Hi "a"

Just a thought -- I think you are concerned with the radiation from the
illuminator adding to (or canceling) the radiation from the reflector.
Normally, the illuminator is designed to establish a pattern that aims only
toward the reflector. But, since you dont wish to shape the radiation
pattern from the illuminator, you might want to consider that the total
pattern from both the illuminator will depend on how big the reflector is
(in wavelengths).
If the parabola is large, the half of the illuminator's radiation in the
"wrong" direction wont seriously effect the total pattern, and can be
ignored. You might assume the radiation from the reflector is concentrated
to be within a 20 degree wide sector (20 dB gain antenna). The radiation
from the illuminator in that 20 degree sector is so weak that it will not be
noticeable.

The fact is - There are many factors that a designer of parabolic
antennas considers. Google

Jerry



Hi Jerry,
Thanks, yes the adding/canceling was what I was trying to address.
I'd Googled on the subject but couldn't find anything that answered that
point. I did come across some interesting sites that described the need
to illuminate the whole of the reflector to make good use of its full
dish size (and hence minimise beamwidth), but I was trying to keep mine
simple.

Treating it simplistically, I guess that, for a given diameter, a deep
parabola (ie the focus well inside the paraboloid) will capture more of
the radiated energy and redirect it in the required direction, than a
shallow parabola would.

I was thinking that I would go for a sheet of aluminium bent into a 2-D
parabola, about 2.5 wavelengths across, giving me a beamwidth of around
0.5 radians Presumably a 2-D parabola would only have gain in 2-D, and
it would be about 2*pi/0.5 = 12, (11dB over a whip).
For a 2.4GHz signal this equates to a parabola 31cm across.

"a" wrote in message
...

Hi "a"

Just a thought -- I think you are concerned with the radiation from
the illuminator adding to (or canceling) the radiation from the
reflector. Normally, the illuminator is designed to establish a pattern
that aims only toward the reflector. But, since you dont wish to shape
the radiation pattern from the illuminator, you might want to consider
that the total pattern from both the illuminator will depend on how big
the reflector is (in wavelengths).
If the parabola is large, the half of the illuminator's radiation in
the "wrong" direction wont seriously effect the total pattern, and can be
ignored. You might assume the radiation from the reflector is
concentrated to be within a 20 degree wide sector (20 dB gain antenna).
The radiation from the illuminator in that 20 degree sector is so weak
that it will not be noticeable.

The fact is - There are many factors that a designer of parabolic
antennas considers. Google

Jerry

Hi Jerry,
Thanks, yes the adding/canceling was what I was trying to address.
I'd Googled on the subject but couldn't find anything that answered that
point. I did come across some interesting sites that described the need to
illuminate the whole of the reflector to make good use of its full dish
size (and hence minimise beamwidth), but I was trying to keep mine simple.

Treating it simplistically, I guess that, for a given diameter, a deep
parabola (ie the focus well inside the paraboloid) will capture more of
the radiated energy and redirect it in the required direction, than a
shallow parabola would.

I was thinking that I would go for a sheet of aluminium bent into a 2-D
parabola, about 2.5 wavelengths across, giving me a beamwidth of around
0.5 radians Presumably a 2-D parabola would only have gain in 2-D, and it
would be about 2*pi/0.5 = 12, (11dB over a whip).
For a 2.4GHz signal this equates to a parabola 31cm across.

Hi "a"

I saw http://www.nodomainname.co.uk/parabolic/parabolic.htm on Google
and thought it fit your line of interest. It appears that your goals are
realistic and dont require extensive research on the design of parabaloids.
Try Googling again and see if you cant find some data on WiFi, instead of
parabola antenna design. I have seen many WiFi antenna designs that use
kitchen pots and pans as the reflectors.

Jerry

"a" wrote in message
...
snip


Thanks for the replies.

I agree that the radiating element must be placed at the parabola focus
to give minimal beamwidth, and that this condition is met when the
radiating element is placed at the focus (which is given by D^2/16d).

The point remains that I can still choose the parabola parameters to set
the focal length to whatever is desired.
Should I choose them so that the focal length is an odd or even number
of quarter wavelengths?

I don't think it matters. The gain signal off the parabolic
reflector will be so very much stronger -- except for a small antenna. Do
you have the gain formulas? http://en.wikipedia.org/wiki/Parabolic_antenna


What I really had in mind was a uniformly radiating element (ie a simple
whip) with a parabolic reflector behind it, like this:-
( x
reflector radiating element

To get the right-going signal from the reflector in phase with the
right-going signal from the radiating element I need to choose the
reflector distance correctly.

This is an unusual concern; it's not common practice to
have an "omni" feed for a parabolic.

I have a feeling that there WILL be a phase inversion at the reflector
but I'm not certain.

The reason that I think that there might be a phase inversion is that
the (radiator plus reflector) could be considered to be a (radiator and
its image). At the (perfectly conducting) reflector the voltage will be
zero and the current will be infinite, which implies that, at the
reflector, the reflected wave must be phase inverted wrt the radiated
wave.

Any thoughts?

On Sun, 28 Oct 2007 09:48:18 +0000, a wrote:
a wrote:
I'm looking at making a parabolic reflector for a wifi link.

I'm trying to work out the distance required between the radiating
element and the reflector.
Should it be an odd number of quarter wavelengths, or an even number?

Thanks for the replies.
I agree that the radiating element must be placed at the parabola focus
to give minimal beamwidth, and that this condition is met when the
radiating element is placed at the focus (which is given by D^2/16d).

The point remains that I can still choose the parabola parameters to set
the focal length to whatever is desired.
Should I choose them so that the focal length is an odd or even number
of quarter wavelengths?

You are getting the use of a _parasitic_ reflector mixed up with what
amounts to a _mirror_ reflector in your case.

It matters _not_ what the distance is from the driven element at the
focus to the surface of the parabolic reflector -- in wavelengths,
inches, centimeters, or furlongs.

What you could do is mount a _parasitic_ reflector outboard of the the
driven element -- and _this_ would be positioned approx 1/4 WL from the
driven element. Tho', how much gain it might add is questionable...

73
Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
38.24N 104.55W | @ config.com | Jonesy | OS/2
*** Killfiling google posts: http://jonz.net/ng.htm

me wrote:
"So...is there a 180 degree phase change at the reflector?"

Yes. The incident and reflected components of electric field exactly
cancel at the surface of a perfect reflector due to the short-circuit.

Best regards, Richard Harrison, KB5WZI

On Thu, 01 Nov 2007 20:58:44 -0400, W2RAC
wrote:

http://www.freeantennas.com/

On Sat, 27 Oct 2007 22:25:35 +0100, a wrote:

I'm looking at making a parabolic reflector for a wifi link.

I'm trying to work out the distance required between the radiating
element and the reflector.
Should it be an odd number of quarter wavelengths, or an even number?

(An odd number of quarter wavelengths would be required if there is a
180 degree phase change on reflection, and
and even number would b required if there is no phase change).

So... is there a 180 degree phase change at the reflector?

Do like we do with he big C and Ku band dishes. Point it at the signal
source, adjust the polarity for max, and then move the feed horn in
and out slightly for max on a signal meter.


Roger (K8RI)

TIA

Richard Harrison wrote:
me wrote:
"So...is there a 180 degree phase change at the reflector?"

Yes. The incident and reflected components of electric field exactly
cancel at the surface of a perfect reflector due to the short-circuit.

But the incident and reflected components of the magnetic field don't.
Is that cool or what?

Roy Lewallen, W7EL

Roy Lewallen wrote:
Richard Harrison wrote:
The incident and reflected components of electric field exactly
cancel at the surface of a perfect reflector due to the short-circuit.

But the incident and reflected components of the magnetic field don't.
Is that cool or what?

It's simply another example of the conservation of
energy principle in action.
--
73, Cecil http://www.w5dxp.com

Roy Lewallen wrote:
"But the incident and reflected components don`t. Is that cool or what?"

Cecil wrote:
"it`s simply another example of the conservation of energy principle in
action."

Yes. When a radio wave encounters a short-circuit, a reflection takes
place with reversal in phase of the voltage but with no change in phase
of the current. Voltage cancels and current doubles at the
short-circuit. Reflection reverses the wave`s direction of travel
because only one of the wave`s components has had a phase reversal but
not both. Sinultaneously reversing the phase of both wave components has
no effect on the direction of wave travel.

Best regards, Richard Harrison, KB5WZI

Roy Lewallen wrote:
Richard Harrison wrote:

me wrote:
"So...is there a 180 degree phase change at the reflector?"

Yes. The incident and reflected components of electric field exactly
cancel at the surface of a perfect reflector due to the short-circuit.


But the incident and reflected components of the magnetic field don't.
Is that cool or what?

Just falls out of the fact that while there's no voltage (the perfect
conductor enforcing a constraint of E=0) there can be current flowing,
so there can be a net magnetic field.

of such cool stuff are things like the reversal of CP sense made.