| Subject: Re: HollStar Dishes |
Author:
Outsider
|
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Date Posted: 03:21:17 01/08/04 Thu
In reply to:
JVT
's message, "Re: HollStar Dishes" on 06:08:58 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
>?
By 'slighly off the vertical' you mean that the dish itself has some 'skew'? Difficult to see how this would cause any problems if correct skew is set by twisting the LNB.
F/D and LNBF being equal, what remains are mainly the mechanical tolerancies, whether it's the LNB positon or the accuracy of the dish itself, difficult to say. Btw, there's a link http://home.online.no/~jensts/Satellite/Antenna/PrimeFocus_vs_offset.pdf to a nice article about the factors affecting the dish efficiency on Bobbens homepage although it may become as a small disappointment to some would-be readers that it's all in Norwegian ;-)
Likely you know all this already but some notes on dish alignment:
* Do NOT use Astra 2D to align the dish but some of the stronger Astra 2A/2B transponders, to avoid unnecessary struggle with loss of sync, they ARE all in the same orbital position!
* Some Eurobird TPs (like the Kingston test card at 11426H) could be nice for optimizing the skew because those have vertical and horizontal transponders at the same frequencies (== even a small error in skew gets lots of crosspol interference from the opposite polarization)
* For fine tuning use a receiver that has a good signal QUALITY indicator. My MM9602S has a signal quality display with a resolution of app. 0.4dB and that's adequate although thre's still quite wide 'flat' area before the quality begins to impair when alignment is changed.
* If you change LNB location, always realign the dish after that because the dish alignment is the most critical adjustement!
* Tightening of the clamps easily ruins the alignment totally, do not tighten them until you are satisfied with the result and then do it carefully making sure you maintain the performance that was achieved. I do not usually open the clamps at all when I have to realign the dish for another satellite, just tap and wedge the whole mount untill it's OK! ;-)
I do not have any VoyUser account, I could PM you in Digital Spy if you have registered there.
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