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an LNB (the thing that goes in the focus of a parabolic antena) is really cheap and easy to power

the probem would be that the cheap dtv (RTL-SDR) only gets up to 2.4MHz of spectrum bandwith, a typical DVB-S mux can be around 20MHz or the DVB-T terrestrial standard, 8MHz

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The LNBs on ebay are pretty cheap but the intermediate frequency some of them use use is a bit too high. And as you say. You'd just be finding some slice of the signal without any hope of demodulation. I still think a microphone or rtl are a great way to play with gnuradio companion. Lime sdr is probably the next step up in price.

How can the DVB-T signal bandwidth be larger than what the 'dtv' receiver is capable of if that device was actually originally designed to receive exactly that class of signal?

The 2.8 MHz limitation comes from shipping raw RF samples off the device over USB 2. When operating as a DVB-T receiver the device decodes the signals before sending them over USB so it uses less bandwidth.

The device was originally intended only as a dtv receiver and not a general purpose software defined radio. The hobbyist/ hacker community discovered the hidden debug mode that allows raw data acquisition and wrote drivers for it.

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