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Figure 3.1 - 5 diameter elements of collinear antenna
It is one type of broadside array using half-wave dipoles. The basic concept is when the number of elements is increase, therefore the gain will increases and the beamwidth will decreases. This antenna will produces more higher gain at higher frequencies (50 to 2500 MHz). Collinear antenna takes the form of two or more half wave in-line and in-phase, resulting in an omni-directional pattern with maximum radiation near the horizon. There are several techniques used to ensure that the half waves in phase and each of these techniques is designed to reduce or prevent radiation from the out-of-phase components of the antenna. Such the techniques are:
1) Interspersing the in-phase half waves with quarter wave stubs. This antenna array behaves like a series of vertical dipoles stacked one above the other. The more stacked sections, the greater gain and the narrower the vertical beamwidth.
2) Interspersing the half waves with resonators to provide the necessary phase reversal.
The collinear makes use of the fact that large diameter elements within an antenna can radiate more readily than small diameter elements. Shown below is a schematic of a 5 elements collinear with 3 large diameter elements which are in phase and 2 small diameter elements are in the opposite phase. Therefore, large diameter elements will dominate the radiation.

Collinear antennas are often mounted with the main axis vertical. They will be omnidirectional in the horizontal plane but will have a narrow angle of radiation in the vertical plane. Based on this characteristic, they make good base-station antennas for mobile radio systems. Figure 3.2 shows a coaxial collinear array and a vertical-plane pattern for this type of antenna is shown in figure 3.3. This antenna often is enclosed in a fiberglass sheath, called a radome, and appears as a simple pole that can be mounted off the side or on top of a mast or tower.
Figure 3.2 - coaxial collinear array
Figure 3.3 - vertical-plane pattern
Typical practice collinear antenna