I was speaking to a friend at work about the problems with these wireless devices and the answer (or the reason) is so simple!
He specializes in radar and communications systems and I basically said that when no signal is being transmitted (and therefor received) I actually get loads and loads of noise on the receiver (by looking at it on a scope)
He said "of course you would!"
The reason being is that all these sorts of devices utilize AGC (automatic gain control)
If you think about it - if you have the transmitter and receiver within centimetres away from each other and you were transmitting data to the receiver, then the received signal would be through the roof - it would be very large indeed. So the AGC of the receiver would reduce the gain to bring it down to the required level.
If you were to move a few metres away from the receiver and transmit again, the received signal would not be nearly as big in amplitude as the previous (because we are further away) So the AGC would automatically adjust the gain to get it to the required level.
So if we are not transmitting anything - the AGC puts its gain upto maximum to try and get some signal - with gain up that high basically any tiny little signal will get amplified and thats why we see so much noise!
So while that does explain the problem, it does not fix it
Now as for the distance that you can transmit there are two key things you need to do. The first is that these require 50 ohm coax to be used as the antenna. If you dont use 50 ohm coax you will get HUGE losses and therefor your transmitting distance is greatly reduced (to just a few metres)
Furthermore, the length of the antenna needs to be spot on to further reduce the loss. I think 24cm is what is recommended for these.
Now as for manchester encoing, that is a great way to overcome the AGC problem of the receiver. As far as I understand, manchester encoding always transmits clock pulses even if we are always sending logic 0's - is that correct?
So this means the receiver is always receiving something, therefor it sets the gain for that particular signal, and doesn't amp it up to allow noise to interfere.