QO100 reception with commercial LNB

[IU2KAC]

I've always wondered why a common commercial LNB (frequency 10.7-12.75 GHz) can receive the Beacon at 10491.5 MHz, which is out of range.
I'd appreciate a technical explanation.
Thank you!

Liborio,IU2KAC

[g0mjw]

Fairly simple, an LNB is an LO and a mixer and some gain. What they don't have is very much in the way of filtering, so although with a 9.75 GHz LO the 10.3-10.5 GHz amateur band where QO100 transmits converts down to a 550-750 MHz IF rather than the minimum 950MHz IF the LNB is designed for operating at 10.7 GHz, it usually still works. The gain is usually quite a bit less, especially at the bottom end but it is still still more than enough. The QO100 beacon comes out at 741.5 MHz with the 9750 MHz LO.

The typical LNB (old type) has 9750 MHz or 10600 MHz switched by a 22kHz DiSEQ tone. It defaults to 9750 MHz which is handy for us as it means no tone needed.

So, for once not much filtering helps us. The front end is wider band limited by the waveguide at the lower end, it can't be too sharp or it would impact the intended band. The lowest frequency that the IF amplifier works at depends on the chip but often down to VHF, so even 10 GHz might get through to some extent.

Mike

[IU2KAC]

Thanks, Mike.

So, in your opinion, the same thing also applies to the S-Band Downconverter?

Practical example: Can a Downconverter that reports 2.5 - 2.7 GHz receive 2.4 GHz?

Liborio

[g0mjw]

No idea, Try it.

[radiogareth]

About 25+ years ago it was popular to use a Drake (branded) downconverter for use on 2320. From memory that DID need some changes to the internal filtering.
I have one of these:http://www.g3pho.free-online.co.uk/microwaves/drake.htm
Gareth