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A satellite dish with LNB connected to a satellite detector when powered by a 18v source could detect radio waves from satellites. I made one using the ppt here. The detector, (when it was not connected to the LNB) detected the radio frequencies from my mobile, but as soon as the LNB was connected the detector did not respond to the mobile phone even in proximity.why?

To convert the analog output to digital should i use a bias tee injector near the 18 v source? Would a 1000 micro henry RF choke suffice?

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The satellite detector is looking for IF frequencies from the LNB, which are typically between 950 - 2150 MHz, depending on how you are powering the LNB (13v or 18v, with or without 22kHz tone). Many mobile bands are also in this spectrum, except the super-high LTE bands e.g. 7, 30, 38, 40, 41.

Without any attachment to the LNB the unterminated F connector on the satellite finder (or if you have the coax attached, the unterminated end of that) acts as a poor antenna, but effective enough to pick up the strong signal from your mobile device.

When the LNB is attached, the input port to the satellite finder is now correctly terminated with 50ohms and will not act as an antenna. As alex.forencich correctly said, the LNB is filtering only for Ku-band signals from the sky, and the IF frequencies will be filtered out in its RF front-end.

You can use a bias tee (you will also need a capacitor to stop the 18v DC from going into whatever ADC/decoder you plan to use), but the scenario does not seem to make sense - the LNB is not useful for mobile signals, the satellite finder itself is not providing you with any gain, and so the signal you will retrieve from the satellite finder output will still be a low-level mobile signal requiring down-conversion and decoding (and likely amplification and filtering). Do not underestimate the complexity of decoding a modern mobile signal... 2G/GSM signals might provide the most interesting starting point due to their time-division nature, you might be able to see it on an oscilloscope once down-converted to baseband.

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  • \$\begingroup\$ Thanks @Mark Ch i am actually trying to build a solar radio telescope and view it in a radio spectrum analyzer software called radio eyes. The above incident was an accident. I was calling my friend and I came near the device and found out this. Thanks for a clear explanation. \$\endgroup\$ – Mathav Raj Oct 9 '15 at 14:06
  • \$\begingroup\$ Good luck with the project, it sounds fun! Out of curiosity, will you be using an RF capture card for the PC, and if so which one? \$\endgroup\$ – Mark Ch Oct 9 '15 at 14:28
  • \$\begingroup\$ you mean the interface device? i don't know much about it, but the site link i referred recommends this RTL2832U dongle \$\endgroup\$ – Mathav Raj Oct 9 '15 at 17:25
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The LNB is a block downconverter with a lot of filtering. Unless your phone transmits at 10 GHz or so, the signal will be blocked by the LNB. If it doesn't block it, then cell phone signals would interfere with your satellite tv.

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