"Nigel M" wrote in message
...
I've been looking to make a linear parabolic reflector for Wi-Fi, I've
found quite a few sources, such as:

http://www.genericgeek.com/index.php?q=node/280

http://www.freeantennas.com/projects...te2/index.html

Those I've found all give templates for a parabola, but without any
explanation as to why they have chosen that *particular* parabola, or
the formula used to draw it. As a result, the drawings are a bit
"sketchy" to say the least!

I know radio amateurs are often knowledgeable on antennae, so I thought
this was a good place to ask. I'd like to know a bit more theory, and
the pros and cons of various parabolic shapes.


--
Nigel M
"Occam's razor is not always sharp"

Nigel

I've been gathering parts so I can make a WiFi site. I dont know enough
about wireless to offer any help with that part. But, I do have some
experience with parabloic antennas. Contact me thru my E-mail if you are
interested in trading thoughts.

Jerry

On Mon, 21 Mar 2005 16:49:13 GMT, in rec.radio.amateur.antenna you
wrote:
Those I've found all give templates for a parabola, but without any
explanation as to why they have chosen that *particular* parabola, or
the formula used to draw it.

Hi Nigel,

All parabolas have the same formula, simply different constants
(size). If you want more gain, build a larger parabola. There is
some diminishing return because your actual antenna is not getting
larger, and a lot of the gain from the ends would be reflecting into
unoccupied space.

I would offer one caution. The "template" offered appears to have a
geometrical distortion from being stored for printing vs. stored for
viewing (it appears to be squashed from either side, as viewed at the
page).

As for a formula, that would be far more complex than construction
instructions:
1. Obtain a large sheet of graph paper;
2. Along its longest edge, rule a line;
3. In the middle of this line, extend a perpendicular line;
4. Along that perpendicular mark a focus point (5 inches would do);
5. Along that perpendicular, halfway between the line in 2. and the
focus in 4. mark one point of the parabolic surface;

This defines the hallmark characteristic of a parabolic surface, ALL
points lie equidistant from a perpendicular line from 2. and from
point 4. Your graph paper should provide a lot of perpendiculars from
line 2. to use as guides. This will be cut-and-try until you get the
hang of it:

A. Select one perpendicular graph line and mark a point out X inches;
B. Using a compass, measure from focus point 4. X inches;
C. If the compass can touch the mark A. you made a lucky guess;
D. Mark this A. as another point along the parabolic surface.

However, luck is not always with us, and the compass either overshoots
A. or undershoots it. Move A. and adjust the compass equally until
they both match. Repeat A through B along other perpendicular lines
and eventually you will have a rough trace of a parabolic surface.
Connect the dots and smooth the line.

73's
Richard Clark, KB7QHC

"Nigel M" wrote in message
...
I've been looking to make a linear parabolic reflector for Wi-Fi, I've
found quite a few sources, such as:

http://www.genericgeek.com/index.php?q=node/280

http://www.freeantennas.com/projects...te2/index.html

Those I've found all give templates for a parabola, but without any
explanation as to why they have chosen that *particular* parabola, or
the formula used to draw it. As a result, the drawings are a bit
"sketchy" to say the least!

I know radio amateurs are often knowledgeable on antennae, so I thought
this was a good place to ask. I'd like to know a bit more theory, and
the pros and cons of various parabolic shapes.

Nigel -

A very good technical resource is the Green Bay Professional Packet Radio
group.
http://www.qsl.net/n9zia/

You may not be aware that amateur radio (FCC Part 97) actually shares a
portion of the
802.11 "WiFi" (FCC Part 15) spectrum allocation.

Parabolic Reflector analysis calculator
http://my.athenet.net/~multiplx/cgi-...bolic.main.cgi


"To invent, you need a good imagination and a pile of junk" -- Thomas Alva
Edison (1847-1931)

Nigel
I have little experience with dish antennas but I do know that to provide
the 'new' it is
one percent insparation and 99 percent persperation, so how about this
aproach:
Determine frequency of use.
Then draw a graph of frequency versus reactance change from a zero point
indicating resonance
of the prime frequency.
This curve is parabolic and would be an ideal starting point as there would
seem to be a connection
If one would move to far away from the point of resonance I would suggest
that the dish it's self
would rise in temperature as current starts to flow instead of a pure 'loss
less' reflective action.
Regards
Art




"Nigel M" wrote in message
...
I've been looking to make a linear parabolic reflector for Wi-Fi, I've
found quite a few sources, such as:

http://www.genericgeek.com/index.php?q=node/280

http://www.freeantennas.com/projects...te2/index.html

Those I've found all give templates for a parabola, but without any
explanation as to why they have chosen that *particular* parabola, or
the formula used to draw it. As a result, the drawings are a bit
"sketchy" to say the least!

I know radio amateurs are often knowledgeable on antennae, so I thought
this was a good place to ask. I'd like to know a bit more theory, and
the pros and cons of various parabolic shapes.


--
Nigel M
"Occam's razor is not always sharp"