You want to convert a surplus parabolic dish for amateur radio use, the gain of this antenna depends on:

The gain of a parabolic dish antenna is fundamentally determined by its aperture area relative to the wavelength of the signal. The larger the diameter of the dish in terms of operating wavelengths, the higher the gain it can achieve. While factors like the feed polarization, focal length, and material composition are important for impedance matching, efficiency, and structural integrity, the theoretical maximum gain is mathematically dictated strictly by the dish's diameter in wavelengths. A larger electrical diameter focuses a narrower, more intense beam.