An abstract from "ARRL Antenna Book"
Collinear Transposed-Coax Arrays
Collinear arrays tend to be tolerant of construction tolerances, making them easy to build and adjust for VHF and UHF applications.
The most popular collinear array is the omnidirectional array of half-wave dipoles constructed of transposed sections of coaxial cable
as shown in Figure 15.10B. The original array of this type is the Franklin array
shown in Figure 15.10A.
The phase-reversing stubs allow multiple half-wave sections to operate in phase, creating gain at right angles to the antenna. An example of this array is the popular Cushcraft Ringo Ranger series of omnidirectional VHF and UHF antennas. While the phasing stubs make the Franklin array inconvenient for vertical stacking of more than two elements, a derivative of this array uses transposed sections of coaxial cable as in Figure 15.10B.
The phasing stub is created by the inside of each coaxial section. The outer surface of the coaxial shield forms the radiating element
. The resulting antenna can be enclosed in a PVC or fiberglass tube, such as the Comet GP-series of VHF/UHF omnidirectional antennas.
The practical limit for gain in this type of array is about 10 dBi. A choke balun or other method of decoupling such as a set of λ/4 radials is required at the feed point of the array to prevent current from being induced on the outer surface of the coaxial feed line.
Collinear Omnidirectional Array for 70 cm
Figure 15.11 shows the basic construction of a transposed-coax array for the 70 cm band with dimensions in millimeters for accuracy. The λ/4 whip at the end of the array is optional
. The gain of this array is approximately 9 dBi (slightly less without the whip). The original design of this antenna is credited to the Radio Amateur Society of Norwich (www.rason.org
). More information is available via the “Projects” page of the RASON website.
The physical length of each λ/2 section of coax must account for the velocity factor of the cable which should be measured accurately before cutting any cable. Once the physical length of λ/2 has been determined, add 8 mm to allow for creating the 4 mm connecting surfaces on each end.
For a VF = 0.66, the λ/2 sections should be 223 mm long plus 8 mm for a total of 231 mm. RG-58, RG-8, RG-8X or RG-213 can be used for this antenna. Do not remove the outer jacket from the cable other than at the connecting ends as this will allow the individual braid strands to loosen, reducing the shield’s effectiveness as a continuous conductor.
Use a 169 mm segment of #16 AWG copper wire for the top whip section.
A λ/4 coaxial sleeve balun
is attached at the feed point of the antenna. (See the Transmission Line Coupling and Impedance Matching chapter.) The balun is made from copper tubing that is soldered to the shield of the feed line using strips of brass or copper shim. If 5⁄8-inch tubing is used, the length should be 160 mm. The feed line should be centered in the balun tubing by using small pieces of plastic inserted between the coax jacket and the tubing’s inner surface. Approximately λ/4 beyond the end of the balun’s closed end add an additional choke balun of three type 43 ferrite beads
(choose the ID to fit the feed line coax).
The entire antenna should be enclosed in a length of PVC or fiberglass tubing to protect it from the weather. If necessary for mechanical stability, support the antenna sections with a length of wooden dowel or plastic rod, secured with electrical tape.