The type of aerial needed depends on the site requirements, so we normally recommend a site survey to identify the optimum type. Factors that influence the requirements include the radio frequency, the range required, whether or not directional aerials can be used and any environmental needs.
A dipole aerial transmits equally in all directions (including up and down), and is used as the reference against which aerial gain is measured. The radiation pattern can be visualised as a sphere.
Colinear aerials and some whips give a radiation pattern like a doughnut, so offer a higher gain in the horizontal plane but don’t transmit up or down.
Yagi aerials are directional and give a radiation pattern like a funnel so focus the power in one direction.
It should be apparent that a base-station that communicates with a number of outstations usually needs to use a dipole, colinear or whip aerial. Each outstation only needs to transmit to the base-station, so can use any type of aerial.