--- In Secomb's <> wrote:
> I have used the site that you mentioned and found it very
> useful,especially the spreadsheet which one change to get the desired
> size and focal length. I have tried using cardboard but found
achieving
> a reasonable shape challenging. How did you go getting the shape using
> plastic and are at the stage of testing it yet?
Hi David and the Group
I also considered using cardboard but decided against it because of
perceived durability/damp problems. I thought I would try Correx.
This is just like the corrugated cardboard that packing cartons are
made of i.e. two sheets of plastic separated by a corrugated piece of
plastic. It is available in various thicknesses - I used quarter inch
(6mm).
After feeding the spreadsheet with my design parameters I very
carefully copied the resulting dimentions of one panel onto 6mm
plywood. This I very accurately cut out and shaped using a coping saw
followed by a bench mounted disc sander. This template was then used
to markout the 12 panels used in the construction. Because the Correx
material was 6mm thick I found the inside joints mated relatively
accurately but those on the outside had quite big gaps.
I used hot melt glue to join the outsides of the panels. This worked
well as it easily filled the gaps and also ran into the corrugations.
A very strong seal was therefore achieved but a lot of hot glue was
needed. The gaps on the inside face were very minor - in many places
no gap at all in others no nore than 1mm. Initially I thought I
would cover these with electricians tape but ended up using wet
fingers to massage Araldite epoxy glue into the gaps. This has
resulted in a good finish for the reflecting surface without resorting
to tape - probably added to overall strength as well.
The last two days have been spent making the handle, the mic
positioning assembly and the mic holder (due consideration being given
to isolating these item from the dish). These have been designed as
they were being built - very seat of the pants and quite a few changes
being required enroute. However they seem acceptable.
I will use the same stereo pair capsules made up for the Jecklin disc.
At 7pm this evening I could not restrain myself. I added the
capsules, guessed where the focal point might be and went out into the
back garden for a first test.
It works. How well I don't know as this is the first time I've ever
used/heard a parabolic. It is certainly directional. Pointing it to
the open door to the kitchen I could hear the England versus Rest of
the World Soccar game on TV. Pointing away from the house - no soccer
game but plenty of birds in a somewhat confusing situation (I could
hear plenty, some very strong, but I could not see any). Clearly a
learning curve is about to be climbed.
The "Parabolic Reflector" album at
m("btinternet.com/my_photos","//uk.pg.photos.yahoo.com/ph/g0sbw");">http:
has had six new photos added which reflect (sorry about the pun) the
current state of affairs.
Clearly the mic lead needs to be taken care of - when it hits the edge
of the reflector there is a mighty knock, I wondered what was going
on. A wind shield is required and I need to spend some time finding
the true focal point and its shape. A member of the Group kindly
emailed me a good tip on how to do this but it needs the Sun which
seems to have gone walkabout these past 10 days.
A good coat of khaki paint would be a finishing touch.
Cheers
Tom Robinson
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