I just found the high gain antenna's GAIN is higher and higher, and the price to too low to be ture in ebay market.

When I talk with my firend who made 5dbi antenna for LINKSYS, he said the high gain antenna's market is in chaos. You can test a CPU's speed, you can test a harddriver's capacity, but it's hard to measure the antenna's gain. They have instrument, but it's too expensive to others (around 5000US$).

And he can confident that it's impossible to make in-door omnidirectional Antenna more than 10dbi(it's must be longer than 1 meter).  My suggestion is: Buy 7dbi antenna made by linksys or d-link. They are real 7dbi antenna. And in most case, 5dbi is good enough

I try to ask him give me some 9dbi antennas with cheap price, he told me the cost of materials is more than 10$, so... I decide buy some linksys 7dbi antenna. By the way, here is also some fake 5dbi antenna in market, so I just suggest you buy brand name antenna.

Here is a simple table can tell you about the gain of antenna:


    Gain       2dbi                 5dbi          7dbi        9dbi
In-Door
   1-50m        O                   O             V           V
 50-70m        V                   V             o           o
 70-90m        X                   X             V           o

Out-Door
   <300m         O                  O              O           O
300-500m       V                  V              O           O
500-900m       X                  V              V           O

O Good            V Good/workable         × out of range