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A power amplifier designed for my ,,555 Timer Amplitude Modulation'' circuit. Will boost the modulated signal and filter it to perfection. The amp works in class A mode and is a normal AF topology. Actually normal AF topologies can be used up to a few MHz effortlessly but after maybe 5Mhz these topologies become tricky to use and others like Class C amps are used. Anyway the output power is around 900mW which in AM will get you easily a few km of range. With the right directional antenna, I've heard that using the skipping techniques, people have been able to transmitt a coherent signal over thousands of miles with only 1W of power. The antenna can be a very long cable. Best case scenario the antenna must be 600m long to match the lambda of the wave. Worst case scenario is 75m which will decrease the efficiency to 12.5%. There is a certain technique used to minimise antenna size. Typically it's used in receivers, but i guess you can use it to transmitt too. I must warn you though, anything less than the full wavelength will result in a diminishing efficiency. You can use a loop antenna with much less loops than normal. The idea is to bring some reactance to the equation to match the antenna Impedance. Such an antenna is very resonant and frequency specific, and can be 8 turns on a 15cm diameter in this case. This way the reactance will be 50ohms, thereby matching the output impedance of the amp. As i said the efficiency will be extremely low and will probably shorten the radius to maybe a few hundred meters. The beauty of this amplifier is that you can build it with almost any transistor. At 500Khz even 2N3055 will work flawlessly. Anyway this is far from perfect. Much more complex systems are used to ensure stability, but if a zombie apocalypse comes, this would have to suffice :)
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