"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

On Mon, 21 Mar 2005 18:58:07 GMT, Nigel M wrote:
All parabolas have the same formula, simply different constants

Yes, but what is the difference (in gain terms) between one that looks
flat and one that looks pointy?

????????

They all look the same: Parabolas.

73's
Richard Clark, KB7QHC

Nigel M wrote:
In rec.radio.amateur.antenna, Richard Clark wrote:

All parabolas have the same formula, simply different constants

Yes, but what is the difference (in gain terms) between one that looks
flat and one that looks pointy?


Assuming equally efficient feeds, nothing.

It is diameter in wavelengths that determines gain.

Changing the "pointyness" just changes the focal point, which may cause
gain problems due to practical problems with providing a decent feed, but
doesn't change gain.

--
Jim Pennino

Remove -spam-sux to reply.

Nigel M wrote:
In rec.radio.amateur.antenna, Richard Clark wrote:

They all look the same: Parabolas.

A flat one is a smaller section of a pointy one, see:

http://mathworld.wolfram.com/Parabola.html

I can envisage that one that ends with its extremities at between 45 and
90 deg to each other would "seem" to envelop the signal better.
--
Nigel M
"Occam's razor is not always sharp"

Nope.

The maximum theoretical gain of any parabola is determined by the diameter
in wavelengths.

The things than subtract from the maximum gain are the surface accuracy,
both in terms of how close the curve is to a true parabola and any surface
"bumps", and the illumination of the feed.

Irregularities in the curve and "bumps" less than about 1/8 wavelength
have little effect, bigger than that and they can have significant effect.

The pattern of the feed (the feed itself is an antenna and has it's own
pattern) determines the illumination.

The feed is mounted at the focal point of the parabola.

A "flat" parabola has a longer focal point than a more "curved" one, to
use your terms.

If the pattern of the feed is such that it just exactly and perfectly
illuminates the whole reflector, you get maximum gain.

If, as in the real world, the pattern of the feed spills out beyond the
edge of the parabola or doesn't fill the whole parabola, you get less
then the maximum theoretical gain.

The ARRL antenna book has a pile of equations, tables, and graphs showing
these relationships and how to make working parabolic antennas.

--
Jim Pennino

Remove -spam-sux to reply.

In article , wrote:

The things than subtract from the maximum gain are the surface accuracy,
both in terms of how close the curve is to a true parabola and any surface
"bumps", and the illumination of the feed.

Irregularities in the curve and "bumps" less than about 1/8 wavelength
have little effect, bigger than that and they can have significant effect.

The pattern of the feed (the feed itself is an antenna and has it's own
pattern) determines the illumination.

The feed is mounted at the focal point of the parabola.

A "flat" parabola has a longer focal point than a more "curved" one, to
use your terms.

If the pattern of the feed is such that it just exactly and perfectly
illuminates the whole reflector, you get maximum gain.

If, as in the real world, the pattern of the feed spills out beyond the
edge of the parabola or doesn't fill the whole parabola, you get less
then the maximum theoretical gain.

In the example being discussed, the feed antenna appears to be an
omnidirectional vertical attached to an 802.11 access point.

In this particular case, because the feed has an omnidirectional
pattern, it seems to me that there would be a definite advantage to
using a relatively "deep" and thus somewhat "pointy" parabolic
section, in which the focal point lies a fair distance back from the
forward-most edges of the actual reflector.

This would tend to increase the portion of the feed antenna's omni
pattern which actually illuminates the reflector and is focused in the
forward direction.

Using a more shallow parabolic section, and getting high gain out of
it, would require a modification to the feed antenna so that it
illuminated the reflector more efficiently, with less spillover. I've
seen some designs for 802.11 which use a fairly shallow dish (with the
focal point well forward of the edges of the reflector), illuminated
by a feed antenna which is either a two-element dipole+reflector or a
"backfire" patch antenna.

I don't know whether it's feasible to make such a non-omni-feed design
as inexpensively as the "cardboard, tin foil, standard access point"
designs posted on the Net.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Mon, 21 Mar 2005 19:19:19 GMT, Nigel M wrote:
http://mathworld.wolfram.com/Parabola.html

Hi Nigel,

As you may observe, their description exactly matches my instructions.
All Parabolas look the same, it is merely a difference in constants,
and the constants in this regard are explicitly stated as the variable
"a."

Change "a" and hold "L" constant, and you have identical gain in the
direction of interest (perpendicular to the directrix).

The only question left unresolved is do you choose graph paper from an
8x11 tablet, or from a newssheet sized tablet? Same instructions for
either, the sheet from the larger tablet (larger "L") yields higher
gain in the direction of interest (perpendicular to the directrix).
Choose any "a" dimension you care to for aesthetics sake only.

73's
Richard Clark, KB7QHC

Dave Platt wrote:
In article , wrote:

The things than subtract from the maximum gain are the surface accuracy,
both in terms of how close the curve is to a true parabola and any surface
"bumps", and the illumination of the feed.

Irregularities in the curve and "bumps" less than about 1/8 wavelength
have little effect, bigger than that and they can have significant effect.

The pattern of the feed (the feed itself is an antenna and has it's own
pattern) determines the illumination.

The feed is mounted at the focal point of the parabola.

A "flat" parabola has a longer focal point than a more "curved" one, to
use your terms.

If the pattern of the feed is such that it just exactly and perfectly
illuminates the whole reflector, you get maximum gain.

If, as in the real world, the pattern of the feed spills out beyond the
edge of the parabola or doesn't fill the whole parabola, you get less
then the maximum theoretical gain.

In the example being discussed, the feed antenna appears to be an
omnidirectional vertical attached to an 802.11 access point.

In this particular case, because the feed has an omnidirectional
pattern, it seems to me that there would be a definite advantage to
using a relatively "deep" and thus somewhat "pointy" parabolic
section, in which the focal point lies a fair distance back from the
forward-most edges of the actual reflector.

This would tend to increase the portion of the feed antenna's omni
pattern which actually illuminates the reflector and is focused in the
forward direction.

Using a more shallow parabolic section, and getting high gain out of
it, would require a modification to the feed antenna so that it
illuminated the reflector more efficiently, with less spillover. I've
seen some designs for 802.11 which use a fairly shallow dish (with the
focal point well forward of the edges of the reflector), illuminated
by a feed antenna which is either a two-element dipole+reflector or a
"backfire" patch antenna.

I don't know whether it's feasible to make such a non-omni-feed design
as inexpensively as the "cardboard, tin foil, standard access point"
designs posted on the Net.

In the case of an omni vertical, it would be easier to construct a
corner or trough reflector with a ground plane and it would probably
work better since the bottom half of the parabola (or full corner
reflector) most likely won't be illuminated anyway.

--
Jim Pennino

Remove -spam-sux to reply.

"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)

"Richard Clark" wrote in message
...
On Mon, 21 Mar 2005 18:58:07 GMT, Nigel M wrote:
All parabolas have the same formula, simply different constants

Yes, but what is the difference (in gain terms) between one that looks
flat and one that looks pointy?

????????

They all look the same: Parabolas.

73's
Richard Clark, KB7QHC

Not even close. Prime focus parabolas ( and offset fed) are defined by their
F/D ( focal length to diameter ratio). A "deep " dish would have an F/D of
0.3 or less. Shallow dishes 0.6 or greater. The real problem here becomes
the ability to properly illuminate the dish with a feedhorn. Typical scalar
feeds will be efficient from 0.3 to 0.5 or so. Under illumination can mean
better G/T but you are likely not using the all the surface. Over
illumination results in seeing warm earth noise and degrading G/T.
Do a Google search of W1GHZ site- an excellent tutorial on passive
reflectors and feeds.

Dale W4OP