--- In "Julian Baldwin"
<> wrote:
>
> Tom,
> I noticed that in the original details which you gave us about your
DIY dish
> you mentioned that the focus was about half an inch from the dish. Later
> posts have suggested that the normal focus point is much further
from the
> dish surface. I'd be interested to know how satisfactory your focus
point
> proves to be.
Hello Julian and the Group
My apologies for the confusion which comes about because I mixed
centimetres with millimetres.
The spread sheet mentioned does not have units only numbers. I set
the parameters for the dish to have a 50 diameter and a 12.5 focal
length.
In my mind I assumed 50cm and 12,5cm would give a nice sized dish with
the focal point at the rim. In my first post I typed 12.5mm instead
of 12.5cm
In the event the dish turned out to have a 60cm diameter - I must have
read one row too far on the spreadsheet when marking out the plywood
template. At the point where the diameter is 50cm the depth of the
dish is approximately 12.5cm, so I think the dish shape is not too
bad. The main problem is that the focal point is inside the dish.
this makes using the sun to find the actual focal point a bit
difficult as I cannot see the point without a mirror. The wishy washy
sun today has also not been perfect. I have put this method to one
side for the moment. I have done a test with the front face of the
capsules 13cm from the face of the dish. I placed a very quiet travel
clock about 20ft. away and moved the dish from directly facing to 90
degres leff. This was repeated several times and was recorded on
HiMD, pcm, high sensitivity, manual record volume 19. The MP3
recording is at
m("btinternet.com/tictocparabolic.mp3","//uk.geocities.com/g0sbw");">http:
The difference is not dramatic but is there. It is to be remembered
that the mic capsules are omnis and there will be room reflections.
Probably a portable radio in the garden, as suggested by Stoatwizard,
would be a better test.
I apologise if you have problems accessing the FTP site. It seems to
object even if it is only lightly used. I tried to upload the file to
this Groups files area but, apparently, it is too long.
Cheers
Tom Robinson
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