| Subject: Re: HollStar Dishes |
Author:
JVT
|
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Date Posted: 06:08:58 01/07/04 Wed
In reply to:
Outsider
's message, "Re: HollStar Dishes" on 05:29:23 01/07/04 Wed
>>>What did you mean by "I'd tried measuring the focal
>>>length and diameter and checked the ratio and there's
>>>not much in it", do you mean that the values were
>>>equal in both dishes? I measured my Triax TD88 (well,
>>>actually a noname one, but the construction easily
>>>reveals the origin) and got F/D=0.6, quite ordinary
>>>value. I was just thinking that if Tursat has
>>>essentially different F/D, it may need different
>>>feed/LNBF to work well.
>>>
>>>Please, note that the axial placement of the feed is
>>>very critical, moving it something like one
>>>wavelength, about 3cm (I do not have any reference
>>>readily at hand to check the exact value) in either
>>>direction makes a complete null, moving it further
>>>away gets other minimums and maximums, you should not
>>>try to move it too much!
>>
>>The F/D ratio values for both the 110 TRIAX and the
>>Tursat 130 were about 0.58 in both cases. I've not
>>tried using a feedhorn at all on either of the dishes.
>>
>>
>>In terms of axial displacement of the LNB on the
>>Tursat I moved the LNB mm by mm along as full a range
>>of positions as was possible, even with main support
>>arm attached at the limit of it's fixings. Needless
>>to say, re-mounting the TRIAX resulted in a return to
>>viewing. The Tursat dish is soon heading for the tip
>>I think ! I'll look into an alternative large dish
>>later in the new year. Thanks for your tips.
>>
>>Happy New Year.
>
>A bit puzzled! An 105cm x 100cm dish with gain 40.2 dB
>(i.e. the specifications for Triax TD110) would have
>efficiency of appr. 66%, an 1.3m wide offset dish with
>equal gain would have efficiency less than 40%, quite
>poor indeed. Sure, the Tursat dish wasn't damaged
>during transport? Larger dents or especially warp can
>totally ruin the performance.
Hi again Outsider,
there was a very minor dent on the LHS outer rim of the dish but it appears not to have distorted the front face of the dish. Don't think that the dent in the rim would have that great an effect. I think the problem is due to the accuracy of alignment of the LNB main arm and the two supporting arms. The other possibility is that the vertical mounting of the dish is very slightly out. I've found that I sometimes get a good "Locked On" alignment but with Zeros in the Ident and Stream fields when the dish is not completely vertical (this happens with the Triax too). e.g if the dish mounting bracket is slightly off centred on the pole mount and is tightened such that the dish is slightly off the vertical. Do you have a "VoyUser email address, I can send you some JPEG snaps ?
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