How to calculate total gain of parabolic dish AND helix feed?

  • How to calculate total gain of parabolic dish AND helix feed correctly ?

    Do I sum the (for example 5 turn) helix gain of 7dB AND the dish gain of 25dB so gain is 32dB?

    Or do I use only the dish gain of 25dB and does not add helix gain as it assumes already proper illumination....?

  • :) The specific question was more like: if the feed has 5dB gain, do we add 5dB to the gain given by a standard parabolic dish gain formula?5dB is not insignificant.

    (how is the efficiency factor in the above formula affected by feed illumination etc).

  • In addition to DJ7GP Peter : the 'gain' of the feed is actually the 'opening angle', i.e. the narrowness/broadness of the lobe/radiation pattern illuminating the dish.

    Thus, it is NOT when you've a 60cm dish, having ~20 dB gain @2.4GHz, and illuminate this with a Yagi having 10 dBi gain, the total gain is 20 + 10 = 30 dBi.

    (Sorry ; -)

  • It is important to optimally illuminate the parabolic dish. Usually, the field strength at the edge of the dish should be about 10 dB lower than in the middle of the dish. Thus, it also becomes clear that deep dishes with a small F/D (e.g., 0.4) require a feed with a larger aperture angle than flat mirrors with large F/D (e.g., 0.7). In fact, feeds with a smaller opening angle have a higher gain than those with a large opening angle. It follows: With the YAGI mentioned by Remco with 10 dB gain a deep mirror can not be optimally illuminated!

    More comprehensive information on this topic can be found at:

    73! Peter, DJ7GP

  • The maximum gain depends on the area of the dish, the actual gain depends on how well it is illuminated. You are doing well if you get 60-70% efficiency. The helix must be matched in gain to the focal length of the dish (and it must be at the focus). There will be additional loss from feed blockage for a prime focus dish. See W1GHZ excellent online papers.

    If you dish gain is 25dB, then expect 23 dB in practice, if your feed radiation pattern is well matched to the dish.

    2.4 GHz Gain

    0.6m 21-22 dB

    0.9m 24.5-25.5 dB

    1.2m 27-28 dB

    1.5m 29-30 dB

    1.8m 30.5 - 31.5 dB

    2.4m 33-34 dB