But my application requires more than 4 channels. I dont know whether rf modules with more than 4 channels are already available or not. But if they are, then the prices will be high.
So i want to build my own rf module so that i can decide the frequency and also increase the number of channels
You cannot just decide on a frequency .... as the freq's you choose MUST still be within the band allocated for such things.
Therefore you will still have the channelling problems
Unless you are a skilful circuit designer and RF technician, then, seriously, you would be nutz to use anything other than commercial modules ...
many of them are designed to cover many dozens of channels that are selectable by a little programming and microprocessor control, eg .
a PIC controller. Many others can also use sets of DIP switches to select one of many channels
trying to design and build from scratch would be a serious job for an unskilled person
there's a zillion multi-channel RF modules, matched TX and RX pairs on eBay for dirt cheap prices
https://www.ebay.com/sch/i.html?_nkw=rf+modules
specifically .....
https://www.ebay.com/itm/2PCS-Ardui...h=item3ad3d47c4e:g:YP4AAOSwLzdWTYYk:rk:3:pf:0
2PCS Arduino NRF24L01+ 2.4GHz Wireless RF Transceiver Module New
Maximum operating speeds up to 2Mbps, GFSK modulation efficiency, Anti-interference ability, Particularly suitable for industrial control applications.
125 Channels, Multi-point communication and frequency hopping to meet the communication needs
Built-in hardware CRC error detection, Multipoint communication address control.
Low-power 1.9 ~ 3.6V, only 1uA on Power down mode
Built-in 2.4Ghz antenna
Available software to set the address, only received local Address when output data(Provide interrupt instruction), can be directly connected to a variety of microcontrollers, Software programming is very convenient.
Built-in voltage regulator
Standard DIP Pitch Interface for embedded applications
size: 34mm X 17mm X 1mm
you could NEVER build it as small or as cheaply as that
Dave