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Re: IC-910 / 3731



Hi Drew,

Whooops! I was thinking of an 18" dish - not 60cm -
I apologize for the misleading comments.

To make up for it, I did a quick model to generate
some comparisons and here is what I get:

1. For a 60cm prime focus, .35 f/d dish with a G3RUH
    patch feed, the blockage is around .7 - .8 dB,
    not really that bad. The overall gain of this dish
    ought to be around 21.5 dB which is pretty close to
    the 21 dB that I read was measured on a G3RUH dish.

2. For a 60cm, .6 f/d offset fed dish with a -10dB taper
    feed, I get around 22.5 dB gain. This is because there
    is no feed blockage and you can illuminate the dish
    a little more without increasing the noise (since the
    spillover is into space.)

So, to compare, a (properly fed) 60 cm offset dish ought to be
somewhere around 1 dB (maybe 1.5 dB) better than a 60 cm prime
focus dish. I agree with you though, the prime focus dish is
much easier to feed and set up.

Cheers,
Tony AA2TX


>I'd say it depends on the size of your feed. The G3RUH 60 cm dish/patch
>combo seems to work well. I think if you modeled it the blockage is so small
>a portion of the total area the difference would be negligible.
>Empirically, I've worked a lot of guys using 60 cm dishes (them). Most of
>them seem to hear fine. Look at the fine results with W9AE's DXpedition
>setup with just a AIDC D/C, no preamp......
>
>I think I'd take noise from the sidelobes of a helix (which you can reduce
>with a cupped reflector) over the noise from overilluminating a offset dish
>with a patch feed.
>
> From my experience, having used a BBQ (K5GNA), Primestars (both normal size
>and 1.2 m), DSS 18" dish (as VP2EAG), and now a 3ft prime focus (pansat Ku
>tv dish), the prime focus is by far the easiest to set up and use. Easy to
>point, easy to not overilluminate with the right F/D, and makes a good
>birdbath after Florida thunderstorms....
>
>73, Drew KO4MA
>
>P.S. I enjoyed the heck out of the symposium, and can't wait to see what you
>dream up for next year...
>
>
>
>
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