Have a look at goTenna (<a href="http:&#x2F;&#x2F;www.gotenna.com&#x2F;" rel="nofollow">http:&#x2F;&#x2F;www.gotenna.com&#x2F;</a>). It uses 151-154 MHz radio waves to communicate upto 4 miles. Doesn&#x27;t require mobile network. It&#x27;s probably too large for your requirement, but may give you some ideas to DIY or adapt its PCB.