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 )

Power Amplifier for the 472KHz/630m Band

🔘This construction project, which I've only just completed, is a transmitter 🗼 power amplifier for the ∿ 472-479KHz 630m medium-wave amateur band. It is a Class C amplifier design, meaning it is a non-linear amplifier.

 Front, on the left, with input socket & indicator LEDs (click to zoom) 

Features of the design include two field effect transistors ( MOSFET's ) connected in parallel, 'RF' sensed automatic antenna changeover on transmit, antenna changeover delay optimised for the data modes I use, -20dB power coupler ( for connecting an oscilloscope, diode probe or power meter ), DC power LED (green), transmit mode LED (yellow), 50 Ohm output impedance. An integral 'L'-Match network matches the MOSFET's load resistance to the required 50 Ohms output impedance of the amplifier.

Underside, cover removed, to show internal layout

Regarding building the amplifier, a two piece 'U'-section aluminium box was acquired for the case, though I replaced one half of the original box with perforated mesh sheet for improved ventilation. A substantial heat-sink has avoided the need for any forced-air ( 🪭fan ) cooling🆒. The coil former ( yellow ), toroid core clip ( blue ) and some capacitor holders ( grey & blue ) were custom made on a 3D printer. The amplifier is physically compact; overall dimensions📏being ( l x w x h ) 230 x 100 x 136mm. The 🏋weight is 1.273 kilograms.

The amplifier produces about 95 watts output when used with a 54Vdc power supply. Approximately 2.5W input is required; i.e.,  gain is 15.8dB.🔘

3-D Printed Holders, Clips & Formers

🔘I've designed and printed a selection of parts some of which are intended for use in the transmitter power amplifier I am currently building for the 472KHz∿/630m wavelength radio 📻 band. So far I've made holders for some large foil capacitors, mounting clips for toroidal cores and coil formers ➿. Both PETG (grey) & PLA (blue or yellow) plastic filaments were used

Toroidal core clips, coil formers & capacitor holders

The toroidal core clip is for a T150-26 core which has an external diameter of 1.5". The coil fomers are ribbed for a close-spaced winding with 1.6mm diameter enameled copper wire. One side of the capacitor holder is open so that the value and voltage rating are visible. Where required, the parts can be panel-mounted using M2.5 nuts and bolts 🔩.

One benefit is that all dimensions and the style can be customised for a 'tailor-made' solution. I am no longer restricted to using PVC tube or pipe fixings for example, in only a few sizes bought from the shop. 

  

Remotely operated Antenna Tuner - in use

In my last post I described the system I have recently put together to remotely tune ➿ the antenna 🗼 I use for the MF 630m ( medium wave) amateur band. The images below show the outdoor parts of the system now deployed and in operation with the antenna.

Base of antenna showing tuner, coil, element & support

A usb rechargeable power-bank 🔋 provides power for the remote unit and motor. I made a protective cover ( yellow object ) for the stepper-motor to keep it dry.

Container cover removed to show remote unit & power-bank

While trying it all out for the first time when using my transmitter I was able to re-tune 🔁 the antenna as I changed transmission frequency, from the operating position indoors.  All parts of the system are performing well. I am very pleased with the results.

Remotely operated Antenna Tuning

My inverted-'L' ( end-fed Marconi ) type antenna🗼for the 630m 📻 amateur band uses a variometer tuned loading coil ➿ at its base, which has to be adjusted to maintain antenna resonance when the transmitter changes frequency. Manual adjustment has always been possible; but never convenient as the coil is situated 35m from the house. So I devised a method for 'tuning' the antenna remotely, building on my experience using  LoRa and BluetoothⓇ 📶 in some previous projects.

Indoor unit (display active), remote unit, stepper motor & coil

The setup comprises an indoor unit for the LoRa transceiver and BluetoothⓇ server and remote units consisting of  another LoRa transceiver and stepper motor. TTGO LoRa Esp32 micro-controller boards are used.

App "Step_Match" opened

My custom App "Step_Match" for a smart-phone📱 connects to the server "Step_Tune" with BluetoothⓇ. Each time 'Step CW +' or 'Step CCW -' is tapped a command is sent to the indoor unit, then via LoRa to the remote receiver to drive the motor shaft clockwise or counter-clockwise by 5ͦ . Cumulative motor steps, number of clicks and degrees of rotation, ( relative to a half-way position at zero ), are updated. The motor actually makes 28 steps for each 5ͦ of shaft rotation, 2048 steps per revolution at a speed of 2rpm. 

The 3-D printed 3cm diameter toothed wheel  ( 24 teeth ) fitted to the motor shaft engages with a 4cm diameter toothed wheel ( 32 teeth ) on the variometer coil adjustment shaft; this gear ratio being 1/1.33. So the variometer shaft rotates 3.75ͦ per click. A initial check just holding the motor in position confirmed that it developed enough torque.

I temporarily placed the 'remote units' outdoors next to the antenna. The 868MHz LoRa link worked perfectly. Everything is now ready for use with the antenna. 

 

LoRa = Long Range ( a low bit rate, low power, long range digital wireless data technology ).

🗼 An image showing the original antenna was posted on 11 September 2012.

Class C transmit amplifier for the 630m MF band

I continue to try different amplifiers in order to improve my station for weak signal digital modes, such as JT9 and WSPR, on the 475KHz, 630m band.

unboxed prototype amplifier with space left for antenna relay

The amplifier posted here is a medium power, and physically small, two IRF640N mosfet class C design. I designed the amplifier so that the 13degC/W heatsinks would provide adequate cooling for up to 50 watts output power when run from a 54Vdc power supply, while still retaining the compact size. With 2.5W drive power at the input, I measured 33W output power, ( gain 11.2dB ).  This was deliberate because at that power level, the optimum mosfet drain load is almost 50 Ohm; so I avoided the need for any output matching circuit. It will be interesting to see what I can achieve on digital modes now.

antenna change-over circuit added (lower-right)

Using it for the first time yesterday evening, my WSPR beacon transmission was received at a record distance 1655kms in France, and I was rewarded with a JT9 digital mode contact with Scotland at a similar distance.

 finished amplifier in home-made enclosure

 

3 February 2023

I received a request from a radio amateur in the USA ( Jeff, callsign WB8RJY ) for the circuit diagram, as he wanted to construct the amplifier. A few days later Jeff sent me several pictures of his completed amplifier, and a summary of his WSPR activity using the amplifier detailing some very long distance reception reports.

amplifier built by WB8RJY based on my design

 

 

 

 

 

 

10 watt amplifier for MF phasing exciter

Internal view, cover off - amplifier built inside old pc psu box

Bias circuit bottom left, antenna changeover circuit top right

In my last post, ( 2nd November ), I described the phasing exciter I made especially for data mode communications on the 630m 475KHz MF band. I have been using it for beacon transmissions and 2-way contacts. I tweeted news of one such contact using JT9 mode. All this has been achieved with an output power of just 100mW. For consistent results, however, particularly when propagation conditions are not favourable, ( e.g., long periods of fading ), more power would be beneficial. So I have built a single-ended class AB rf amplifier rated for 10W max output which is connected externally between the phasing exciter, low pass filter and antenna.
Once again I have used a metal box from an old pc power supply, complete with cooling fan, for the enclosure. A type 2SC3039 power transistor is fitted to a 5K/W heatsink directly in front of the fan. I have included a variable bias circuit, based on a 2SC2958 transistor; actually it's the same transistor type as the driver in the exciter as I had spares. I have used this to set the current for my chosen no signal operating point for the 2SC3039 for class AB operation.
The amplifier has made a huge improvement to the reception range of my WSPR signal; so far up to 1632kms.

Phasing exciter for the MF 630m band

DDS interface dongle left, antenna c/o top, main board below

antenna change-over circuit removed to reveal main circuit board

So that I can join in the fun of using weak signal data modes such as JT9, JT65, FT8 and WSPR on the 475KHz 630m band I have built a phasing exciter for a low power single-sideband transmitter. Audio input signal is generated by pc data mode software and sound card. The rf signal source is a cheap AD9851 DDS module bought at an online auction site. Its frequency of operation is changed using the usb dongle and interface software obtained from www.spectecs.com. The rf phase shifter is a dual J-K flip-flop chip, af phase shifter comprises two quad opamps, and the mixer is a dual 1to4 mux/demux fet bus switch.
To give the project a smart appearance I chose a Hammond type 1455N1201BK box for it. Even before the prototype was finished, I couldn't resist the temptation of connecting it to my inverted 'L' antenna and trying it out. The exciter on its own produces only 1mW ( 1 milli watt, 0dBm ); but this was sufficient for my WSPR beacon signal on 474.2KHz to be received with SNR -23dB at a distance of 11kms by a monitoring station.
The prototype was completed with the addition of a class A rf driver amplifier to increase the output power to 100mW, and an automatic rf sensed antenna change-over switching circuit. Since then my signal has been received in Norway, distance 1071kms, Estonia, distance 816kms and Germany, distance 700kms, which surprised me as the antenna's directionality favours the east. Sadly there don't seem to be any foreign receiving stations in the east at the moment.

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 ). 

Half-wave filter for the MF amplifier

Two T200-26 iron powder toroids are used

It's called a half-wave filter because it functions like a half-wavelength of transmission line, ( e.g., coax cable ), regarding the terminating impedances at the design frequency of 475.5KHz. Using lumped circuit elements is a much more practical proposition than 200m of cable. It also has a low-pass frequency response; theoretically 2nd harmonic 951KHz -27dB, and 3rd harmonic 1426.5KHz -47.8dB. This is a useful amount of extra attenuation. So I built it for use with my MF amplifier, ( posted 6 December 2015 ), to provide additional suppression of harmonics in the transmitted signal. It is connected externally in-line with the amplifier output.  

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.

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%.

Getting ready for 630 metres

Experimental amplifier for 475KHz using a single 32N12 mosfet

Just a few days ago I read that the 630m / 475KHz band will be released to radio amateurs in Poland from 18 February. Having no transmitter for that band and wanting to radiate a signal from the first moment the band becomes available, I had to make something very quickly. It was a race against time and all I could hope to complete was an amplifier, ( based on a type 32N12PV2 mosfet device ), to boost the output signal of my frequency synthesizer by about 26dB.
For the antenna, I plan to use my existing 46m end-fed wire, ( posted on 12 August 2011 ), and part of the loading coil I already use on the 2190m / 136KHz band; ( posted on 19 February 2010 ).
With this setup I shall be amazed if I can even make contact with local radio amateurs, ( let's say those up to 10Kms away ), assuming there will be any of them also equipped and active on the band at the start. Still, it's worth a try.