The Cycloid Collinear, a CP Omni
Despite the advantages of circular polarization, amateurs have made little use of it. One of the main reasons is the difficulty in making a good circularly polarized omni-directional antenna (cp omni). This paper describes a wide band, high gain, multi-element cp omni with a single 50 ohm feedpoint with instructions for building a 2½ element cycloid collinear for 2 meters.
There are no commercially available cp omnis for amateur use , although there are many available for the fm broadcast band . A common design is the cycloid which consists of a horizontal loop approximately lamba/2 in circumference, with short vertical extensions at the loop ends . The radiation from the horizontal loops is approximately omni-directional and approximately equal to the radiation from the vertical sections (and is 90 degrees out of phase), resulting in circular polarization. The impedance of this antenna is not 50 ohms and requires a matching section . If more gain is desired, then two or more elements can be stacked collinearly, with each element fed separately.
A simpler way to combine elements is to extend the vertical sections of each element until they touch. However doing this increases the vertical radiation and the antenna becomes mostly vertically polarized. Simulations have shown that this makes a very good vertically polarized omni with a small amount of horizontal polarization. The vertical polarization radiation can be reduced by reducing the diameter of the vertical wires . The antenna described here is just such an antenna.
A test antenna was modeled for 146 MHz consisting of three horizontal loops made of 3/8 inch diameter soft copper tubing and two #18 (0.040 inch diameter) vertical wires. Mininec is required for the modeling because nec cannot simulate an antenna made of two wires of very different diameters [5,6]. The design goal was to design a right hand circularly polarized antenna with 50 ohms impedance at 146 MHz.
A loop drawing is shown in Figure 2. For the top and bottom loops, L = 33 9/16 and S = 1 ¾. For the center loop, L = 28 and S = 1 ½. Cut the copper tubing to the lengths shown and bend them to approximate a circle with a gap as shown. All dimensions are in inches.
Then support the elements in some way. I used a ten foot length of ¾ inch schedule 40 pvc pipe. The spacing between the elements (and the length of the vertical wires) should be 57 7/8 inches. Figure 3 shows how the vertical wires and the loops are connected together. This figure is drawn from the point of view slightly off axis from the top or bottom of the antenna. The polarization from an antenna assembled in this way is right hand circular polarization (rhcp). The antenna is fed in the center of the center loop through a balun.
Figure 1 shows the assembled antenna on the roof. No patterns were taken, but the good agreement between the simulated and measured SWR (Figure 4) gives confidence that the antenna is working properly.
The simulated gains at 146 MHz are:
Right hand circularly polarized gain = 4.54+/-0.59 dBi.
Vertically polarized gain = 2.76+/-0.05 dBi.
Horizontally polarized gain = 0.63+/-1.45 dBi.
Adding more elements will improve the gain. If you decide to change the design goals using different diameters, more elements, or different frequency, you should resimulate using mininec. If you decide to make a mostly vertically polarized omni using the same diameter for the vertical and horizontal elements, then nec (which is more accurate than mininec) can be used.
I have not said anything about why you would want a cp omni. Please refer to the references for discussions on circular polarization.
Ross Anderson W1HBQ firstname.lastname@example.org April 19, 2004
July 24, 2006
 GBPPR Tech Bulletin #2 - Advantages of Circular Polarized FM Repeater Antennas http://www.qsl.net/n9zia/cir_pol_rpt.html
 The WA7X Beacon The Cycloid Dipole http://www.wa7x.com/cycloid_info.html
 The Simple Collinear http://home.comcast.net/~ross_anderson/sc.htm
 When MININEC is Superior to NEC http://www.cebik.com/amod/amod56.html
 I use K6STI’s AO. His e-mail address is bb at tns dot net. Other sources for mininec can be found on the web.
Circular Polarization References:
My homepage “Ross’s Antennas” with links to my other pages is http://home.comcast.net/~ross_anderson