can you help me i need to buy access point with only 1 km diameter
No AP can have a 1 km diameter radius coverage because this will be against telecommunication laws.
802.11a and 802.11b technology leverages the ISM bands and these are free.
You will need a lot of APs to cover 1 square kilometres. If you don't know what you're doing, then I recommend you need to get a wireless site survey done.
The Cisco 1300 Series could do a point to point link over 1km but if you are looking to actually provide client coverage to devices such as laptops, smartphones, barcode scanners, VoIP phones. There is no access point that is legal that will allow you to accomplish that and actually even if you were able to cover that large of an area with a single access point the devices themselves wouldn't have sufficient transmit power and antennas to communicate back to the access point. You are likely going to need atleast 1,000 access points to cover that area.
Take a site survey : https://learningnetwork.cisco.com/thread/69268
There are some other factors that play into like retries,etc.... but that's why the typical rule of thumb is 20-25 users per radio on an access point.
you need to consider several factor when you plan for wireless such as bandwidth, protocols, frequencies, Determine density, cell sizing, antennas, coverage.you need to Determine application and device requirements
User density is the critical factor in the design. Aggregate available bandwidth is delivered per radio cell, and the number of users and their connection characteristics (such as speed, duty cycle, radio type, band, signal, and SNR) occupying that cell determines the overall bandwidth available per user
The required per connection bandwidth will be one of the imp factor in determining the number of AP's needed. you need to keep in mind that this is shared medium and available bandwidth is shared so AP placement is crucial. For more insight check the following link
And one access point will not be enough for 1km diameter . getting a site survey done will be a good idea