burts said:
i think it was on uhf satcom in the antenna gallery that i saw a dish being used for the 255mhz with what looked like a whip mounted in the middle of the dish ,
so my question was really does any old dish refect any old frequencey into a point of focus to produce a directional gain.or am i being stupid
The reflector is quite broadbanded with a couple of caveats... As PRC stated, the high frequency is limited by the accuracy of the survace of the dish. The low frequency is limited by the size. The larger the dish, and the higher the frequency, the higher the gain. A 6 foot dish mught have 30 db of gain at 2 GHz, 40 db of gain at 10 GHz, and 10 db of gain at 800 MHz. The feed antenna is usually MUCH more narrow banded than the reflector. A 6 foot grid dish could have the same 10 db at 800, 30 db at 2 GHz, and be completely useless at 10 GHz. That's where the surface accuracy part comes in.
The long answer to your question is complicated. The short answer is no, any old dish won't work on any old frequency.