Fixed antennas are designed to mount on poles or sides of structures. Omni-directional antennas provide 360° of coverage around the antenna and are usually pole-type in shape. They are useful when equal coverage is required in all directions.
One of the drawbacks of omni-directional antenna is that unequal power levels from two or more wireless carriers will have equal amounts of gain applied through the antenna, which may cause one carrier to over-power another when the antenna's signal is applied to an amplifying device (amplifier or repeater). Directional antennas (panel or yagi antennas) direct most of the antenna's gain in one direction; the gain of a directional antenna would be shaped like a lobe or bubble from the antenna (unlike a circular pattern around an omni-directional antenna) and the width of the lobe is called the antenna's "beamwidth". Usually, the higher the gain of the directional antenna, the more narrow the beamwidth of the antenna.
If you think you need a directional antenna on the outside of the building, see Fixed Panel Antennas or
Fixed Yagi Antennas.