Is there such a thing as a combination DSS dish and OTA antenna capable of receiving HD signals???
Not really. Some vendors make "clip-on" OTA antennae that attach to DSS dishes; these sorta work in good areas, and really are a second antenna.
If not, why not??? Thanks.
Well, DSS and OTA signals have distinctly different characterisitics, which translate into very different antenna requirements. DSS signals have a frequency of over 2000 MHz, and thus very short wavelengths. The signal source is a satellite at an altitude of 22,000 miles, located over the equator. The transmitter power is a few hundred watts at most. To receive such a signal, you need a very high gain antenna, line of sight, and a low noise front end. Fortunately, with the wavelength so short, a high antenna gain is possible. With the satelite in orbit, line of sight is also usually feasible. Finally, DSS dishes put the Low Noise Block converter right at the focus of the dish.
OTA frequency ranges are much lower, from 54 to ~700 Mhz. Thus, wavelengths are much larger, and it is much more difficult to build a high gain antenna (unless it were really huge!). Line of sight is not always possible; but at least the lower wavelength signals stand some chance of difracting around objects (simlar to water waves will difract around a pier, but a flashlight shown at the same object will simply be blocked by it). Fortunately, OTA transmitters are only 50-70 miles are so maximum distant, and transmit ERP is up to 1,000,000 watts. So for for OTA antenna system gain usually isn't the main objective, but directionality, in order to antenuate extraneous reflected signals, is.
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