useful insulators of this type, both ceramic and glass, by looking on the ground at the bottom of telephone poles.
I have just cleaned the insulator, ( a white, ceramic one ), checked and weather-proofed the connections to the antenna, confirmed that it is resonant on my preferred frequencies and impedance-matched to the 50 Ohm coaxial feeder cable from the transmitters.
The matching networks, ( a tapped coil for the 80m band, a L-C 'L' network for the 160m band ), are located inside the storage container with lid, otherwise known as the "Matchbox", at the foot of the antenna, with the exception of the loading coil for the 2190m band which is too large to fit inside.
Perhaps this year I will reach my target of making contact with 160 different countries on the 160m band.

Loopy thoughts

I've been sorting through my stock of coax cables. If I join together several lengths of the same thickness of 6mm, I could make one length of 70 metres. Perhaps it will be enough to construct a reasonably effective rectangular loop antenna for transmission on the longwave 2190m band.
So let's see.
I could support it vertically from two trees in the garden 20 metres apart, with the vertical plane running N-S. The area enclosed by the loop would be 300 square metres.
After doing some quick calculations, I predict Rrad = 121 micro Ohms, and efficiency = 0.0076%, assuming rf losses = 1.6 Ohms.
In terms of efficiency it will be nearly 4dB worse than my existing longwave Marconi antenna. Another limitation is the loop's bi-directionality, ( Marconi omni-directional ); so radiation broadside ( E-W ) could be 30dB down on end-fire direction ( N-S ).
I wonder if making it will be worth the effort.
Has anyone made a longwave loop antenna of a similar size ?
If so I would be pleased to hear of your experiences with it.
I won't be buying a single 70m length of new coax just yet !