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The disadvantage of most radio receivers is that they cannot receive anything below 50 MHz (e4000 - does not work below 50 MHz, R820T - below 24 MHz), and in this range - 3/4 of all interesting things that can be heard on the radio due to the fact that short waves are reflected from the ionosphere - and the communication range is no longer limited by line of sight.
This drawback can be eliminated by adding a frequency converter that shifts the frequencies for the SDR radio receiver to work with a range below 10 MHz.
The principle of operation - from the incoming signal from the antenna we cut off everything that is higher than 50 MHz, mix it with the 125 MHz frequency obtained on the crystal oscillator, again filter out the excess from the signal - and we get our signal shifted upward by 125 MHz. Those. what came from the antenna at 10 MHz - in the receiver we will receive at a frequency of 135 MHz.
The choice of 125MHz quartz is very important, many converters use 100MHz quartz and the received signal is received just in the area of powerful FM stations, which can be a serious problem.
Converter (UpConverter) Ham It Up 1.3 makes it possible to work with frequencies from 0.1 to 60 MHz.