Which feed for f/D 0.25?

  • Hi folks,



    I have tried to improve my uplink with a 120cm dish. This is an old commercial dish with a low f/D 0.25.

    Now I am using linear polarised WiFi grid antenna PW 24 dBi,


    Due the low f/D I have choose K3TZ patch feed. I was calculating with a gain of 3dB for circular polarisation and 6dB for mechanical dimensions.


    Test shows only +3dB.


    I can confirm that th feed was placed correctly in focal point and SWR was not bad (indication on LZ5HP's XVRT), The reason why I did not reach the estimated gain is probably that the feed has not so wide radiation angle as I need for f/D 0.25.


    To imagine that 120cm dish has 30.8cm depth and focal point is 29.2cm from it depth.


    Has anybody an idea for circular feed for this parabolic dish, please let me know.



  • om0aao Hi Viliam, when looking at the middle photo with the assembled patch, I can add two more things that possibly affect performance:

    • grounding contact of the N-connector body. It seems to be on "stand-offs" and would be better when attached over a large contact-area
    • the black isolating/support (material?) in between the radiator and reflector may cause degradation. Small Teflon bushings would be better perhaps.

    73 Ed

  • PA1EW Hi Ed, thanks for suggestions! You have rights, the N-connector body stands off the reflector a little bit. Good point!


    It is small picture - the black material is 3mm drill used for correct spacing. There is nothing, the plate is soldered and holded by the live pin of the N-connector.


    Probably this type of feed is not suitable for f/D 0.25, the parabolic dish effeciency is only 50% depending on the study:


    [Blocked Image: http://2ingandlin.se/432_MHz_ES5PC_patch_feed_in_dish.gif]


    Probably the dish was designed originaly to feed with a dipole and rod reflector spaced 0.24 lambda or so. There is teoretical effeciency more than 70%

    http://www.w1ghz.org/antbook/chap6-2.pdf


    I rather changed my plans and order 100cm TV dish with higher f/D and will try helix. Think that it should work.


    Thanks!

  • om0aao

    rather feed the TV-dish with a patch feed than with a helix.

    Today I tested the 5 turn Helix from WINKLER (which I adjusted for 2400 MHz, because from factory it was horrible) as a feed against the POTY-feed. The difference measured via QO-100 was 3dB in favorit for the patch. (I tested with a 1,2m offset dish)


    73 de Johannes

  • Unfortunately the dish was damaged in transport. But Santa (in June, hi) brings me a present - D6 analyser. Not calibrated yet, but it helps me a lot.


    There are 2 screens, one is before aligment, second after. It was tuned toho high.



    Did anybody try to Fred deep dish with a moxon? Or 2 moxons phased fór circular polarisation?