I agree with you about the homework part. But just speculating what may have happened, and just listing the end result would be interesting to see what people think may happen. Heck off the top of my head I can only think of two reasons.
The kid seems to have decent internet skills as he found this news group.
So lets assume he has indeed tried to find the answer. I suspect you won't find the answer to the question as worded as such. Insufficient information and it appears its an essay required answer instead of multiple guess... this would allow some leeway in explaining his answers (and if answered correctly, point out the poorly worded question).
Granted, a client on the back side of a directional access point would loose coverage, but we don't know which antenna was changed out - the AP or client side.
How much coax loss was introduced into the system? Did several or all of the client points loose connection? Which antenna was replaced, the AP side or client side?
Incidentally, the original question came from the Soroush Resaneh Institute in Tehran, Iran via mellat.tehran.sinet.ir
Well, now that you've given away 2 out of 3 answers, I can become part of the problem instead of the solution. Might as well give up any pretense of at being helpful. However, in the future, I suggest you avoid answering student homework questions as they are expected to do their own research and usually have much better learning resources at their skools than are available on the internet.
3) Increased antenna gain causes distant source of interference to now become a problem.
So obvious I overlooked that as an answer. But then my two answers were obvious as well. I kind of dissagree with the homework thing, especially if you only give generic answers like we did, and leave it to the original poster as to find out why those answers may indeed be the answers they are looking for.
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. 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.)
thanks alot for ur help. i searched myself 'n here is what i came up with (i'm sorry if i'm not able to explain well in english):
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.
3.maybe the the propagation bandwidth of the transmitter antenna is not equal to the receiver's bandwidth.
Well...at least you found a third answer to a a questions that was rather ambiguous to begin with.
But in all practicality, I seriously doubt you will find an antenna with so much gain that the bandwidth is degraded.
Increasing the gain will narrow the beam width or vice versa, narrowing the beam width increases the gain. *It also narrows* the band width of the antenna, but not to any degree that you need to worry about in actual practice.
Ad hoc off the top of my head numbers, a 6 dB gain antenna has about 5% bandwidth at 1.5:1 SWR. Therefore in the 2412-2462 MHz band, a 6 dB gain antenna cut for a center frequency of 2437 MHz will be acceptable from 2300 Mhz up to 2560 MHz...well within specs. Again, these are off the top of my head numbers.
The antenna by itself would not have a bandwidth.(as described in your reply #3) When you talk about the propagation bandwidth of a transmitting antenna being different than the receivers, I think you meant to say the frequencies were different.
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.
I'm not familiar with the book. Antennas cannot be learned in 30 minutes.
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:
Mis-aiming the antenna.
Obstruction in the line of sight.
Too close and causing receiver overload. The last one might be too obscure for a beginning class.
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.