if antena and wifi card both have 25 db gain, does it means that sumary gain is 50 db?
if i use two dual polarizational antenas, for p2p link, can i configure same frequency on both AP and use diferent polarizations for rx/tx?
wifi card does not have gain… What you are seeing probably from specs is the power output!
eg 20dBm = 100mW
23dBm = 200mW
1dBm= 1mW
dB is from the days of log maths.
10dB = factor of 10
20dB = factor of 100
30dB = factor of 1000.
Hint… every +3bB is double power.
Thus 13dB= 20mW.. etc etc etc
Antenna gain is somewhat similar. And we deal in ERP. Effective radiated power.
Lets keep it simple and suggest that the antenna has a gain of 26dB and the power of the card is 26dBm.
26dBm= 400mW, coupled with an antenna of 26dB gain.. factor of 400.. then 400 x 0.4W is equal to 64Watts of Effective Radiated Power.
(actual transmission power is never increased, but the power density is) Another words immagine an antenna made from a 6" nail, but the nail is positioned in the center of a balloon filled with water.. You can manipulate the balloon into all sorts of shapes, but it still holds the same amount of water… Now pinch the edge of the balloon and pull it outwards or stretch it in one direction.. In effect your imaginary radiating element is now offset within the balloon. Look at the shape that you have now, you now have a larger shape pointing in a specific direction)
That is what we call gain!, you have removed some of the unwanted power density from one direction (that you did not want to transmit to) and concentrated it to another direction.
That’s it! in very basic laymans terms.
Regarding the other question, I didn’t really understand it. I assume you want to do a PtP link using a dual feed antenna, that has a horizontal polarisation and vertical.
Yes, thats possible and appears to be the newest advance that we have in PtP links, using the N protocol which has moved along from Nstreme dual.
There should be no problem, so long as you have sufficient isolation from the 2 feeds (again this will be measured in dB, but by a negative value) and that the frequencies that you use are as far apart as you can get them.
You will not be able to use the same frequency on the same feeds as there wouldn’t be enough isolation.
wirelesswaves
thank you very much for so clear explanation… 
now i’m interesting : is it possoble to interferrence between two different polarization signal with the same frequency ? does this event degradate the signal?
and also how to calculate how much mW i need for 1 km p2p link?
how to count maximum amount of clients that can connect to the aP?
can i calculate aproximately the coverage area in meters of my ap using output power and antena gain values?
thanks ahead.. i have so many questions 
i’m becoming wifi addicted 
is it possoble to interferrence between two different polarization signal with the same frequency ? does this event degradate the signal?
Answer= Yes there will be too much interference… The most reliable links are formed by the greatest degree of difference between signal to noise.
Using a dual polarised antenna will not give you enough attenuation or rejection to the unwanted signal.. Unless the channels are seperated to opposite ends of the same band.. ie one signal at 5200Mhz and the other at 5750Mhz.
how to calculate how much mW i need for 1 km p2p link?
Answer= 1km at 2.4Ghz is roughly a free space loss of -100dB.
So -100dB + (+20dBm 100mW radio card) + (+20dB gain of 1 antenna) + (+20dB gain of other antenna)
Will give you a net result of -40dBm… So since the target was -60dBm, you can afford to drop the gain of each antenna by 10dB.
Rule of thumb: You can safely use the 100dB argument for 2.4Ghz for 1Km… And then each time you double the distance, you can add another -6dB.
For example: 2Km = -106
4kM = -112
8kM = -118
http://www.k5rmg.org/tech/FreeSp.html
In answer to your question more directly.. The radio card could be set to 10dBm with 2 antennas, each having 15dB gain and still achieve the target of -60.
how to count maximum amount of clients that can connect to the aP?
answer= hundreds in theory, but in practice things start getting a little shaky when you have 20 connected, THAT ARE NOT USING ANY MECHANISM FOR INTELLIGENT POLLING.
If you use Nstreme as one alternative or play around with the RTS/CTS values (cpe only) together with hardware retries, then you should manage double that or more.
It will also depend on the data rate ie: 11b, 11g or 11n… The faster that you can get the thruput across will then give the AP time to pause and listen or poll..
Before anyone shoots me for saying that, at your level its the easiest way to understand it, but its far far more complicated than I have expressed.
can i calculate aproximately the coverage area in meters of my ap using output power and antena gain values?
Answer, yes, of course, by using the link to the calculator above..
Assuming the antenna is omni-directional (very basic setup and far from ideal)… Say it has a gain of 15dB.
Say the CPE units have a gain of 20dB
Say the AP is set so that the power is 20dBm.. Then the overall system gain is 55dB.
If then you were trying to reach the optimum of -60dB at the fringe of your coverage zone, then you could afford to go out as far as a path loss of -115dB.
Remembering the rule of thumb for above!!! -100dB = 1Km, then -106 = 2Km, -112kM = 4Km and -118 = 8Km.
So roughly and without referring to tiresome equations, I would say 6Km..
When you work out your path loss figures, link distances etc, you must remember that you are dealing with system gain, which equals, the power of one radio, and the gain of both receiving and transmitting antenna. DO NOT INCLUDE THE POWER OF THE OTHER RADIO CARD IN THE EQUATION. (Its one way only). Ie, compute for link from A to B using the radio card power of A +both antennas, then recompute the link from B to A using the power of the card in B + both antennas.
This is of course based on 2.4Ghz only, no feeder or connector losses and a clear fresnel zone, adequate height and no atmospheric ducting etc etc.
And based on what I have to say, others on the forum may have differing views!!
Hope that helps.
thanks, thanks and again a lot of thanks…
now i’m gonna start reading this book :OReilly - 802 11 Wireless Networks - Definitive Guide, and time by time try to post my questions here..
hallo again everyone, i’ve read some info about the wireless antenas.. and i have question about antena polarization..
what is advantages and disadvantages of dual and single polarized antennas?
Which is better for long distance p2p links?