Enlargement of the reflector on ​​a PF dish ?

  • It is theoretically the case that if the antenna area is doubled, the gain of the antenna also increases twice. (3dB)

    How is that with a Prime Focus Dish, which is already very well illuminated with a patch (POTY) at f / D of 0.45?

    In this case, is there a profit if the reflector surface is enlarged as shown in the picture?

    I can not imagine that in the "over iluminating" of the dish are still 3dB performance.

    73´s de Robert, DD4YR

  • To double the surface of a dish it is necessary to increase the diameter, however the focal length stays unchanged. Therefore the f/D will be lower. In your example f/D will be something like 0,3 ..0,35. A POTY feed will probably not work well in this configuration. To justify the effort of enlarging the dish, a suitable feed would be needed.

  • Hello,
    as DH1DA has mentioned you will reduce the f/D of the dish when enlarging the diameter while keeping the focal length the same.

    This will be very beneficial especially on the receive side because the round waveguide has an opeing angle of about 130 degrees.

    In receive mode you actually want to optimize G/T and not G. The deeper dish will pickup much less ground noise.

    I am using a 1.8m PF dish with a f/D of 0.29 and even though I am underilluminating the dish I am very happy with the results.

    NB transponder noise floor is about 8dB and WB beacon MER is about 9.8dB,

    The POTY is certainly much better for dishes with a low f/D (my f/D=0.29 is clearly on the low end of the recommended range).

    Kind regards



  • Armin and Matthias, thanks for the feedback. Now I understand the relationships f / D and focus. I am currently considering upgrading my 1.2m PF dish, will check the dimensions again to see if that makes sense with POTY as feed.

    73´s de Robert

  • In the case of my second dish it would probably not be a good idea to increase the diameter from 1.2m to 1.8m ...



    The f / D would decrease to 0.25, and the PATCH feed certainly doesn't have an opening angle of 180 degrees....

    maybe useful for a dipole ... :-)

    73´s de Robert

  • Hi Robert,

    yes, that is a bit too deep.

    However my 1.8m dish has an f/D of 0.29 and really works well in reception.

    Please also note, that the precision of the extension needs to be very precise on 10 GHz. it is not only the mesh size which need to be about 3mm max but especially the precision of the curvature.

    I had used before a mesh dish with 1.9m diameter and f/D of 0.4 and when changing to the solid dish especially the reception improved a lot.

    Kind regards



  • Robert

    you can make a mall on a foam plate, calculating X and Y axis values and write them down on the foam. You will get a nice parabole and cut it out carefully. Check the shape off your dish and use the mall for resizing your dish to a bigger reflector. Also the position of the feed changes. When using mesh on 3cm, theory says 1/10(3mm) but that is too big. In practice 3mm*3mm is maximum use on 3.4GHZ. Try to use 1mm*1mm or smaller. Also the shape of the complete dish surface must be within 1.5mm.

    I made a mall for a mesh dish extension from 1.90m to 2.7m,but doing the extension will be in a few weeks when it's getting warmer.

    73's Jerry