"Art Unwin" wrote in message
...
What is the main factor that prevents HF radiation from focussing
for extra gain?

1, Focusing the radiation involves using a parabolic antenna whose surface
area is several wavelengths in length
2. Since our definition of HF includes frequencies between 1.8 Mhz and
50mhz, the physical size of the reflector required would be too large for
the average ham to handle.
3. However, that does not prevent HF radiation from being focused, it just
address the practicality concerning costs.
Therefore the answer is: There's nothing to prevent HF radiation for
focusing for extra gain, as long as you build the antenna or beam in
proportion to the wavelengths. If the wavelength is 10,000 times larger than
that old TV dish, then simply build your "dish" 10,000 times larger, and in
the same proportions.