Skewed Path vs. Antenna Polarization

Radio signals are not always propagated on a direct great circle route between two stations. When it happens it can be surprising and so we remember it. Yet it occurs less often than what we might believe. The ordinary act of determining direction (azimuth or bearing) by where we point our antennas to maximize the signal can be misleading. 

This is particularly true when we determine direction by comparing antennas with different polarities. We are misled because antenna patterns depend on the polarity. For HF antennas these are the linear polarity modes -- horizontal and vertical -- referenced to the ground. Other than local effects such as ground loss, Faraday rotation ensures that antenna polarity is often not terribly important. A received signal can have any linear polarity, no matter the polarity of the transmit antenna, and it can change quite a lot in mere seconds.

A good example of a polarity-dependent antenna pattern, and how it can be misunderstood, or exploited when properly understood, is the lowly inverted vee. Yagis have a similar behaviour, as shown above. Off the sides, there is substantial vertically polarized radiation at high elevation angles. The elevation plot is for a 3-element 20 meter yagi up 15 meters at 85° to the side. I didn't take it to 90°, directly off the element ends, because the horizontal component is too small to see. There is never infinite signal rejection under real world conditions.

With some antennas the effect of apparent direction and polarity can be even more surprising. This is common on the low bands when we use different transmit and receive antennas. We use directive receive antennas to better copy stations that can hear the signal from an omni-directional transmit antenna.

These are the 1.8 MHz azimuth and elevation patterns of a terminated 150 meter long Beverage antenna 2.5 meters high. Radials are added in the NEC2 model to emulate the behaviour of ground rods -- NEC2 doesn't permit direct ground connections. The elevation pattern is for the centre of the forward lobe where the horizontal component is nil. The main lobe is vertically polarized, which is exactly what is expected of a Beverage. The gain peaks at 40° elevation. The lobe is narrower and the peak elevation lower when the antenna is longer than 1λ, as a 150 meter long Beverage would be on 80 meters and up.

The 160 meter patterns for the Beverages in my station are little different since they are no longer than 175 meters. Notice the substantial horizontal polarization 45° off the forward direction. At an elevation angle of 40°, in some directions the gain is higher for horizontally polarized signals. As we'll see, that is relevant to the main thrust of the article.

From this discussion of polarization and antenna patterns we proceed to consider why hams might believe a path is skewed when it is not. Skew propagation is relatively rare because it requires a horizontal ionization gradient in the lower ionosphere to refract or reflect the signal to the side. 

Active aurora is responsible. There is a narrow area adjacent to the active aurora region, between where E-layer absorption is high and further distant where the gradient is low. It's a transient phenomenon.

There is an ordinary horizontal gradient in the ionosphere since solar insolation is latitude dependent. It is too shallow by itself to significantly skew signals from the direct (great circle) route. Forward and backward scatter are common, but the scatter signals are very weak and, although useful at time, are not skew propagation.

My choice of speaking about Beverage antennas is no accident. I am regularly presented with the appearance of skew path when using my Beverage system on 160 meters.

The occurrence of apparent skew path that motivated my exploration is the sunrise opening to Japan and the Far East. Since it is common when geomagnetic activity is low, the auroral mode of skewing is unlikely. With the Sun conveniently rising around the time I normally wake up -- 7:00 to 7:30 AM -- there are many mornings that I turn on the radio to see what propagation there might be to Asia and the Pacific.

As many have lamented, this has not been a great season for top band. With increased solar activity the absorption of the D-layer and polar regions has increased. Unlike during solar minimums, signals from the Pacific and Asia before and after winter sunrises have been far weaker or totally absent. With limited CW activity, or at least other than that from a handful of regular operators, I most often flip to 1840 kHz and monitor FT8 activity. That way I can easily check propagation while having breakfast.

The Beverage directions of note in this case are the north and west selections of the north-south and east-west reversible Beverages. It has been quite common this winter that signals from JA, HL, HS and a few others are strongest or best copied when pointing west. This is interesting since the great circle bearing for those countries ranges from 330° to 0°, which is right in the centre of the north Beverage's forward lobe. The north-south Beverage points a little west of north, at about 350°. The west Beverage points almost exactly 270°.

Very often this winter the signals from the Far East are best with the west Beverage. About half of the time the north Beverage is best. Pacific and west coast stations are always strongest on the west Beverage. KL7 and VE7 are often a toss up because the bearings to both are in between and the main lobe of the Beverages is pretty broad on 160 meters.

Is it skew path? The question is often asked by those with directional antennas on 80 and 160 meters. The answer is not obvious in many cases. What I am about to present is not novel, but it is worth taking the time since many hams fail to consider the interplay between antenna polarity and ionospheric behaviour.

First, I think it appropriate to reiterate a fundamental rule of antennas and propagation.

The ionosphere, not the antenna, determines the propagation path and its apparent azimuth and elevation direction. Our job is to choose and use antennas to best exploit the path or paths that are present.

Solar irradiation on the D-layer causes it to absorb lower frequency signals. The lower the frequency the higher the absorption. On 160 meters the process is rapid and begins before sunset when the high ionosphere sees the Sun many minutes before its rays strike the ground. Propagation, if it exists, must have a higher elevation angle so that absorption is minimized; at low elevation angle the path through the D-layer is longer and therefore radio waves are more attenuated.

If there is no high angle path, signals will be very weak or absent. At high angles our vertical and vertically polarized antenna patterns are a poor fit. Let's have a closer look at the pattern of the 150 meter long Beverage on 160 meters.

The pattern at an azimuth of 0° was shown earlier and it the polarization was entirely vertical, as expected for a low travelling wave antenna such as a Beverage. The horizontal component grows as we turn away from the main lobe, becoming equal to the vertical component at around 45°. Keep in mind that the antenna itself is, of course, firmly affixed to the ground; it doesn't rotate! That's why we must have several Beverage antennas for 360° coverage.

The great circle route to Japan and Far East falls between 60° and 90° off the side of the west Beverage antenna. In those directions the horizontal component is stronger than the vertical component, although 5 to 20 db weaker than the vertical main lobe. As we approach 90°, directly off the side of the Beverage, the vertical component is particularly weak at all elevation angles.

It is not enough that we must aim high at sunrise for the west Beverage to perform better than the north Beverage. The polarization must also be horizontal, and close enough to being exactly horizontal for the high gain vertically-polarized main lobe to appear deficient at high angles. 

Much has been observed and written on polarization of MF signals that travel through the ionosphere. There are effects, especially the electron gyrofrequency, that favour vertical polarization in most areas of the globe and horizontal polarization in other areas. I am in the majority of hams that live where vertical polarization is dominant on 160 meters. On HF, including 80 meters, Faraday rotation is more dominant and thus polarity has a strong time variability. Faraday rotation is slower on 160 than on HF.

Assuming the path is not skewed, it must be the case that the signals are horizontally polarized. Only that way will the weaker minor lobes compete with the main lobe. In the example of the Far East that I am highlighting, there has been little difference between north and west Beverages this winter, on average; sometimes west is superior and sometimes north is superior. I don't remember this happening as often last year. The east-west Beverage hasn't been up long enough to compare to earlier years.

Either way the signals have been weak and marginally workable with a kilowatt to my full size vertical. A vertical antenna strongly favours vertical polarization in all compass directions. Receiving a horizontally polarized signal well on the west Beverage is no assurance that I'll be heard since the path may favour horizontal polarization. For skew path, when using the Beverage for the apparent direction, reception should be full strength and vertically polarized. A successful QSO becomes more likely. 

A vertical directional antenna for transmission and reception could help to answer the question since these antenna have no more than a tiny horizontal polarization component at all azimuth and elevation angles. An example is my K3LR array on 80 meters; there are a number of these "big gun" antennas and 4-squares on 160 meters. They are huge antennas and I'll never have one; my aspirations are more modest. An alternative is extensive testing with a suitably equipped partner in the Far East. For me, these are both unlikely. I may have to remain ignorant on the matter.

Where this leaves me...

After this long-winded analysis, what should I conclude? Nothing definitive I'm afraid. The path is likely direct and the polarization is more horizontal than what I'm accustomed to experiencing. The signals may be weak for no greater reason than the off axis horizontal component of the west Beverage has a gain 10 db lower than the vertically polarized main lobe. 

Whether skew path or not, there is a valuable lesson. It helps to have more than one antenna, with a selection of horizontal and vertical polarization. That is impossible for most hams but there are alternatives. For example, a low band dipole can be an effective receive antenna on 160 meters. After lightning took out my Beverage system last year I could often get by with the XM240 as a receive antenna. The few decibels of relative gain it has in the forward and reverse directions can be enough to make copy an otherwise unintelligible signal.

When we cannot get directivity or polarity diversity from our low band transmit antennas, it is very worthwhile to install a receive antennas or use a low band dipole or yagi, even though they are far from ideal. Be flexible and try what you have. You might be in for a pleasant surprise. As the saying goes: You can't work 'em if you can't hear 'em.

You may have noticed that several of the links in this article go to K9LA's 160 meter page. If you have the time, it is worthwhile to browse the site. There is a collection of articles by him and by others that delve deeply into top band propagation and antennas. ON4UN's Low-Band DXing book also contains a lengthy discussion of low band propagation.