LNB De-sensing Question

  • From another thread, there has been a question raised about whether an LNB will suffer desensing or overload when the 2.4GHz uplink transmitter is active.

    2.4GHz should not propagate into the LNB input, as the waveguide is well below cutoff at 2.4GHz, although evanescent coupling may be possible? What may be of greater concern is whether there may be leakage into the high gain IF stages in the LNB.

    I wonder if anyone has tried this yet (while not pointing the dish anywhere near to the satellite!).



  • Hi Charlie - and Petra

    made a lot of tests during the last days. My first setup for fixed operation (portable/travel setup under construction) I am lucky enough for a spacious housewall.. 1.4m dish vor RX and Kuhne LNB 1139 MHz IF RTLSDR. 0,60m dish with the famous G3RUH patchfeed 2m separated horizontally from RX-dish. With 20W RF absolutely no desensing - barely nothing.

    Yes I remember the first attempts in L/S-mode on AO-40. YOUR 100dB notchfilter solved the problem.

  • Hi Rolf and Peter

    Thanks for your observations. I was wondering mainly how immune the downconverters are to a strong local field from the transmitting antenna, particularly in the case of dual band feeds where the downconverter is physically very close to the TX antenna.

    As Peter has found there is very high attenuation of the 2.4GHz signal at the RF input to the LNB.



  • I guess we'll get a lot of experience with this once the transponder comes on.

    It should not be too hard to develop a filter (eg low pass or tuned notches) that can deal with such harmonics, if they turn out to be a problem.



  • I guess we'll get a lot of experience with this once the transponder comes on.

    When the transponder was on the weekend before christmas, it was very very strong... we'll have to see how it will be when there are 50 users talking at the same time, and what it will be for "weak stations" when the "alligators" are cranking up the AGC, but it looked a lot like there will be no receive issues at all.

  • Penso che la quarta e la quinta armonica di 2400 MHz, quando presenti, potrebbero essere un problema.

    I believe that the fourth harmonic may give some minor annoyance on the rx part for those who use the classic octagon LNB system to pll given that its OL is very close to the frequency 9750 Ghz. But a good filter certainly should restore proper operation. the previous tests I think are not big problems in any case we'll see.

  • Honestly, did you ever hear so called alligators on L/S mode AO-40? at least DK2ZF never. 1269 MHz was hard to reach for most of the people in 2002

    No, I didn't, but it has changed quite a lot over those years. We will have to wait and see what happens.

    But anyhow, the level of a single station is of course a bit difficult to derive from the tests that they did. I saw both tests where they saturated the transponder and were very very strong, and tests which likely were at the normal operating point, where I could see the noisefloor of the transponder very clearly above the noise of my receiver (15-20dB).

    As long as that remains the case, we should not be to worried about receive performance.

    I never was able to achieve such performance on Oscar 10 and 13 with my "small" system (3m crossed yagis on 2m and 70cm) so I was more in danger of being an alligator because of not hearing enough (but I did not have much uplink power either).

    The main danger are the stations that cannot hear themselves (for whatever reason) and overdrive the transponder without knowing it.