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The Propagation Corner Winter VHF Propagation
by Tomas Hood, NW7US |
 “Halo” Coronal Mass Ejection of August 16, 2002. (Source: SOHO, http://soho.nascom.nasa.gov) |
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In last month’s column, I touched on mediumwave (MW) propagation during the winter season. This month, let’s take a look at the other end of the High Frequency (shortwave) spectrum, the frequencies right above 30 MHz (known as “low VHF”). In the last few years, some rather strong and exciting long-range VHF DX openings have occurred during late December and early January. Paging, television picture and audio, and other signals have been received from over 2,000 miles or more via F2-layer refraction. These unexpected openings surprised many DXers, since F2-layer propagation is unusual during the winter season. Long-range reception of DX VHF signals tends to occur most often by modes such as Sporadic-E (Es) or tropospheric propagation. F2-layer propagation is less common at these higher frequencies, being possible only when the Maximum Usable Frequency (MUF) between the station and the receiver is high enough to reliably refract these signals. Having an MUF high enough during the middle of the winter season is rare. Why VHF Openings Occur There are several theories about why conditions were perfect for these winter VHF openings. The foundation of each theory is that these F2-layer openings are occurring during the peak years of Solar Cycle 23. During the years of a solar cycle maximum, solar activity is high enough to cause MUFs that would support VHF signal refraction. One of the theories goes on to explain that these openings were created by a perfectly timed coronal mass ejection (CME). When the energy from such an occurrence reaches our magnetosphere, it creates high MUF spots at locations determined by the relative position between the sun and earth at that particular point in time. If the orientation of the magnetic field lines in the CME’s plasma cloud are such that they combine with the magnetic field lines of earth’s magnetosphere, the plasma penetrates into the atmosphere and ionospheric layers, causing an increase in the ionization. Sometimes this will create so much ionization in the lower levels (D- and E-layers) that MW and HF signals are absorbed and attenuated. In addition, geomagnetic disturbances increase in magnitude and aurora is observed. At higher frequencies, such as the low VHF range, this increase in ionization can create an F2-layer opening between you and some distant transmitter. Return to January 2003 Highlights Page
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