๐It has been a while since I last made some equipment for my amateur radio station; according to this blog that was on๐
8.10.2023.
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 ( ๐
22.02.2013 ), for the LF 135.7-137.8KHz 2190m longwave ๐ผamateur radio band, when being driven from the phasing-exciter (๐
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, ๐น-filter.
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| Schematic diagram of the filter |
The input is connected to the low power RF amplifier, and the output to the antenna ๐ผvia any swr/power meter. Input and output are interchangeable as the filter is symmetrical and bi-directional.
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| 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 ๐tone of 1400Hz ∿ by interfacing with my DdsModTerm software (๐
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.
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| 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 ๐ 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 regarding๐ป reception distance๐. To find out, I shall have to wait until the ❄winter when propagation conditions on the 2190m band are most favourable. ๐
