"Ikkunaprincessa" <Ikkunaprincessa69@gmail.com> hath wroth:
>i'm so terribly sorry i didn't read group's guidelines.
>i didn't know i wasn't allowed to ask for homework assignments.
It isn't the groups guidelines. It's a general usenet guideline to
not ask about homework problems. You don't learn anything by simply
asking for the answers. However, it appears that you want some
understanding of the antenna problem, which is quite different.
>the problem is that our proffesor didn't teach antenna well. he spoke
>about them only 30 minutes.
>also the text book he intorduced us doesn't explain much about antenna.
>(the book is "Data Communications, Computer Networks and Open Systems"
>by Fred Halsall.
>we have only studied chapter one & two.)
I'm not familiar with the book. Antennas cannot be learned in 30
minutes.
>1. maybe the main lobe of our directional antenna is not directed
>toward the receiver.
>(the end user is not in the direction of coverage)
>
>2.maybe polarization of the new antenna is not as of the receiver''s.
You seem to be interpreting the question as "How can I misuse or
misintall the antenna". Direction and cross-polarization are just
two. Excessive coax cable losses, diversity reception failure,
positioning the antenna in a null area, and line of sight problems are
some others. However, I don't think that this is what the professor
wanted. I think he's assuming a properly designed antenna that should
work correctly in a wireless system, but doesn't. That means the
source of the 3 failure has to come from outside the radio and the
high gain antenna. That leaves:
1. Mis-aiming the antenna.
2. Cross polarization.
3. Obstruction in the line of sight.
4. Too close and causing receiver overload.
The last one might be too obscure for a beginning class.
>3.maybe the the propagation bandwidth of the transmitter antenna is not
>equal to the receiver's bandwidth.
Good thinking, but not possible (as others have mentioned). The
required receiver bandwidth for 802.11 is about 25MHz. In general,
the higher the gain of the antenna, the narrower the bandwidth. At
about 24dBi, the antenna bandwidth is about 80Mhz for a VSWR < 1.5:1.
That's the entire 2400 to 2483.5MHz band, which is nice because the
antenna does not require any adjustments for different channels.
However, if the gain of the dish antenna is much over 24dBi, the
bandwidth will be less and the antenna will need to be either selected
or tuned for a specific channel. Even if this were the case, the
antenna would need to be so narrow as to exclude signal from the
adjacent RF channels, which is highly unlikely. The system should
still work even with a mistuned antenna.
I'm not sure any of this helps with your homework problem. It's
difficult to guess what the professor is expecting for an answer.
However, it might explain some of the terms you've been using.
--
Jeff Liebermann
jeffl@comix.santa-cruz.ca.us
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
replacing omnidirectional antenna with a directional antenna 
Linear Mode
