| Subject: Re: HollStar Dishes |
Author:
JVT
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Date Posted: 06:19:06 01/08/04 Thu
In reply to:
Outsider
's message, "Re: HollStar Dishes" on 03:21:17 01/08/04 Thu
>>>>>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/PrimeFo
>cus_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.
Hi again Outsider,
Thanks for the advice. I'll have a look at Bobben's site - My otherhalf is Norwegian and I can understand a little Swedish so I'm sure there'll be something there I can understand. I'm not registered on Digital Spy.
I'll hang fire on tipping the 1.3m dish then and see what I can discover from your tips. Thanks.
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