At 11/30/2007 09:09, you wrote: >HI, guys! > >I'm not sure I'd agree that the theory is sound. Remember that a >satellite dish is working in what's called optical mode. This means that >the reflector is creating an image of the source, much as a Newtonian >telescope reflector forms an image consisting of concentrated star light >(when used as an astronomical telescope). In the case of the satellite >dish, it normally forms a "bright" concentrated spot of microwave energy >at the focus of the dish. The feedhorn is placed here to collect this >concentrated energy, and the attached LNB receives it and converts it to a >lower range of frequencies that the IRD or satellite receiver in the house. > >To create an image, a reflector needs to be at a minimum of a certain >number of wavelengths of the energy it is attempting to collect. I don't >remember the exact number, but it's on the order of a dozen or so. Larger >diameters are better, hence reflectors such as the one a Aricebo, Puerto >Rico, which is about a mile in diameter. As the reflector gets smaller >and smaller, the quality of the reflected image gets worse and worse, >until it's simply acting as flat reflector, with no image at all. > >UHF TV has wavelengths on the order of a meter in length (400MHz has a >3/4meter wavelength for example). Thus to act as a true optical >reflector, the dish would really need to be on the order of 12 meters in >diameter, at the minimum. > >That said, such a use for an old TVRO dish would result in somewhat better >signal strength (if implemented properly) than the flat reflector >typically used in a bowtie antenna setup, but I don't know if I'd consider >the difference worth the effort of mounting such a dish on the roof of my >house.
I cranked through the equations for the gain of a 10 foot dish @ 600 MHz, assuming 50% illumination efficiency, & got around 21 dB of gain. Not bad, but in practice the hard part is properly illuminating the dish with your feed antenna. If the feed pattern is too broad, too much energy spills over the edge of the dish & is lost; if the feed pattern is too narrow, the effective area of the dish is reduced, along with the actual gain. You really need to know the beam pattern of your UHF feed before you can successfully integrate it into a microwave dish reflector. Bob
