Originally, I used a DHT22 as the external T/H sensor but these devices don’t survive well outdoors. They work for a few weeks and then the humidity sensor starts giving strange readings. Normally the temperature sensor is OK. I replaced the DHT22 with a SHT7x which is rated for outdoor use. My only worry is that I am using around 8 metres of ordinary 4-core telephone cable between the sensor and the Tx, but it appears to working OK. I embedded the SHT7x in a blob of Sugru inside an old Fine Offset transmitter.