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
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
Figure 3.3 - vertical-plane
practice collinear antenna