Flat scalar feed function on PF dishes ?

[Vipersan]

Hi Friends ..
I'm Sat here trying to get a mental image of how a multiple ring flat scalar feed actually works ..
That it does is not in question ..but how ?

I fanally came to the conclusion that It works in 2 ways ..
First as a wave block and as a 'lens' of sorts ..
I have drawn a rough diagram of how I belive it functions ...but any additional info and comments would be most helpful and enlightening ..
In my diagram the red line indicates a direct beam from the bird ...which would obviously hit the back of the lnb assembly and be blocked..
The green beams indicate signal which is more direct and will be almost certainly bounced straight into the lnbs waveguide ..
The blue beams ((weaker blues to the dish extremeties - darker the closer to the centre dish 'sweat spot' ))...indicate waves which may or may not bounce directly into the guide ..but represent near miss beams which are focussed to a point outside the waveguide ..
I assume that these near miss beams are collected by the scalar rings and set up standing waves within the valleys between the rings (fequency specific for 'accept' due to the distance apart of the rings..
These standing waves eventually escape and are bounced back to the dish surface only to be relected again ...but this time relected from nearer the centre 'more focused' part of the parabola ...and therefore eventually into the waveguide of the lnb ..
For the weaker 'outer' near misses ..this may happen quite a few times ..back and forth until eventually entering the waveguide ..
Thus the rings act as a lense to focus the wanted beams/wavelengths..
Now on my diagram I've represented unwanted signals from electrical background radiation etc ..as a Brown line ..
This too will be reflected and arrive close to the lnb waveguide ...but since it is the wrong wavelength ..ie unwanted signal ...it cannot set up standing waves within the scalar ring valleys ...and therefore are not accumilated ...bounced back and forth ...and 'amplified...
Therefore these signals disperse and die ..
Those unwanted signals are effectively filtered out ..
The net effect is amplification of wanted signals and rejection of unwanted signals ..
and therefore an increase in the signal to noise ratio ..
Ok ..I'm done ...but if my crude theory is wrong ..then please would some kind soul enlighten me ...and stop me scratching a hole in my head ..
;)
rgds
VS

 

[Channel Hopper]

_http://web.archive.org/web/20050324030215/http://astron.berkeley.edu/ral/ata3.pdf

 

[Vipersan]

Thanks CH ..
A very useful pdf ...
Sadly I never was much good at maths ...so a lot of it zips over my head ..
It certainly indicates the relative usefulness and criticality of the scalar plate (choke) relative to the lnb feed ...and of course these again are both relative to the whole assemblys position relative to the focal point of the dish..
Sadly it still doesn't explain the actual focusing action of the rings and just how this is acheived ...
Does it use a back and forth reflecting action to the centre of the dish ....or are standing waves created like a cloud accross the rings 'surface' ...
The math is interesting but I'd love to see an atual diagram of the relective physics ...
Just so I can get a mental picture of whats actuallay going on ..
On final thing ..
Is it correct to assume the lnb should be nounted first without the choke ..
Its mouth postioned at the focal point ..
Then the choke added and moved back and forth along the waveguide to optimise signal to noise ...ie signal quality ..
Or should the choke be nounted a fixed distange from the waveguide mouth ..then the whole assembly moved back and forth to acheive best results..
In my case ..I have no references ..
I'm using an lnb that was never deigned for my dish specifically ..
On a homebrew mount ..as the dish never had a cband mount ...
And using a multi ring choke canabalised from a corotor...
So its all a bit hit'n'miss ...but I'm desperate to get maximum gain and signal quality accross the band ...as I lurrv the dx-ing for cband fringe stuff..
rgds
VS

 

[Channel Hopper]

The link does confirm the focal point of the dish is altered by the introduction of the scalar rings.

I would really have to delve back into my technical journals from the 90's to give some answers on exactly how it works. Without going into the maths though, you won't get any answers on how scalar rings alter the radiation pattern at the focal point. Suffice to say though, the linear distance of the rings (and the depth of them) is linked to the amount of incoming signal being reflected, and/or absorbed, you want more of the original transmission, and less of the noise, the scalar feed achieves both to some degree, which is why it is often called a choke.

In the meantime though you could return to the other thread on depolarisers as there were some interesting developments in signal improvement, which I believe has relevance to your idea of signal 'alignment'.

http://www.satellites.co.uk/forums/...ope/156485-dielectric-plate-4.html#post880738

 

[Vipersan]

Many thanks CH ..
Very informative ..
The one think I can definately confirm from practical testing ...ie trial and error ..
The 'borrowed' scalar feed from the corotor definately outperforms the other 2 I own ..
The Pauxis came supplied with a single ring plate approx 1.2 an inch deep ..
The BSC421 came supplied with a 2 ring plate approx 1/2 inch deep valleys ..
The borrowed corotor plate has 3 rings and the 'valleys' are approx 3/4 inch deep ..
Needless to say this borrowed plate ouperforms the BSC421 plate ..
..which in turn outperforms the Pauxis single ring feed..
The dramatic improvements and fringe captures of late are soley down to the addition of the corotor plate ...but optimising its positioning is critical and crucial ..
hence my interest and questions..
rgds
VS

 

[Channel Hopper]

Just as an aside, if you do have some radiator reflective tape (self adhesive) try sticking it over the petal gaps tomorrow to eke out that extra 4% or so.

You could also give the whole dish a shakedown test if you have a couple of spare hours, the 1.8m Alcoa really does benefit from one.

 

[Vipersan]

Looks like our minds think much alike CH ..
I already washed the dish ...and ordered some 1 inch aluminium tape off ebay ...
..last night ..
In order to do the very thing you just suggested ..
Should I have ordered wider tape ? ...or do you think it will be adequate..
?
Rgds
VS

ps
..also sent an email to inverto to ask about this product :- IDLR-TWNSRC00-CLASC-OPP
It being a dual feed lnbf with matching feed plate ..
I do use 2 receivers with my dish ..so switching required atm ..
This presumably inserts some losses ..though minimal

_http://www.inverto.tv/products/product.php?section=1&id=65

Im assuming the inverto having a twin output will negate this need for switching ..but as usual ..
No one appears to sell it ..nor is there much info around..
Are you in any way familliar with it CH ?
or indeed anyone else ?

 

[Channel Hopper]

1" tape should be fine, though I would use double thickness to improve Cband reflection.

A shakedown is very important before you apply the tape though.

As for the Inverto, 13k sounds about right (you would have to drop down to 7K or so to improve the nF by 0.1d, though there are many more specifications that make a difference, and the picture looks just like thousands of other brands.

Other countries have the same LNB down as the 'pro' version

_http://translate.googleusercontent.com/translate_c?hl=en&sl=ru&u=http://www.inverto.com.ua/index.php%3Foption%3Dcom_content%26view%3Darticle%26id%3D133%253Aidlb-twnsnc00-oopro-opp%26catid%3D39%253Ablack-pro%26Itemid%3D63&prev=/search%3Fq%3DIDLR-TWNSRC00%26hl%3Den&rurl=translate.google.co.uk&usg=ALkJrhg-K9uihoE8bNGr9qJScf1RFIX-BA

 

[pride21]

hi vipersan i have tried different scalers,i have 12mm depth rings and 20mm depth rings the signal was much better across the band with the 20mm depth than the 12mm.