Choosing a Band for a Path That Won't Close
Two amateurs, one in the northeastern United States and one in the upper Midwest about 1,200 km away, want reliable evening contacts in mid-winter. They have been trying 80 m SSB at 21:00 local time and find the band noisy with weak signals and frequent fading. They also tried 20 m at the same time and heard nothing. Their antennas are low horizontal dipoles, about 10 m high, with no gain. Both run 100 W. They ask whether they should switch bands, change antennas, or change operating time, and want to understand why 20 m failed while 80 m was marginal.
1. Explain why 20 m was silent and 80 m was marginal at 21:00 local winter time for a 1,200 km path.
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At 21:00 local winter time, the ionosphere over the path is in darkness. The F layer's maximum usable frequency drops after sunset, so 20 m (14 MHz) may be above the MUF for a 1,200 km single-hop path in winter, especially at low sunspot activity - hence silence. Meanwhile 80 m (3.5 MHz) is well below the MUF, but at 1,200 km the required takeoff angle is high and a low dipole at 10 m radiates poorly at high angles; absorption is also greater at night on the lower bands, and atmospheric noise is high. So 20 m is closed by MUF, and 80 m is marginal because of antenna takeoff angle, absorption, and noise. The judgement is to connect band choice to MUF, path length, and antenna pattern rather than trying bands at random.
2. Recommend a band and time change, with reasoning.
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A better choice for a 1,200 km evening path in winter is often 40 m (7 MHz), which sits between 80 m and 20 m: it is usually below the nighttime MUF, and a 10 m high dipole has a more useful takeoff angle for that distance than on 80 m. The operators could also try earlier in the evening, before the MUF falls further, or try 80 m with a higher or more efficient antenna. If conditions are poor, 60 m may also work. The reasoning: match the band to the path's MUF and to the antenna's radiation angle. A 1,200 km path is roughly a single F-layer hop with a moderately high takeoff angle, so a band that supports that angle and is below the MUF is the best bet.
3. Would raising the dipoles from 10 m to 20 m help on 80 m for this path? Explain.
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Raising a dipole changes its radiation pattern. At 10 m height on 80 m (about 0.12 wavelength), the dipole radiates strongly at high angles, which can favor short skip but is inefficient for a 1,200 km path. Raising it to 20 m (about 0.24 wavelength) lowers the main lobe somewhat and can improve low-angle radiation, but on 80 m even 20 m is still a small fraction of a wavelength, so the change is modest. The bigger gains on 80 m usually come from reducing ground loss, using a more efficient antenna, or accepting that 80 m is a nighttime regional band. For 1,200 km, 40 m is generally the more productive choice. The judgement is to weigh antenna height against wavelength and against the path's required takeoff angle.