|
Phased Vertical Arrays at W1KOK |
|
This webpage describes my antenna system and its construction and performance. If you have any comments or suggestions, please feel free to e-mail me. Six years ago I began to experiment with vertical arrays because a beam up on a tower has several serious drawbacks for my situation. We have a lot of ice here in the winter and, of course, ice is tough on beams and towers. A beam on top of a tower is an expensive proposition, too. Having reached an age where climbing towers no longer appeals to me, I hoped that vertical arrays would allow me to work some dx. Vertical arrays have several aspects which I like. They are inherently good low angle antennas and properly situated, they don't have to be put up in the air to perform well. The systems described here are several orders of magnitude less expensive than a tower and a beam, and they can switch direction instantaniously. Also, I want to be able to easily work on my antennas, and you can't beat a ground mounted vertical for accessability. At present I have two 6 element inline arrays on 17m, one on a N/S axis and another on an E/W axis. Since each array can be fed to radiate from either end, beam headings of roughly 0, 90, 180, and 270 degrees are achieved. I have an identical set of arrays for 20m as well. Using dot notation, I describe these arrays as ...... With the help of EZNEC, it appears that these inline arrays (......) do not work as well as 3 x 2 (: : :) arrays. The 3 x 2 array produces about 3 dB more forward gain (see the bottom of this page). Using this configuration, a four element array (: :) will outperform the six element inline array. |
|
The vertical antennas described here can be put together at a cost of about $50-75 (U.S.) per element buying materials from local suppliers. The elements are made from "L" type copper plumbing tubes available from the local hardware store here in the U.S. Each tube is 10' in length. Each element consists of two tubes, one 3/4" in diameter and a second 1/2" in diameter. The tubes are soldered together using a standard copper coupling. I use a propane torch for all soldering. Before soldering I always burnish all copper surfaces to assure a good electrical connection. After the two tubes are soldered together, the element can be cut (from the top) to resonance. I use an MFJ-259B for this purpose. Each element should be as close to identical as you can make it. These copper pipe elements are quite sturdy for frequencies at 14 MHz and higher; the element I made for 10 Mhz needed guying or perhaps a taller support. The tubes have a high copper content and make excellent radiators. |
|
Each antenna has a simple Marconi feed using a feed-through coax connector available from various suppliers (Radio Shack part #21-961). These elements are connected with 52 ohm coax. The ground side of the connection goes to a flat piece of copper which serves as a solder plate for the ground radials. I use 4 radials per element. Each radial is one-quarter wavelengh long. My radials slope from the base of the element to the ground; ideally they should be pegged just above the ground. The antenna side of the connection bolts through the bottom of the antenna element. The antenna is mounted on a hardwood stake about two feet above the earth. Plastic pipe supports are used to insulate the radiator from the wood and u-bolts hold it in place. The 6 element inline array performs very well and will take up about 65 feet from end to end. Properly placed, because of the thin profile of the elements, the antenna has little visual impact from a distance. When an array of vertical elements are placed one-quarter wavelength apart in a line and fed so each one is 90 degrees out of phase with the one before it, an endfire pattern is produced. My array has elements placed every 13'7". The theoretical gain for a 6 element array of this type is about 6 dBi at an elevation angle of 25 degrees and with a beamwidth of 75 degrees. The f/b ratio is 25 dB or better based on on-air testing. A random length of 52 ohm coax connects each end of each array to a coaxial switch (I use an Ameritron RCS-8V) providing rotary coverage on four compass headings. The system is located at a distance of about 300' from the shack. The phasing lines for these arrays are 52 ohm coax cut in electrical three-quarter wave lengths. My tests showed that cutting these by formula produced reasonably accurate results, but I used a MFJ-259B analyzer to get as close as possible. Make sure you do a good job putting the PL-259 connectors on the coax. Each element is fitted with a t-connector. The feed line, 52 ohm coax of any length, attaches to one side of the t-connector of the first element. The other side of the tee attaches to the phasing coax and leads to the next element. Each successive element is connected in a daisy chain. The open end of the tee on the last element is attached to another feedline. Both feedlines can be brought back to the shack or to a coax switch in the field. The array will have maximum gain in the direction of the first-fed element. Actual on-air performance has been very good. DX contacts consistently give excellent reports and frequently state that my signal is the strongest or only one being heard. My experience has been that my reports are comparable to those of other hams using a typical tri-band beam at 65 feet. Some hams believe that a vertical will not perform well if the station on the other end is horizontally polarized, but this is not true for DX work. Each time the signal bounces off the ionosphere the polarization is randomly shifted, so the signal arriving at the DX station has an unpredicable polarization. Noise is very low on these antennas, much lower than a single vertical. This system is clearly not going to work well in a situation where the horizon is obstructed. A few trees will not cause too much trouble, but neighboring objects and structures should be considered. This antenna works well because it radiates at a low angle; if there's a mountain sticking up 20 degrees in front of the antenna it's not going to work very well. My QTH is on a fairly level hilltop and I have a good shot to the horizon in each direction, giving the radiation the chance to travel as far as possible before bouncing. |