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In case of an apocalypse, first thing you gotta do is contact other survivors. The best option is not VHF/UHF, because these waves are limited up to a 100km or so. Anything above, and you gotta spend tremendous amounts of power to increase the distance. At MW/LW, the range is almost unrestricted by power or terrain, but mostly by receiver sensitivity and EM noise. Other advantages include simplicity and frequency stability in regards to the simple design. Terrain is almost a non-factor, because these wavelengths are ,,ground waves'', as they use the ground conductivity to propagate themselves along the terrain, and mountains and such are not a difficult obstacle to overcome. Ground conductivity is an important factor though. It may severely deteriorate the range if poor. The two major disadvantages are the antenna requirements, and the bandwidth restrictions, where at these frequencies, you can only encode low resolution voice. To radiate successfully and efficiently a 1MHz wave you need an antenna the size of the Eifel Tower (as it was historically used as such during WWII). Luckily almost every major city has an antenna that fits the requirements, mostly used for long range aeronautic communication or AM radio transmissions.
Okay now to the circuit design. Luckily the design is pretty simple and straightforward and can easily be built with salvaged materials. The input can be supplied by a simple function generator, one that any lab has. The rf transformer is air wound, quite simple actually, and the audio transformer is a 1:1 audio transformer, one that almost any old UK tube television set contains. The output filtering is a bit more complicated, but frankly you'd be better off without it, as the harmonics and subharmonics will populate a very wide band and increase the odds of your transmissions being heard by someone. All you need to do is supply power and hook up the antenna. With a 140W, a proper antenna, a good receiver and the right atmospheric conditions, a 1MHz wave could easily DX across an entire continent, maybe even a successful transatlantic connection could be established. I've heard of instances where as much as 20W have made a successful intercontinental connection.
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