Horizontal verses Vertical Polarization
The question as to which is better, horizontal or vertical polarization, has not been satisfactorily answered in the available literature. That is a question that cannot be readily answered in a few words, therefore this article presents data in a graphical format to answer that question. This article deals only with the HF spectrum where the ionosphere is used as a reflecting medium, and is primarily concerned with beam antennas. This article also presents a concept that offers an extension of range of communications at a small cost, i.e., by varying the polarization of an existing antenna.
Background
Although the Ham desires a range of communication from very close to extended DX coverage, he is typically constrained to a single antenna at a single height. The height of an antenna above ground has a major influence on the radiation pattern, and in particular, the lobe structure relative to the takeoff angle. There are numerous references that provide the optimum height for a desired takeoff angle, which in turn is directly related to the communications distance of HF radio systems. To illustrate this effect, the elevation pattern of a 3 element 20 meter beam is presented in Figure 1. The beam is placed at a height of 63 feet, typical of many beams on 60-foot (18.288m) towers. At 14.3 MHz this is equivalent to a height of 0.92 wavelengths (l). On Figure 1, markings are provided to illustrate the typical communications distance relative to the takeoff angle, for the average height of the ionosphere.
Note in particular that the radiation pattern of a horizontal antenna in Figure 1 has a null that prevents communications at a range of 400 to 1000 miles (644-1609km), unless the ionosphere is other than at its average height. The pattern of the same beam, but mounted vertical, i.e., the beam has been rotated to cause the elements to be vertical rather than horizontal, is also presented in Figure 1 as a shaded area. By rotating the antenna we now have a nice pattern to fill in the null of a horizontal beam. By simply rotating the polarization of the antenna, the effective communication range of the antenna has been greatly enhanced!
As a real world example, for a long period of time I kept a schedule with antenneX editor, Richard Morrow, K5CNF, in Texas, USA (about 1,000 miles (1.6094km) from Florida). We both had beams on 60-foot (18.288m) towers and used 20 meters. Many times, due to the height of the ionosphere, we were unable to communicate at all. It would have been nice to have another antenna to switch to, but a more practical answer is to control the antenna polarization.
The above is a specific case. To provide data a Ham may use for his particular situation, a set of curves is presented in Figure 2.
To use this information, determine the height of the antenna in wavelengths. This is readily done from Figure 3 for the common Ham band frequencies. Next, determine the distance you are interested in using, and find an equivalent antenna pattern takeoff angle (available from Figure 1). Follow that line on Figure 2 (or extrapolate between them) and determine which provides the maximum signal, horizontal or vertical polarization. If the curve is to the left of center, then vertical polarization is preferred. The amount (dB) one polarization is preferred over the other, is plotted along the horizontal axis. The plot provides a quantitative comparison between vertical and horizontal polarization, not an absolute value of pattern gain for any particular antenna. Therefore, the plot applies to any antenna from a dipole to a multi-element beam.
In general, if the interest is only in long haul DX—takeoff angles below 10 degrees—and the antenna height is above 0.5 wavelength, then horizontal polarization is preferred. If the antenna is greater than 0.8 ohms and both DX and short range communication is desired, then a polarization rotator on the beam is recommended, or a second beam at a height of about 0.5 ohms. The rotator is much less expensive. To illustrate, assume a 15 Meter beam (21 MHz) is mounted at 60 feet (18.288m). From Figure 3, this is 1.25 ohms From Figure 2 there is a 6 dB advantage for vertical polarization at a takeoff angle of 20 degrees, corresponding to a distance of approximately 1,800 miles (2,897km). This should explain the “holes” in range encountered with typical beams, but a relatively simple change can be made to fill that hole.
Implementation Concept
Figure 4 presents a concept for changing the polarization of a beam. A rotator would be mounted above the beam and coupled via an arm. The rotator need only be a light weight TV rotator, since there is only a small mass to rotate, and the length of the arm helps multiply the available torque. The clamp for the beam must be modified to include bearings to allow the boom to rotate.
I hope some company will produce a kit to allow Hams to modify their existing systems. Those Hams that are mechanically inclined can readily make their own.
Footnote
A large amount of effort was required to collect and plot the data. This could only be done with the use of a program like ELNEC, due to the excellent human engineering that went into making that program exceptionally user friendly, and very easy to move and rotate antennas. There was a lot of time saved using a 486 PC computer. It was these two factors that enabled the derivation of the plot in Figure 2. Without both, I would not have attempted this task. Thanks to Don Klein KC3RQ for assisting in this effort.
Originally posted on the AntennaX Online Magazine by Ted Hart, W5QJR
Last Updated : 23rd April 2024