Line of Sight in Signal transmission

Line-of-sight is where there is no visual obstruction between two antennae, meaning that there is the best possible opportunity for a radio signal to pass from transmitter to receiver. At some frequencies, a visual obstruction can constitute a barrier to radio communication, whereas at others the signal will arrive via a reflective path, having bounced off something else by nature of its frequency and the reflectivity of the something. The Apollo astronauts, for example, were out of radio contact with Earth while their craft was passing around the far side of the moon, the moon presenting an obstruction to line-of-sight communications and no suitable reflective surface being available (see propagation below).

http://en.wikipedia.org/wiki/Noise is often refered to in the context of the strength of the signal in relation to it. For a signal to carry any meaning, it has to have a recognisable strength over and above the background noise level. Antenna height, design, alignment, and polarisation all affect the signal-to-noise ratio, as does the power dissipated by the transmit antenna, the sensitivity of the receiving equipment, the number of obstructions and multiple signal paths between the transmitter and the receiver, among other things.

http://en.wikipedia.org/wiki/Radio_propagation is affected by transmission carrier frequency, the weather and also sunspot activities - this produces varying degrees of ionisation in the upper atmosphere. Some combinations will allow round-the-world radio communications. It is not unusual for high-frequency [HF] radio signals to bounce on the underside of various ion layers in the atmosphere several times to reach the other side of the world, whereas these layers are unreflective at VHF and UHF frequencies. The direction of transmission is also significant: New Zealand is a very large target at these frequencies and communication between there and Britain is relatively easy; New Zealand conveniently lies on a number of http://en.wikipedia.org/wiki/Great_circles that can be drawn joining them. Australia, though a bigger land mass, is a smaller target by reason of its longitude and latitude allowing fewer Great Circles to pass through both.

Further study of radio signal propagation is a recommended topic and the http://www.rsgb.org.uk/ is a good source of useful reference material.

There is a great site for line-of-sight for TV transmissions, giving a graphic output for terrain and signal. While it might not cover where you are, it helps to visualise the problem.