LNB modification for use in dual band (LNB+Helix) feed. Is it critical where on the neck to cut the waveguide?

  • I need to cut off the outer section of the LNB (marked X) to make it easily stick thru the reflector of my helix antenna since I am making a dual band feed. Is the length of the waveguide section in front of the LNB pick up antennas critical?

    Can I cut at A, or B freely without affecting any "correct lengths" of the waveguide?

    What is the function of the outer part with the larger diameter? Choking section to prevent ground reflections? Impedance matching? Other?

  • The location of the cut is not critical. Position A would make the most sense as it gives you more to make the connection with.

    The large diameter part is the feed horn which is designed to have a radiation pattern which will illuminate the dish. In a dual band feed this needs to be replaced by a plastic dielectric lens (removed from a ‘rocket’ LNB ) which does the same thing.

  • OK thanks for info. That makes it much simpler to implement a dual feed DD0KP . Did you quantify the difference? For example, did the signal to noise ratio on the downlink become degraded without the dielectric lens or was still the transponder noise floor rised above the receiver noise (the "noise hump" was still present)?

  • My result was trying with or without lens was more or less the same. I only checked the result with sdr#. If I will find the pictures I will post this later for you..

  • With a rocket LNB lens I receive(d) the Es'hail-1 11199.8H beacon with 5 - 6 dB more SNR compared to the 'raw' waveguide. With a home brew Nylon lens the difference is 3 - 4 dB in favour of the lens.

  • For the narrowband transponder the signal is strong enough that the lens may not be essential, especially if you are using a dish larger than 60cm.

    For the wideband transponder and TV reception you will need every dB you can get.