( Odd thing about this dish -- this one is a DishTV brand -- is that in
the
original design for satellite reception, the driven element [ the "can",
the
input to the LMB ] was located low on the dish. In other words, the focus
used in this design was off-center, about 30 degrees below the center axis
of
the dish. This would mean that aiming would be off-axis, too, yes? What is
the reasoning behind this design? )


1. Prevents the LNB converter from shadowing a major portion of the
dishes surface with loss of gain.

2. Since the angle of the received signal is offset by the same number
of degrees as the feedhorn offset, this results in a more vertically
plumb dish surface that more readily sheds snow and ice buildup.

Pete K1ZJH

Working with 2.4 GHz 802.11b wireless signal. Using USB adapter (one of those
thumb-sized products) and an old 18-inch satellite TV dish.

Calculating dish's focus point as f = ( d^^2 ) / ( 16 * c )
f = focus point above center of dish
d = diameter of dish
c = depth of dish at center

With a circular, symmetrical parabolic dish, I presume that the focus is
directly above the center. I've located the tiny antenna within the adapter
as close to this point as i can.

( Odd thing about this dish -- this one is a DishTV brand -- is that in the
original design for satellite reception, the driven element [ the "can", the
input to the LMB ] was located low on the dish. In other words, the focus
used in this design was off-center, about 30 degrees below the center axis of
the dish. This would mean that aiming would be off-axis, too, yes? What is
the reasoning behind this design? )

Yes, these dishes are designed/intended for an off-center LNB. They
have a different radius of curvature in the horizontal and vertical
dimensions. You won't be able to place an LNB (or access-point
antenna) at an on-axis location and get a clean focus.

The reason for an off-center focus arrangement is that it keeps the
LNB or other driven element, and its support strut(s), out of the path
between the satellite and the dish. This reduces diffraction effects,
and results in a stronger signal and cleaner focus.

I find 2 things:
1. It doesn't really matter where I point the dish. The signal varies a
little bit when I approach the direction of the access point, but no "leaps
and bounds" in the signal strength between being "dead on" and way off.

2. I pick up signals that aren't even in the direction of the access point.

I'd guess that this is due to a combination of three effects:

- You aren't getting a clean focus, because you're trying an on-axis
focal position for a dish which isn't circularly symmetrical. Try
re-attaching the LNB support arm which came with the dish, and
mount the access point on this arm so that its antenna is at the
location of the original LNB.

- The dish was designed for frequencies higher than 802.11b - it's a
bit small (in terms of wavelengths) to get a really clean focus at
2.4 GHz. Some people have had better results using the larger
dishes originally intended for use with the lower-frequency
Primestar system.

- The access point's antenna is probably rather omnidirectional in at
least one dimension - you're getting significant reception "off the
back" of the antenna, via direct radiation which isn't being
reflected from the dish.

Commercial parabolic-dish antennas for 802.11b use are usually
circularly symmetrical, with an on-axis feed arrangement consisting of
a dipole or (often) a two- or three-element Yagi arrangment aimed
towards the dish.

I've gotten reasonably good results using a Dish Network dish and LNB
support arm, with the LNB replaced by a homebrew biquad antenna and
sheet-metal reflector.

--
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!

That is one of the most ingenious sites I have ever seen those guys in NZ
are quite creative!
Xanax.
"DaveC" wrote in message
al.net...
Working with 2.4 GHz 802.11b wireless signal. Using USB adapter (one of
those
thumb-sized products) and an old 18-inch satellite TV dish.

Calculating dish's focus point as f = ( d^^2 ) / ( 16 * c )
f = focus point above center of dish
d = diameter of dish
c = depth of dish at center

With a circular, symmetrical parabolic dish, I presume that the focus is
directly above the center. I've located the tiny antenna within the
adapter
as close to this point as i can.

( Odd thing about this dish -- this one is a DishTV brand -- is that in
the
original design for satellite reception, the driven element [ the "can",
the
input to the LMB ] was located low on the dish. In other words, the focus
used in this design was off-center, about 30 degrees below the center axis
of
the dish. This would mean that aiming would be off-axis, too, yes? What is
the reasoning behind this design? )

Setting up on the roof, finding the wireless access point's signal, I
moved
the wireless adapter around the focus point a little to maximize signal
strength (I use the word "maximize" loosely...). I then aimed the dish
around
in the general direction of the access point, looking for a leap in signal
strength (using a signal strength utility program to verify my aim ).

I find 2 things:
1. It doesn't really matter where I point the dish. The signal varies a
little bit when I approach the direction of the access point, but no
"leaps
and bounds" in the signal strength between being "dead on" and way off.

2. I pick up signals that aren't even in the direction of the access
point.

I tried rotating the dish on its axis to account for polarity mismatch (
would this really be an issue? ). No joy.

The wireless adapter can receive signals well, generally, w/o a dish, but
because of my location, reception of the desired signal is not great.

I'm aware that reflections can be strong from nearby objects, so that
could
account for some pickup of signals in directions other than from their
origin. Also, I suspect that the shallowness of the dish is a contributor.
But the lack of directionality of the dish, in general, has me stumped.
Have
I miscalculated the focus? I understand that the ratio of
focus-to-diameter
of the dish is important; for 2.4 GHz, is best in the 0.25-0.55 range
(this
one is 0.69). Is this contributing to my problem?

I used this web page as a reference:

http://www.usbwifi.orcon.net.nz

Any help would be greatly appreciated.

If this is off-topic for any NG here, please let me know what is a more
appropriate forum. Are there mailing lists for such topics?

Thanks,
--
DaveC

This is an invalid return address
Please reply in the news group



---
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DaveC wrote:

Working with 2.4 GHz 802.11b wireless signal. Using USB adapter (one of those
thumb-sized products) and an old 18-inch satellite TV dish.


In addition to the discussion[s] on focal point, the received signal
gain from the 18 inch antenna is down approximately -14 dB from the 12
GHz nominal frequency for the satellite tv signals. There will be a
further reduction from the off center feed [It's been 15 years since I
ran the calculations and I'm too rusty at age 66 to do them now grin].

From old experience, I would expect somewhere around a 10 +/- ? degree
beamwidth at 12 GHz and somewhere around 30 to 40 +/- ? degree beamwidth
at 2.4 GHZ. So, a fairly broad pattern that you describe does not seem
unreasonable. However, I invite a microwave engineer with more recent
experience to comment and I'll freely accept a better analysis and
conclusions.

http://www.qsl.net/n1bwt/chap5.pdf

On Wed, 12 May 2004 16:09:13 -0700, DaveC wrote:

|Working with 2.4 GHz 802.11b wireless signal. Using USB adapter (one of those
|thumb-sized products) and an old 18-inch satellite TV dish.
|
|Calculating dish's focus point as f = ( d^^2 ) / ( 16 * c )
| f = focus point above center of dish
| d = diameter of dish
| c = depth of dish at center
|
|With a circular, symmetrical parabolic dish, I presume that the focus is
|directly above the center. I've located the tiny antenna within the adapter
|as close to this point as i can.
|
|( Odd thing about this dish -- this one is a DishTV brand -- is that in the
|original design for satellite reception, the driven element [ the "can", the
|input to the LMB ] was located low on the dish. In other words, the focus
|used in this design was off-center, about 30 degrees below the center axis of
|the dish. This would mean that aiming would be off-axis, too, yes? What is
|the reasoning behind this design? )
|
|Setting up on the roof, finding the wireless access point's signal, I moved
|the wireless adapter around the focus point a little to maximize signal
|strength (I use the word "maximize" loosely...). I then aimed the dish around
|in the general direction of the access point, looking for a leap in signal
|strength (using a signal strength utility program to verify my aim ).
|
|I find 2 things:
|1. It doesn't really matter where I point the dish. The signal varies a
|little bit when I approach the direction of the access point, but no "leaps
|and bounds" in the signal strength between being "dead on" and way off.
|
|2. I pick up signals that aren't even in the direction of the access point.
|
|I tried rotating the dish on its axis to account for polarity mismatch (
|would this really be an issue? ). No joy.
|
|The wireless adapter can receive signals well, generally, w/o a dish, but
|because of my location, reception of the desired signal is not great.
|
|I'm aware that reflections can be strong from nearby objects, so that could
|account for some pickup of signals in directions other than from their
|origin. Also, I suspect that the shallowness of the dish is a contributor.
|But the lack of directionality of the dish, in general, has me stumped. Have
|I miscalculated the focus? I understand that the ratio of focus-to-diameter
|of the dish is important; for 2.4 GHz, is best in the 0.25-0.55 range (this
|one is 0.69). Is this contributing to my problem?
|
|I used this web page as a reference:
|
|http://www.usbwifi.orcon.net.nz
|
|Any help would be greatly appreciated.
|
|If this is off-topic for any NG here, please let me know what is a more
|appropriate forum. Are there mailing lists for such topics?
|
|Thanks,

I should have added:

http://www.qsl.net/n1bwt/app-5a.pdf

In alt.internet.wireless DaveC wrote:
Working with 2.4 GHz 802.11b wireless signal. Using USB adapter (one of those
thumb-sized products) and an old 18-inch satellite TV dish.

With a circular, symmetrical parabolic dish, I presume that the focus is
directly above the center. I've located the tiny antenna within the adapter
as close to this point as i can.

Oops. It's not circular or symmetric.
I've heard that it is a portion of a parabola, rather than a parabola in
itself, as if the oringal mounting point of the LNB were mounted in the
predicted focal point of a larger piece of metal that did form a parabola.

The proper focal point is wherever the antenna element was on the original
LNB. The frequency doesn't matter. That's still the focal point.
The next problem is knowing exactly where the antenna is in your USB
widget, so that you can put it in the same spot.

( Odd thing about this dish -- this one is a DishTV brand -- is that in the
original design for satellite reception, the driven element [ the "can", the
input to the LMB ] was located low on the dish. In other words, the focus
used in this design was off-center, about 30 degrees below the center axis of
the dish. This would mean that aiming would be off-axis, too, yes? What is
the reasoning behind this design? )

I think it's 22 degrees, but it is certainly "off". The old Primestar
dishes often looked like they were pointing at the ground, instead of into
the heavens.

I tried rotating the dish on its axis to account for polarity mismatch (
would this really be an issue? ). No joy.

My DLink DWL-122 USB adapter improves noticeably when the orientation is
correct. I tried the face facing the WAP, horizontal and vertical, as
well as edge on, horizontal and vertical. I didn't try end-on.
For my setup the face on - horizontal was best.

The wireless adapter can receive signals well, generally, w/o a dish, but
because of my location, reception of the desired signal is not great.

I put my USB widget 1" in front of a 9" pie tin. The 9" pie tin isn't
really quite large enough. 1" was discovered empirically, and appears to
be good for about 6db, according to Netstumbler. It is also far more
stable with the pie tin, where without it the signal seems to wander.

http://www.usbwifi.orcon.net.nz

His wok is a parabola. That doesn't count for you.

I was going to use a reflector like Trevor's BiQuad to feed the USB to a
Dish-Dish, but my son cleaned out his garage and threw the dish away about
a week before I asked for the dish ;-(.

I think the reflector behind the USB widget, painting a parabola, should be
a good combination. And the right focal point for the odd-shaped Dish-dish
is important.

http://www.trevormarshall.com/biquad.htm
http://martybugs.net/wireless/biquad/
http://63.142.46.238/buck/CantennaUSB.html

And my favorite site:
http://www.nodomainname.co.uk/cantenna/cantenna.htm
Where David uses a full sized USB adapter, pulling the antenna loose and
sticking it through the side of a can.

I might put my mini-usb inside a can.

--
---
Clarence A Dold - Hidden Valley (Lake County) CA USA 38.8-122.5

Actually, (gain wise), as, from memory, double the SIZE, (or conversely),
Double the Frequency, you gain 6 dB! lets see: these for 18 GHz, , lose 6
dB, for 9 GHz,
Another 6, to 4.5 GIG, and another 6 to 2.25 GIG ! total, 18 dB loss!
Now, as to the dish, IF the round one, what was stated about getting the
feed centered is true,
HOWEVER, if you have the ELEPTCIAL dish, these are made to view 2 SATELITES!
and, even
Directv is useing them!! Reason?? Has to do with "OTHER" services, like
internet connects, and
LOCAL TV feeds!! In esscence, the dish is aimed between 2 sattelites,
allowing it to "see both
sattelites at the same time. This allows aprox 2-3 HUNDERD more (local )
channels to be direct
broadcast to your home (ABCBSNBCPBS) !! These are NOT very effecient
antennas at 2.4 GHz,
but, if you place your 802.11 feed, where the original feedpoint was for 18
GHz, most likely will be
at the most effecient point. Also, buried in the RECIEVERS for satelite
Recievers, they have
aiming instructions for these dishs, by zip code, and you will note the
different pointing instructions
for them, both round and eliptical!! Hope this helps, Jim NN7K



"Dave Shrader" wrote in message
news:SCzoc.36401$xw3.2418212@attbi_s04...
DaveC wrote:

Working with 2.4 GHz 802.11b wireless signal. Using USB adapter (one of
those
thumb-sized products) and an old 18-inch satellite TV dish.


In addition to the discussion[s] on focal point, the received signal
gain from the 18 inch antenna is down approximately -14 dB from the 12
GHz nominal frequency for the satellite tv signals. There will be a
further reduction from the off center feed [It's been 15 years since I
ran the calculations and I'm too rusty at age 66 to do them now grin].

From old experience, I would expect somewhere around a 10 +/- ? degree
beamwidth at 12 GHz and somewhere around 30 to 40 +/- ? degree beamwidth
at 2.4 GHZ. So, a fairly broad pattern that you describe does not seem
unreasonable. However, I invite a microwave engineer with more recent
experience to comment and I'll freely accept a better analysis and
conclusions.

In addition to the discussion[s] on focal point, the received signal
gain from the 18 inch antenna is down approximately -14 dB from the 12
GHz nominal frequency for the satellite tv signals. There will be a
further reduction from the off center feed [It's been 15 years since I
ran the calculations and I'm too rusty at age 66 to do them now grin].

From old experience, I would expect somewhere around a 10 +/- ? degree
beamwidth at 12 GHz and somewhere around 30 to 40 +/- ? degree beamwidth
at 2.4 GHZ. So, a fairly broad pattern that you describe does not seem
unreasonable. However, I invite a microwave engineer with more recent
experience to comment and I'll freely accept a better analysis and
conclusions.



For a circular aperture--a good approximation-- the gain equation approximates
to :

10 log (10 D(L)^2)

where D(L) is the diameter of the aperture in wavelengths.

If the dish is 0.45 meters, we get the following for the gains (assuming
(naively) 100% aperture efficiency):

Freq gain
12 GHz 35 dB
2.45 GHz 21 dB

Thus Dave's accurate answer.

Going off-axis lowers the aperture efficiency anywhere from 1-3 dB from ideal,
although it usually is only 1 dB or so down from the losses of vignetting from
the (centered) feed. So you're probably seeing about 18+ or so dB of gain.

The FWHM of the beam is approximately

57/D(L)

degrees. So at 2.4 GHz this is about 16 degrees to the 3 dB points. Possibly
broadened by the feed arrangement and dish curvature , but the 30-40 degrees
sounds a little high Dave.

Not a microwave engineer; will a radio astronomer do:-)?

73,
Chip N1IR

Oh, and one other consideration-- your 802.11 most likely , (for the want
of a better word)
is "Omnidirectional" which is a lousey way to illuminate a dish!! A dish is
much happier when it
is illuminated with a beamwidth of say 60 degrees (for the sake of
arguement), any other power
outside that beamwidth isn't even hitting the dish (wasted energy, further
decreaseing the gain )!
Translation: if even a horn ,or a small yagi, ect. would make a much more
effecient feed than a
point source (radiating in ALL directions) You are starting with little
gain, and overlapping (wasteing
power) that is 2 strikes against you for directional gain!! Jim NN7K


I Sent:
Actually, (gain wise), as, from memory, double the SIZE, (or conversely),
Double the Frequency, you gain 6 dB! lets see: these for 18 GHz, , lose 6
dB, for 9 GHz,
Another 6, to 4.5 GIG, and another 6 to 2.25 GIG ! total, 18 dB loss!
Now, as to the dish, IF the round one, what was stated about getting the
feed centered is true,
HOWEVER, if you have the ELEPTCIAL dish, these are made to view 2
SATELITES!
and, even
Directv is useing them!! Reason?? Has to do with "OTHER" services, like
internet connects, and
LOCAL TV feeds!! In esscence, the dish is aimed between 2 sattelites,
allowing it to "see both
sattelites at the same time. This allows aprox 2-3 HUNDERD more (local )
channels to be direct
broadcast to your home (ABCBSNBCPBS) !! These are NOT very effecient
antennas at 2.4 GHz,
but, if you place your 802.11 feed, where the original feedpoint was for
18
GHz, most likely will be
at the most effecient point. Also, buried in the RECIEVERS for satelite
Recievers, they have
aiming instructions for these dishs, by zip code, and you will note the
different pointing instructions
for them, both round and eliptical!! Hope this helps, Jim NN7K