Refer to figures.
The VHF band extends from 30 MHz to 300 MHz. The frequency range normally used for Aeronautical Mobile Service (AMS) voice communication is defined between 118.000 and 136.975 MHz. This is a subpart of the VHF band. Any VHF transmission has the form of direct waves (space waves).

Space waves travel in a straight line. For efficient VHF transmissions, line-of-sight is required between the transmitter and the receiver. In other words, the straight line joining the transmitter and receiver must be relatively free of obstacles (i.e. mountains) since they might block the signal. Low level flight reduces greatly the range of reception, due to the presence of hills, mountains, bumps on the surface of the Earth and the roundness of the Earth itself. Consequently, high level flight provides better chances to establish communication on a VHF frequency.

When the aircraft is further away from the aerodrome, there is a danger of existing obstacles between the transmitter and the receiver, affecting the reception of the VHF signal. What is more, VHF space waves travel in a straight line, becoming weaker as distance increases. Consequently, being closer to the aerodrome provides better chances to establish communication on a VHF frequency.

Therefore, you may expect a solid reception when the aircraft is at high level in the vicinity of the ground station.