Mad science time again ..

[Vipersan]

Probably a total mistake ...but an impulse buy made me jump on this ebay item ..
130592222969

Its a roll of aluminium insect screen mesh ..
but seemed like the kind of thing I needed to own ..
With a view to playing at making wings for my 1.8 pf
The holes /perforations are fine so shouldn't be a problem at C band frequencies ...and can easily be cut n shaped to follow the dish curvature ..
negatives include ...woven not flat ...and will require additional perimeter support ...
but ..
at £25 the lot ..I figured it fun stuff for experiments ..
Next year of course ..as its getting too cold to play out..
what do you guys think ?
Too ambitious ?
Remember ..I am a mad scientist ...and this may well be a experiment too far ..
..but ..if ya dont try ..ya don't answer those niggling questions ....What if ?
rgds
VS

 

[futumsch]

Yeah that would be interesting to see if you how far off the side of the dish you have to extend the 'wings' before you stop getting any more increase in gain. You'd probably have to 'do it by eye', keeping the shape of the dish f/d ratio etc. Or maybe use a bit of old skirting board and cut out the dish radius shape along its length and use it as a guide to extend the parabolic shape outside of the dish area.

And then as your dish is a prime focus, you could extend the legs of the lnb holder all equally, so the lnb will 'see' a larger area, and see what benefit you get by increasing the dish diameter to say 2m, and compare the gain to a conventional 2m dish. Have to do some sums there obviously to work out the focal distance required.

 

[Vipersan]

Thanks for that futumsch..
Interestingly my Alcoa pf is designed to have extension petals fitted ....without moving the tripod legs ..
I've spent years looking for a set ..but suspect they no longer exist except abroad ..on existing setups.

The FD question has arisen before ...and is debateable as to wether it changes by much with the petal extensions fitted ..
I've no doubt there would be some ajustment required ...but the pictures I've seen ..show the original tripod legs in use.
The Alcoa is a deep dish ...so may not require major changes since I only plan to make 8 inch extensions at most.
Input and suggestions are as usual ..most melcome.
rgds
VS

 

[Vipersan]

Ok ...
been seriously thinking about how this might be done ...
and the answer I came up with ....
paper..
or to be more precise ..Papier-mâché...
Im thinking of using the existing dish curvature to produce a Papier-mâché reverse mold ...or pseudo skeleton ..
Which once hardened and built up to at least 6 inches wide ...and a depth of about 3/4 inch ...covering one petal segment ...
Could then be treated with laquer ..and used to form the aluminium mesh around ...
the resulting mesh would then be covered on its outerlayer (non Papier-mâché side) with layers of fibre glass matting ...and allowed to harden ...
Once the Papier-mâché skeleton is remove ...I should in theory have a 6 inch wide petal extension ...in aluminium mesh ..backed permanently with fibre glass..which could then be drilled and mounted ..
Repeat x6 ...and job done ..
Maybe a bit oversimplified ...but what do you guys think ?
rgds
VS

 

[Topper]

Well I think if you have the patience (and I know you do) it will work although I am unsure how adjustments to alignment will be made and will an outer ring be applied around all petals when finally assembled correctly?

 

[PaulR]

Oh lordy VS. You don't like an easy life do you?

Good luck!

 

[Terryl]

The paper mache idea wont work out that good as you wont be following a true parabolic line, try this (on a small scale) if you have a 1 meter square piece of plywood (or a cork board) you can use it to trace out a parabolic, cover the plywood (or other flat object) with paper, you then find the center and drive in 1 nail, then along one axis you drive in another nail about 20 to 50 centimeters from the center, then make up a loop of string long enough to almost touch the outside from the center nail, loop it around the 2 nails and then using a pencil you can trace a true parabolic line.

The second nails position increases or decreases the depth of the parabolic, to get a round perimeter you remove the second nail, as you will see that the outer shape of the parabolic is different then the inner.

 

[pride21]

hi vs i also thought of that idea of extending a dish,i already obtained twelve 200mm by 100mm pieces of 1mm ali from e-bay and obtained brackets from lidl,i wont try it on the 1.4mtr dish i still have the 1.2mtr dish to experiment with,this is a project for the spring as you say if you dont try you will never know.
cheers

 

[Vipersan]

Thanks for your encouragement guys ...
I certainly will try something in the spring ...
@Terryl ...
I agree that the curvature isnt neccessarily uniform and so emulating the last 6 inches of the pf (ie cloning it) ...then moving it into position beyond the dishes current rim isnt perfect ...
..but was hoping that over such a short distance ie 6 inches ..that the deviation wouldn't be so dramatic as to make it unworkable..
Since I am working with C band frequencies ...I also assumed the variations to be perhaps less critical ..
the extension panels could be angled and ajusted for maximimum gain once in place ...the smooth curvature being the most difficult aspect to achieve.
..this being the reasoning behind the 'cloning' idea ..

As pride says ..if you don't try ..you are forever wondering ..
rgds
VS

 

[Channel Hopper]

The f/d ratio should not change dramatically when extension panels are used, however a different feed will be necessary to cope with the increased illumination required on the much larger reflector. You can continue to use the same feed arms.

From the GDSatcom manual (Prodelin pirme focus range)

The focal lengths are, 35- 13/16” for the 2.4M & 3.0M, 46-7/8” for the 3.4M and 54” for the 3.7M & 4.5M.
The F\D ratios are: .30 for the 3.0M & 4.5M, .36 for the 3.4M, and .37 for the 2.4M & 3.7M.

 

[Vipersan]

The f/d ratio should not change dramatically when extension panels are used, however a different feed will be necessary to cope with the increased illumination required on the much larger reflector

...and therein lies a mystery CH ...
Since the feed I currently use was never designed for this dish anyway ...
As you know its all homebrew ...and made to perform
I did notice though ..after trying the various scalar plates supplied with varios c band lnbs ....and a borrowed one from a corotor ...there was little variation between them ..
Are you saying that the scalar plate might have to be 'shaped' ...ie convex in some way ? ....or simply repositioned and remain flat with concentric rings ?
rgds
VS

 

[Channel Hopper]

Ah, that is the $64k question. With dish upgrades it is all a matter of testing once assembled.

 

[Vipersan]

Thanks CH ..
I suspect the feed I currently use isn't correctly matched anyway ...
As you know the only ajustment on these lnbfs is the ability to slide the lnb feed within the scalar plate to prefixed marker points ..
..and this combo is aimed more at shallow dishes anyway ..
I've ofted wondered if I should try grinding down the outer rings on a scalar plate to form a convex curve ..the inner rings being higher ..thus doming the feed plate ..
Sadly the only one deep enough to try this is the corotor feed ...and I dont have a spare ...so am loathe to destroy it in the name of mad science.
;)
rgds
VS

 

[Vipersan]

Excellent discussion in Satguys US forum archive ...relating to this very topic ..

_http://www.satelliteguys.us/archive/t-227093.html

But ..a picture is worth a thousand words ..and I'd love to see a picture of a commercialy designed C band pf feed specifically for a deep FD dish ..
Mine is around 0.42
rgds
VS

 

[Huevos]

if you have a 1 meter square piece of plywood (or a cork board) you can use it to trace out a parabolic

That's an ellipse. With a parabola the second point of focus (or in your case nail) is at infinity.

Since I am working with C band frequencies ...I also assumed the variations to be perhaps less critical

You still need to get the error lower than 1/10th of the wavelength at the dish perimeter, which in C-band is around 9mm.

The f/d ratio should not change dramatically when extension panels are used

The focal length would stay the same so that means there is a linear relationship between the diameter and the f/d ratio. If the starting point were 0.4 f/d on a 1.8m dish adding 6 inches to the radius would make it 2.2m and give it an f/d ratio of 0.33, which I think is going to require serious thought on matching the feed to the dish.

 

[Vipersan]

Thanks for the extra info Huevos ...
Does this mean then ...that the once commercially available petal extensions for my Alcoa ....would have been supplied with a modified feed ?
I wish I could find more info on this old sat dish ...
Sadly it appears to have vanished into the mists of time..
rgds
VS

 

[PaulR]

Surely extending a parabolic curve of a dish will inevitably alter the f/D ratio? For this very reason I've not understood the rationale behind so-called dish extenders as, if the LNB's feedhorn was correctly matched before, then there will be no increase in signal. It's only it were incorrect that signal would increase.

What is needed is a feedhorn with adjustable scalar rings (and that's adjustable in and out - not one-off inwards with an angle grinder!).

 

[Vipersan]

What is needed is a feedhorn with adjustable scalar rings (and that's adjustable in and out - not one-off inwards with an angle grinder!).

Agreed Paul
Such a thing exists for ku I think ...made by invacom ..
Sadly no such thing exists for C band ..
Perhaps I should give some thought to making one ?
rgds
VS

 

[Huevos]

oes this mean then ...that the once commercially available petal extensions for my Alcoa ....would have been supplied with a modified feed ?

I don't know anything about the Alcoa dish, my comments are just generic. In the post by CH he mentions some examples where the f/d ratios are the same or similar on dishes of differing diameter. The reason for this is the whole dish is being scaled up, i.e. not only is the diameter getting bigger, but the focal length is getting longer, keeping the f/d ratio more or less the same. In your case though the focal length (distance between the dish face and LN stays the same while the dish diameter gets bigger, hence in your case the ratio changes.

The problem you have got is that the scalar ring is fixed to the dish and the LNB gets moved in and out, but what you need when adding extensions is the opposite. You need to find the focal point, fix the LNB at this point and then move the scalar ring set in and out until it matches the dish (or just simply follow the guide on the side of the LN.

And even if you leave the feed without any modification you will still gain from a bigger dish because illumination roll-off is gradual.

 

[Vipersan]

...Ahhh ...
Not in my case Huevos ..
I designed my mount and feed with maximum flexibility ..
The body of the lnb (waveguide) is ajustable in and out around the focal length ...and can then be clamped at max signal level ..without the scalar plate fitted ..
The scalar plate is then added and is free to move along the length of the lnb waveguide ...so is also adjustable..
rgds
VS