Best Wireless Modules for Arduino Star Network 3500m Range, 20 Slaves, 1 Master

Hello Dears , 

I want to make a wireless connection projects that have a star network. Or, Point to Multi-Point.

With Arduino I want to make it, for agricultural purposes, for a distance about of 3500 meters.

I have 20 slave node and 1 Master. 

With regard to the question of what kind of modules do you recommend?

is suitable NRF24L01 module for this project?? 

Thanks for your Help. 

Have you looked up the datasheet? I'm not sure it can work 3500meters. Maybe multipoint connection would do. The NRF24L01 module is quite cheap but can still do the work you require.

3500m is going to be a stretch for most 2.4ghz low power radios. Depending on where you live (allowable frequencies and power levels vary by country) and what your data rate requirements are you might find 900/868Mhz LORA radios are a good fit at that range.

Development boards like this: http://www.mouser.com/new/stmicroelectronics/stmicro-stm32-lorawan-discovery-board/ are a good place to start. Since the Murata LORA module has a 32bit ARM processor (STM32F0 series) you might be able to eliminate the arduino and just use the STM32F0. These boards do have the standard Arduino shield connectors (3.3V signals only I think).

some of that seller say its work too 3500 meter with Booster , But I'm not Tested it.

so thanks a lot for your help ,

what frequency this Development boards use?? 900/868Mhz ??

Yes, 868/900. There is a similar board available for the 433Mhz band.