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?

TIA

a wrote in news:4723accd$1_2@mk-nntp-
2.news.uk.tiscali.com:

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?

It is a little different to that, the feed would normally be placed at the
focus of the parabola so that rays emanating from the feed travel to the
reflector, and are reflected in a parallel beam.

For optimal results, the feed should illuminate only the reflector, and the
illumination should be such that the reflected waves have a constant power
density over the projected area of the reflector.

Owen

"a" wrote in message
...
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?


It is not a mater of odd or even wavelengths. The antenna needs to be at
the focus of the reflector. The feed needs to be adjusted so it will put
most of the signal on the reflector without too much over spray.

All the above will depend on what the shape of the dish is.

"a" wrote in message
...
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?

Here it is, exactly:

f = D-squared divided by 16-times-d,

where f is the focal length of the dish (your "distance between the
radiating element and the reflector"),
D is the dish diameter, and
d is the depth of the dish

Example; If your five foot dish is a foot deep, D-squared is 25 and
16-times-d is 16; Divide 25 by 16 and mount the radiator (or "feed" in dish
parlance) 1.56 feet away, about 1' 6 3/4". The example is a fairly deep
dish; the shallower the dish, the longer the focal length.

See also http://www.satsig.net/focal-length-parabolic-dish.htm but note he
uses letter "c" for the depth.

"Sal M. Onella" wrote in
:


"a" wrote in message
...
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?

Here it is, exactly:

f = D-squared divided by 16-times-d,

where f is the focal length of the dish (your "distance between the
radiating element and the reflector"),
D is the dish diameter, and
d is the depth of the dish

Example; If your five foot dish is a foot deep, D-squared is 25 and
16-times-d is 16; Divide 25 by 16 and mount the radiator (or "feed"
in dish parlance) 1.56 feet away, about 1' 6 3/4". The example is a
fairly deep dish; the shallower the dish, the longer the focal
length.

See also http://www.satsig.net/focal-length-parabolic-dish.htm but
note he uses letter "c" for the depth.

Sal, more more complicated for offset feed... Owen

"Owen Duffy" wrote in message
...
"Sal M. Onella" wrote in
:


"a" wrote in message
...
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?

Here it is, exactly:

f = D-squared divided by 16-times-d,

where f is the focal length of the dish (your "distance between the
radiating element and the reflector"),
D is the dish diameter, and
d is the depth of the dish

Example; If your five foot dish is a foot deep, D-squared is 25 and
16-times-d is 16; Divide 25 by 16 and mount the radiator (or "feed"
in dish parlance) 1.56 feet away, about 1' 6 3/4". The example is a
fairly deep dish; the shallower the dish, the longer the focal
length.

See also http://www.satsig.net/focal-length-parabolic-dish.htm but
note he uses letter "c" for the depth.

Sal, more more complicated for offset feed... Owen

Yeah, I thought of that and I have no ready backup for offset. I have
played with both kinds and I'm happy that my two offset feed dishes each
came with a feed-mounting arm already in place. ;-)

My two offset feed dishes have the feed 22.5 degrees offset from the
structural centerline of the dish; the angle of peak signal is therefore
22.5 degees off in the other direction. (Avoiding UP, DOWN, LEFT & RIGHT.)
I think that's a standard.

Given that an offset dish is, I think, more properly described as a
parabolic section, rather than a parabola, I would use the edge of the dish
nearer to the feed for the "f" measurement. (This is intuition with no
backup. I'm naked here, so don't look.)

Speaking of dishes, I have two, a 7-foot and a 10-foot, to give away. San
Diego, California area. So far, no takers as I cast the net ever wider.

"Sal M. Onella" wrote in message
news

"Owen Duffy" wrote in message
...
"Sal M. Onella" wrote in
:


"a" wrote in message
...
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?

Here it is, exactly:

f = D-squared divided by 16-times-d,

where f is the focal length of the dish (your "distance between the
radiating element and the reflector"),
D is the dish diameter, and
d is the depth of the dish

Example; If your five foot dish is a foot deep, D-squared is 25 and
16-times-d is 16; Divide 25 by 16 and mount the radiator (or "feed"
in dish parlance) 1.56 feet away, about 1' 6 3/4". The example is a
fairly deep dish; the shallower the dish, the longer the focal
length.

See also http://www.satsig.net/focal-length-parabolic-dish.htm but
note he uses letter "c" for the depth.

Sal, more more complicated for offset feed... Owen

Yeah, I thought of that and I have no ready backup for offset. I have
played with both kinds and I'm happy that my two offset feed dishes each
came with a feed-mounting arm already in place. ;-)

My two offset feed dishes have the feed 22.5 degrees offset from the
structural centerline of the dish; the angle of peak signal is therefore
22.5 degees off in the other direction. (Avoiding UP, DOWN, LEFT &
RIGHT.)
I think that's a standard.

Given that an offset dish is, I think, more properly described as a
parabolic section, rather than a parabola, I would use the edge of the
dish
nearer to the feed for the "f" measurement. (This is intuition with no
backup. I'm naked here, so don't look.)

Speaking of dishes, I have two, a 7-foot and a 10-foot, to give away. San
Diego, California area. So far, no takers as I cast the net ever wider.


Hi Sal

Tell me more about the 7 footer. I'm in Los Alamitos and would drive to
San Diego to get a 7 footer that'd work for 2.4 GHz.

Thanks
Jerry

"Jerry Martes" wrote in message
news:jORUi.1945$R%4.223@trnddc05...

Tell me more about the 7 footer. I'm in Los Alamitos and would drive
to
San Diego to get a 7 footer that'd work for 2.4 GHz.

Thanks
Jerry


See private reply to your email address.

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?

TIA




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?

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.

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?

"a" wrote in message
...
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?

TIA




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?

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.

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?

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