It could be achieved with ISM band radios. Please let me know if 433MHz/868MHz/915MHz are allowed at you country. This is free band radios for personal use though. There are high power radio modules available to communicate up to 2Km and the power could be maintained to get the 500 yards range. My bid includes designing the schematic, development of the PCB layout, development of firmware for it as well as building one set of prototype that will be shipped to you once tested. There are plenty of options to get the same result, so we can discuss and select the best suitable option that is available at your country without any implication of legal frequency usages. It would be an interesting job indeed. Thank you very much for you time and I am looking forward to hear from you. Best Regards... Roy