I have been using the RFXCOM RFXtrx433E USB 433.92MHz Transceiver for a while now and I have noticed that the reception range is sometimes bad. Even without moving it I have found that the weather have a lot to say about how the reception range is. After some experimentation I found that the best placement of the device is high up, of course, but also so the antenna goes parallel with the ceiling. I taped it close to the ceiling so that the antenna pointed out into the room.
After this placement I noticed that the signal level of most sensors increased one or two levels. Also, the sensors that used to be unreliable earlier became reliable and the device was now able to read every transmission.
I decided to take this even further so I purchased what I believed to be a better antenna. I bought a GSM GPRS Antenna 433 Mhz 2.5dbi cable 90° SMA male Universal DAB Patch Aerial on eBay for $4.40. Here is the technical specifications:
Frequency Range(MHz) : 433MHz
Gain(dBi) : 2.5
Input Impedance(ohm) : 50
Antenna Length(mm) : 115
Cable Type : RG174
Cable Length(m) : 1.5
Connector Type : SMA plug right angle
Mounting : Adhesive
Antenna wide.(mm) : 21
Since this antenna has a 1.5m cable attached to it, I was able to take the RFXCOM down from the wall and put it back on the shelf. The initial tests with this antenna is very good news. The signal levels from all the sensors went up one step further. Even the test transmitter I have out in the shed went up, and the signal has to travel over the yard and trough several walls.
I will highly recommend this antenna. Here is a link to where I bought it: