Minimal Mods Make Commodity LNBs Work For QO-100 Reception

A word of advice: If you see an old direct satellite TV dish put out to the curb, grab it before the trash collector does. Like microwave ovens, satellite dishes are an e-waste wonderland, and just throwing them away before taking out the good stuff would be a shame. And with dishes, the good stuff basically amounts to the bit at the end of the arm that contains the feedhorn and low-noise block downconverter (LNB).

But what does one do with such a thing once it’s harvested? Lots of stuff, including modifying it for use with the QO-100 geosynchronous satellite (German link). That’s what [Sebastian Westerhold] and [Celin Matlinski] did with a commodity LNB, although it seems more like something scored on the cheap from one of the usual sources rather than picking through trash. Either way, these LNBs are highly integrated devices that at built specifically for satellite TV use, but with just a little persuasion can be nudged into the K-band to receive the downlink signals from hams using QO-100 as a repeater.

The mods are simple — snipping out the 25 MHz reference crystal on the LNB board and replacing it with a simple LC bandpass filter. This allows the local oscillator on the LNB to be referenced to an external signal generator; when fed with a 25.78 MHz signal, it’s enough to goose the LNB up to 10,490 MHz — right about the downlink frequency. [Sebastian] and [Celin] tested the mods and found that it was easily able to detect the third harmonics of a 3.5-ish GHz signal.

As for testing on actual downlink signals from the satellite, that’ll have to wait. For now, if you’re interested in satellite comms, and you live on the third of the planet covered by QO-100, keep an eye out for those e-waste LNBs and get to work.

15 thoughts on “Minimal Mods Make Commodity LNBs Work For QO-100 Reception

  1. This type of mod is also useful if you wish to have more frequency stability and precision while listening to stuff in the microwave bands using LNB’s, using a precision frequency source in one’s radio shack.
    Only question I have is what the common LO crystal frequencies for LNB’s for different satellite bands are.

      1. Ha! It’s not quite *that* simple as you also need to adjust the arm length / the focal point position for the LNB to receive the optimal 4G signal. I’m absolutely no expert on it but I spent a while looking at different antenna designs and had been tempted by a single yagi, only to then read that it would be better to have two at perpendicular angles to each other, only to *then* come across the math for repurposing a satellite TV antenna for receiving 4G.
        I have a 4G modem stick (with an SMA connector for external wand antenna) and figured I could mount that external wand antenna to the appropriate focal point in front of the parabolic dish, with the modem plugged into a router running IPFire. Haven’t actually got round to it yet but that’s the plan.

        IIUC it needs to be high up (above other properties and trees), ideally with direct line of sight to the nearest tower.

        These concepts are explained in far better detail across various YT videos and websites I came across when searching DDG for a combination of search terms related to 4G parabolic satellite TV antenna conversion but I don’t have those references to hand right now, I’m afraid.

    1. It could also be quite a neat way of ensuring you get decent quality service as, from what I heard RE Brent’s adventures on Jupiter Broadcasting, the network provider actually prioritises your coverage if you have a stronger signal, compared to the coverage of those who have weaker signal.

  2. Old TV sat dishes can also be made into stealth slot antennas for 2m or 70 cm. Doesn’t magnify the signal with the dish though, it works the same with a flat piece of metal. About the same performance as a dipole or quarter wave whip, except your HOA allows this because they don’t know it’s a ham antenna and are required by federal law to allow sat tv dishes! Google “2 meter dish slot antenna” for details…

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