The HO Collinear, a Horizontal Omni
This paper describes a high gain, multi-element horizontal omni with a single 50 ohm feedpoint with instructions for building a 4 element array for 2 meters.
There are several ways to make a horizontal omni [1,2,3,4,5]. One popular type is the HO which is a half wave dipole folded into a square or a circle [6,7]. 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 through power dividers.
A simpler way to combine elements is to connect the loops together by adding vertical extensions to the ends of each loop. The lengths of the loops and vertical wires are adjusted for maximum horizontal gain consistent with a 50 ohm feedpoint in the center of the bottom loop using an antenna modeling program. Since I decided to use 3/8 inch diameter soft copper tubing for the horizontal loops and # 18 wire for the vertical wires, I used mininec because nec cannot be used for antennas made of two wires of very different diameter [8,9]. It is possible to build this antenna with only one diameter wire, in which case nec can be used .
A test antenna was made for 144.5 MHz consisting of four horizontal loops made of 3/8 inch diameter soft copper tubing and two #18 (0.040 inch diameter) vertical wires. A loop drawing is shown in Figure 2. For the top and bottom loops, L = 31 7/16 and S = 1 5/8. For the middle loops, L = 28 5/16 and S = 1½. 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 piece of ¾ inch schedule 40 pvc pipe. The spacing between the elements (and the length of the vertical wires) should be 20 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 off axis from the top of the antenna. The antenna is fed in the center of the bottom loop. As with other off center fed antennas, the feed point is not balanced, so a current balun is required. See my Quadix page  for balun references.
Figure 1 shows the assembled antenna on the roof. No patterns were taken, but the SWR (Figure 4) agreed with the simulated SWR (except for a 1% frequency offset error) which gives confidence that the antenna is working properly. The simulated gain is 3.27+/-1.22 dBi.
Adding more elements will improve the gain. If you decide to change the design using different diameters, more elements, or a different frequency, you should resimulate using mininec. Mininec typically has frequency offset errors, so the final design may have to be tweaked a bit. I increased the vertical wire lengths from 20 to 20 7/8 inches from my original simulation (keeping the loop lengths the same). The frequency can also be adjusted by varying the gap at the end of the loops. Closing the gap lowers the resonant frequency (and also the impedance). Increasing the gap increases the gain variation (i.e., makes it less omni). If you decide to use the same diameter for the vertical and horizontal elements, then nec can be used, but it may not be possible to get to 50 ohms.
One further comment: The spacing between the loops is smaller than is usually used in collinear arrays. This is because the electrical length between the centers of each loop is a half wavelength. The radiation from the loops is kept in phase by reversing the wire direction between adjacent loops (See Figure 3).
Ross Anderson W1HBQ firstname.lastname@example.org May 21, 2004
July 24, 2006
 The Eggbeater EB-144 http://www.wb0w.com/m2/2mtr/2_meter_antennas.htm
 The Batwing http://www.qsl.net/kd2bd/batwing.html
 The Turnstile http://www.cebik.com/wire/turns.html
 The Wheel http://www.hamtv.com/wheel.html
 Omniangles http://www.parelectronics.com/omnis.htm
 2M HO Loop http://www.wb0w.com/m2/2mtr/2_meter_antennas.htm
 KB6KQ Loops http://www.kb6kq.com/products.html
 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.
 I use 4nec2 by Arie http://home.ict.nl/~arivoors/Home.htm
 The Quadix http://home.comcast.net/~ross_anderson/quadix.htm
My homepage “Ross’s Antennas”, with links to my other pages, is http://home.comcast.net/~ross_anderson