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'd rather
get the whole antenna assembly up higher in the air if I had the choice.
Yes, I'm saying that the three or four foot parabolic UHF antennas one used to
see on fringe area houses and bars weren't really working like satellite
dishes, and were only marginally better than a good Yagi style antenna. Look
at the polar pick-up patterns in their spec sheets to see.
Hope this helps!
Guy Clark
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