Another antenna is officially not allowed on the RFXtrx433 transceiver because this product is certified with the supplied antenna but for the RFXrec433 receiver it should not be a problem.
To increase the receiving range you can connect for example a www.conrad.com
order nr 190073 433MHz ground plane antenna with BNC coax cable,
Or a http://www.radioshack.com/product/index ... PerPage=60
scanner antenna. This antenna needs a coax cable with PL-259 and SMA or BNC connector.
This adapter can be used for SMA- to BNC http://www.ebay.com/itm/BNC-female-SMA- ... 2a03ea8391
The location of the sensor and the RFXCOM is also important if the RF signal has to pass through walls. See chapter 2.4 in the RFXtrx User Guidehttp://www.rfxcom.com/documents/RFXtrx% ... 0Guide.pdf
It is expected that RF interference is happening if sensor signals disappear for several hours. RF interference will occur if sensors of the same type are configured to the same channel.
Here is the list of Oregon sensor types TEMP1, TEMP2... http://www.rfxcom.com/oregon.htm#Oregon
If sensors disappear for a longer period the source can be another transmitter that is transmitting continuously or with short intervals. If X10 MS13 sensors are in use check the battery in this sensor. When the battery becomes almost empty the MS13 starts transmitting a weak continuous RF signal but just enough to disturb the sensitive RFXCOM receiving.