The waveguide that feeds the horn is the bottom of a Tetra Brik. The horn is made of cardboard covered inside with kitchen aluminium foil. Four pieces of cardboard covered with aluminium foil make the horn.
The box is 42x25x25 cm (16-1/2 x 9-7/8 x 9-7/8 in.). The horn’s aperture is about 25×25 cm². The Tetra Brik bottom is about 8.5 cm (3-11/32 in.) long (the ~3 cm (1-1/8 in.) rod is at about the middle). Four pieces of cardboard covered with aluminium foil make the horn. They are about 32 cm (12-9/16 in.) long. Their big side is 25 cm (to match the box side), and the small sides match the milk box (6.5 cm and 9.5 cm, or 2-1/2 and 3-3/4 in.).
Of course, maybe it would be better to use something other than cardboard, but it was so easy. I seem to get 12 dB more than with a Ricore can (diameter ~10 cm or 3-15/16 in.). When one knows that with 6 dB one gets double the gain, here I multiply the distance by four (with one horn). With two Ricore cans, we made more than 5 kilometers.
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