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.

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.

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

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

Fractenna wrote:

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

73,
Chip N1IR


Hi Chip! Sure a radio astronomer will do just fine. grin

Glad to hear from you!