[MLB-WIRELESS] xdB offset cast parabolic dishs by the hundred?

[Julian Featherston]

Here we go, I'd hoped to right into antenna books before posting this
but amazon order moving slow, and you guys move quick.

I hope to make a negative mould to produce offset cast parabolic dish's,
and hope that by making whatever size grid (off the same mould) and
appropriate feed one can achieve varied gain? Tony? Hamish?
I don't say this flippantly it's a long haul, but I'm confident we could
do it. Buy all means, tell me I'm crazy.
This would be available for anyone to use for free. If I get it right we
can make more negative moulds.
I'm also happy to build with others to distribute, I'd hope through
melb-wireless so as to generate cash for expenses. I see it is policy
for melb-wireless not to own any infrastructure, but I think the
propaganda squad could use the cash (promo = nodes) or covering cost of
meeting place or cost of domain reg.

Practical Antenna Handbook and The Arrl Antenna Book 19th Ed. are on the
way, I'll dive into them next week when they get here. First task is to
establish focal length to diameter ratio, then how much of the parabola
to use and the largest dish size required, I'm not equipped to decide as
yet but if someone can guide me, those three figures shouldn't be hard
to pick, this would allow me to keep the jig construction moving, for
which I'd be most grateful.
I guess we should adhere to the "standard" that small offset dishes
include the centre of the full parabola. 

Build process so far - contributions please

I'll just run through the build quickly as if one was making a regular
dish as opposed to an offset one. Basically the same build but difficult
to describe until I've drawn it.

1>Machine, with router and jig, a large block of foam (1000x1000x300mm)
such that it has a concave parabolic form. (it's a big block with a
shallow dish in one side)
2>Apply epoxy to the concave surface, allow to set, sand smooth. (we now
have a positive of the form we want)
3>Apply silicone mould release agent to concave surface (this is a
spray, won't bugger shape)
4>Using fibreglass and cheap polyvinyl resin lay up the concave surface
- this negative mould we are forming now is going to be used repeatedly
so it needs to be very rigid and heavily reinforced internally, allow to
set.
5>Now hopefully the negative will come free of the foam/epoxy without
having to destroy the foam/epoxy positive. 
5a>Now what your holding looks like a huge fibreglass wok with fins
running across the inside holding the shape, and a smooth accurate domed
surface the other side.
6>Set it up with the domed surface facing up, decide how big the first
dish is going to be - put some size guide marks on the domed surface.
Now the tricky bit (ignoring the feed). I've come up with a couple of
options here.
7a>Create a mesh using woven fibreglass strips about 3x10mm, saturated
with polyvinyl resin that has magnesium? Powder or fine flakes added to
reflect signal.
7b>Create a solid dish using standard bi-axial fibreglass sheet
saturated with polyvinyl resin that has magnesium added or a reflective
sheet adhered to the surface or even imbedded if it was perforated
8>Reinforce the grid or dish with fibreglass buttresses providing a
mount point for the feed and for the dish itself, allow to set.
9>Mark the grid/dish for trimming, pull the dish/grid off the mould and
trim/cleanup attach feed mount pole.
10>Learn how in hell to feed it.


Notes:
We don't need the foam/epoxy positive to survive step 5 in order to make
lots of dishes, but we do need it to make negative moulds for others.

Method of finding the outline of a offset dish - using a laser pointer
mounted on the end of a shaft place the pointer at the focus of the dish
and point the shaft at the centre of the offset dish. adjust the pointer
so it points at the edge of your finished dish, when the shaft is
rotated the pointer should draw the outline of the dish, allowing it to
be marked for trimming.


Jules
 


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