Beware of Skip Zone while working HF radio
The issue that my friend was facing was a typical ionospheric skip zone situation. Although on the 20m band, he could work stations in Moscow, various parts of Italy and Spain which are all quite a distance away, he was unable to copy or work into nearby countries like Germany, Netherlands, Czech Republic, etc. I soon realized that in his hurry he had completely forgotten to account for the prevailing “Skip Zone” around his QTH. All of us make such mistakes and then scratch our head over inexplicable results.
For those who would like to visualize what was happening on the 20m band from the location in Poland, the illustration below will make it quite clear.
For all the readers who are new to HF propagation, I will try to briefly explain the concept of Ionospheric Skip Zone. In the illustration, the QTH of the operator is marked with a red dot. The surrounding area in blue color with green boundary is the Skip Zone area for that specific QTH. The size of Skip Zone will be different on different frequency bands and also vary with day/night as well as seasons.
Skip Zone is an area surrounding the QTH of the transmitting station. This zone is circular or elliptical in shape and may have a radius of several hundred Kilometers. Higher the frequency, the greater would be the Skip Zone radius. The Skip Zone radius as I mentioned earlier will vary with the Sun-spot Number (SSN), time of the day (day/night) and seasons. If the receiving station is situated in this blue colored Skip Zone, then no communication is possible. Whereas, radio communication with any station situated outside the Skip Zone, shown in yellow or red color will be feasible. Areas shown in red will receive stronger signals than the yellow areas close to the Skip Zone.
In other words, in the HF communication scenario, the RX station that is nearby will not be workable whereas the RX station at a greater distance will receive a strong signal. Now, one mak, Why is it so? The whole thing seems to appear so illogical and senseless at first. However, the fact is that every physical phenomenon has a clear scientific logic and reasoning. I will briefly try to explain the scientific reason. Most of us Ham Radio operators know that HF ionospheric skip communication occurs due to the reflection (or refraction to be more precise) from charged layer canopies that exist in layers at heights varying from 50-350 Km above the surface of the Earth called the Ionosphere.
The RF power emitted from the antenna of TX station travels towards the sky at various angle and are intercepted by an ionospheric layer (termed D, E, F1 or F2). After encountering a layer of sufficient charge density, the radio wave-front bends back to return back to earth at a distance quite far away from the TX location. A receiver at the distant location picks up the returned signal and thus communication is established. If the RF signal from the TX antenna strikes the ionosphere at a long distance away from the TX, then the angle at which it strikes the ionosphere is a shallow angle.
Similarly, when an RF wave-front strikes the ionosphere at a distance close to the TX station, then the angle of incidence is larger. The RF wave from the TX must be projected skyward at a higher angle to strike the ionosphere at an angle that is closer to vertical. Such high angle wave-fronts that strike the ionosphere at a near-vertical sharp angle would be the ones that would be expected to reflect back to locations closer to the transmitter. Therefore the ionosphere must be able to reflect back these high-angle signals back to earth if near range ionospheric communication were to be possible.
However, this does not always happen. When the ionospheric layer density is not very high and the layered slabs are not very thick, then the incident RF wave-fronts at high angles tend to cut through and penetrate the ionosphere to escape into outer space. When this happens, the nearby RX station that was expecting the reflected wave to reach its antenna never gets it. The wave that was meant to reach it did not reflect back but penetrated the ionosphere. Hence a Skip Zone is created with a certain radius around the TX station where high angle reflected signal returns were expected.
Now, if the prospective RX station location is moved further away from the TX location, the distance between them increases. This, in turn, requires the TX wave-front to strike the ionosphere at a shallower angle to cover a larger reflected distance span. The required incident angle becomes shallower with the increase in TX-RX distance. Finally, we arrive at a situation when the angle of incidence of the RF wave striking the ionosphere becomes low enough. This is the transition angle below which the radio waves will no longer penetrate the ionosphere but begin to reflect (refract) back towards earth. An RX station located at a corresponding distance is a point which falls at the edge of the Skip Zone. As the TX-RX distance increases further, the ionospheric incidence angle becomes even more shallow thus producing strong and workable signal strengths at all RX locations beyond the Skip Zone.
It is beyond the scope of this post to explain various factors that determine the Skip Zone radius or when to expect Skip Zone occurrences and when not. I will dwell on these factors in a subsequent post as and when I have specific queries to trigger a new post. However, at the moment I would state the following thumb-rules.
- Higher the frequency, the wider is the Skip Zone.
- Lower the SSN, wider the Skip Zone.
- Skip Zone is wider during evening, morning and night.
- It is more prominent in winter than in summer.
- 10-12-15-20m have more pronounced Skip Zones.
- 40-30m have minimal Skip Zone often obliterated by Ground-wave.
- 160 to 80m normally have no tendency to produce Skip Zone.
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