Dual band feed with LNB 21 type

  • I have recently received an RX kit from DXPatrol (CT1FUU). It. contains an LNB and a 25Mhz reference box. The LNB box says LNB 21. This will be for the downlink of my Z21NRT station which should be on the air in about three weeks time.



    Has anyone used an LNB of this type and fitted it into one of the dual band feeds which are very popular (like the PY3FYM one for example). I do not want to risk damaging it if it is not modifiable.

    Otherwise, I am not sure how else to make a single dish solution for Zimbabwe. I am going to use a BU-500 up converter with a Chinese WiFi amplifier.

    I'd appreciate advice on making this type of LNB work with a dual feed or other suggestions to achieve a one-dish solution. I may have to buy a second Tx antenna (a 2.4ghx 24db grid antenna for example and I do have a 40 turn Wimo helix antenna which I was thinking of using. Most folks have put me off that idea simply because I will not have a strong enough signal.

    Thought please?


    73 de David (G4NRT & Z21NRT)

  • Hi David,

    I did not use this LNB model before, but perhaps have some hint how to use it without further modifications to make it work as a dual band feed.

    I have done some measurements with an Octagon Twin PLL (27MHz) and a 5,25 turns LHCP helical combination (G3RUH/G6LVB design). The interaction is very small between them: SWR of the helical, mounted as depicted below, is abt 25dB and watching the strenght of a beacon the LNB signal degradation was <1dB when the Helical with reflector was slid over the LNB frontcover. All this was done at the focus-point of an offset dish. Yes, I know the helix is not exactly in focus, but the error seems acceptable.

    I was not able to finish actual transmission tests as my Kuhne MKU2424A Up-converter currently is back to the company for the modifications...

    Several people do have good results with similar constructions.


    73 Ed PA1EW

  • The above solutions are good. The easiest, non-destructive and safest way, is to simply use 2 dishes ?

    Dave Cawley | Ex G8EAO & G6ANG/T |
    (when you had to have a separate TV license !)

  • So is adding a helix in front of the LNB considered to be a better solution than chopping the feed horn off and making it fit into a dual feed (such as the PA3FYM one)?

  • G4NRT It is not absolutely necessary to chop the horn off. You can also drill a 22mm hole into the plastic cap and slide it over the tube until it makes contact with the inner parts of the horn/waveguide. Some (acid free!) silicone to make it weather proof, et voila : -)

  • Hallo,

    here is another example. It is an Octagon Twin. The feed horn of the LNB is removed. The coupling between the LNB and the 22mm copper tube is done with a brass disc. Meanwhile I am not sure if the thermal coupling with all the copper and the LNB is a good idea really? It is all soldered with hot air. The S-Band antenna seems to work, 2 Watts and a 80cm dish give a satisfied transponder response.

    My home made return loss brigde has to be improved before showing a pattern.

    73s
    Andreas, DL5CN