A collinear antenna, also called “co-linear” or “collinear antenna array”, is a set of dipole antennas aligned along their length so they form one long line. Each element of the dipole antennas is 1/2 wavelength long. Properly aligned and phased, a collinear array can provide a 3 dB or more increase in signal strength. A 3 dB increase in signal strength is about equal to doubling the signal strength. This increase in signal strength occurs perpendicular to the length of the array, so the arrays are usually mounted vertically to increase the signal strength in the horizontal plane.
Collinear arrays usually have a maximum of four elements as a result of problems with trying to keep six or more elements in phase. A four-element collinear antenna consists of two separate antennas, each with two elements and each element 1/2 wavelength long and mounted vertically so all the elements are in line. They are connected using a coaxial cable cut to be 1/2 wavelength long, calculated using the velocity factor of the cable. It is this coaxial cable that keeps the elements of the antenna in phase.
Coaxial cable has what is called a “velocity factor” that is based on the type of insulation used. The velocity factor should be marked on the outside of the cable. The length of the coaxial cable needed is found by taking the length of 1/2 wavelength and multiplying it by the velocity factor. For example, if the velocity factor of the cable is 0.66, or 66 percent, multiply the length of 1/2 wavelength by 0.66.
In the 144 MHz range, a popular variation used by amateur radio operators is called the “Stacked-J” or “J-Pole” collinear antenna. In this antenna, the braid of the coaxial cable is connected to a 1/2 wavelength element pointed down and folded into a “U” figure, with each long section about 1/4 wavelength long. The center conductor is connected to a vertical 1/2 wavelength element, another 1/2 wavelength element at the top, with both connected by a 1/2 wavelength element bent into a “U” shape and mounted horizontally. This element serves to isolate the two vertical elements.
At frequencies above 300 MHz, it is possible to use lengths of coaxial cable to make a collinear antenna, using the velocity factor of the cable to determine the lengths and alternating the connections of each section. The center connector of Section 1 connects to the braid of Section 2; the braid of Section 1 connects to the center conductor of Section 2, and so on. The phasing between the sections is determined by the distance between each section.