Environmental Instability

Does your picture consistently breaks up after 6pm -7pm but is ok during the day? You may be experiencing tropospheric propagation.

Radio Signals used for digital TV travel in a straight line So if you could use a telescope on the top of your roof and see the top of the transmitter mast this is called “line of sight” and gives the strongest signal if you don’t have line of sight from your antenna on the roof to the tv transmitters then the signal is not as good. radio waves are similar to light so If you imagine the TV transmitter on the Hill to be very powerful light source, you would still expect to see the light even though there is not a straight line between your eyes and the light source. This is because light will reflect and refract and follow the curvature of the earth. if the signals are traveling over 100 km or over the substantial body of water, the temperature changes in the atmosphere, varies the density of the air. This causes the signals to bend either towards the Earth or away from it. This is why some long distance reception occurs near cold fronts and at the end of summer days on the coast.

Troposphere ducting of TV signals is relatively common and can affect your reception during the summer and autumn months, especially if your not in line of sight and down by the beach somewhere its caused by a change in the refractive index of the atmosphere at the Boundary between air masses of different temperatures and humidity.