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 converts 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 90 watts output when used with a 54Vdc power supply. Approximately 2.5W input is required; i.e.,  gain is 15dB.🔘

Antenna L-Match network for the 160m band

🔘For many years the 🗼antenna which I have been using for my amateur radio activities on long-wave and medium-wave bands ( 2190m & 630m ) has been a vertical, tubular aluminium pole connected at its top to a horizontal wire. Both the pole and the wire together form the antenna element;  the familiar 'inverted-L' configuration  |''''''''''''''' .  There is a picture of it during construction in the post dated 11.09.2012.

Vertical section of the antenna - 
the storage box at the base

However, I also wanted to use the antenna on the 160m band ( 1810 - 2000KHz ); but it wasn't resonant ( less than half-wavelength long ) on that band and also presented a severe impedance mismatch to the 50 Ohm coax cable which connected it to the transmitter.

I solved the non-resonance and mismatch problems by making an L-C "L-Match" network inside a storage box and connecting it to the antenna's vertical pole at ground level; see image below - the cover has been removed to show the  ➿coil  (L), capacitor  -∥- (C), coax cable and wire interconnections.

L-Match network installed at the base of the antenna

After a very short time spent adjusting the variable coil and variable capacitor I was able to obtain a perfect match to the coax at the point of connection to the antenna on my chosen frequency  ∿1840KHz, although the antenna does exhibit a very narrow bandwidth.

After using the modified antenna 🗼 for just a few evenings, I have contacted 18 countries on 160m band.🔘

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 +15.6dBV, 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 -27.2dBV; an unwanted  spurious distortion signal that has been generated at approximately 3x the carrier frequency i.e., 420KHz. The difference in amplitude of these two signals is 42.8dBV. All the other spurious signals are more than 42.8dB 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 20.5V peak to peak. This equates to an output power of just 1 watt. 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. 🔘

More features for DdsModTerm software

🔘I have added some important new functionality to my DDS programming software, "DdsModTerm", ( see 📅31 December 2024 ); updated to version 1.0.0.rev16. It is now possible to generate audio 👂frequencies, as well as program the AD9850 and AD9851 DDS chips, using one application. For example, the 'Phasing Exciter' ( see 'featured post' in side-bar, 📅 02.11.2017 & 23.12.2024 ) requires both audio frequencies (👂AF) and radio frequencies (📻RF). So having the same software able to generate both is very convenient while carrying out testing, alignment, and repair tasks. This has required some changes to the lower half of the program window, (see image below, click to enlarge, compare with 📅 31.12.2024).

Audio tones can be generated from the lower-right panel 

The audio 👂 tones are produced entirely independently of the DDS. Free downloads of audio .wav files are available online. I chose files of single frequency sine-waves ∿; 1200Hz, 1400Hz, 1500Hz & 1600Hz being best suited to the data transmission modes I use on the 472-479KHz amateur band. I imported these files  as resources into the DdsModTerm software, and wrote some code so that the tones are played back through the 💻 pc sound-card 🔊by clicking the appropriate button. The individual .wav files are quite small ( about 1.3Mb ) and playback duration ⏳is only 15 seconds. I added a second progress bar to track the playback, and a button to play back in a continuous loop, if preferred.

While the software was being revised, this opportunity was also used to calculate another pair of alias frequencies  ( for 2 x 🕒clock ), and make some minor cosmetic changes to the window's appearance.🔘

Free .wav files downloaded from 🔗 OnLineSound  with thanks.

DDS = Direct Digital Synthesiser

AD9850 & AD9851 are Analog Devices Inc. parts.