Prop on 75/80 meters

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Prop on 75/80 meters

Postby wm7o » Mon Dec 08, 2003 8:08 pm

I would like to know how propagation works on 75/80m. I am curious what causes the changes between daytime and night time propagation. I am also curious what it is that makes the band generally good for short and long range paths.

This is probably stuff that I learned at one time and have forgotten.
wm7o
 

Postby ku4a » Tue Dec 09, 2003 2:58 pm

During the daytime, the D-layer of the ionosphere is extremely active. The D-layer is an absortive layer, although the absorption decreases with increasing frequency. But on 80 meters, the absorption is high.

At nighttime, the D-layer disappears, so the refractive layers (primarily the F-layer) are exposed. This allows propagation on 80m at night most of the time, assuming the MUF of the F-layer is high enough (which it usually is for 80).

Most amateurs think of 80m as a "local" band, because it is extremely difficult to have a high-efficiency, low radiation angle antenna on that band. But worldwide propagation is available for those who are able to construct a suitable skywire. I have a 1/4-wave sloper on 80 with the top end a about 40 ft. This is not a good DX antenna on 80.

Returning to daylight hours, note that there is some "skywave" possible during the day. But the attenuation of the D-layer is such that your signal can only go through the D-layer a few times before it is reduced to an unusable level. That is why communication distances are limited to a just a few hundred miles during daytime.
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