 |
The Propagation Corner Mediumwave DX: Why It’s So Hot Right NOW by Tomas Hood, NW7US, |
||
|
|
|||
|
On December 22, the Northern Hemisphere experiences the longest day of darkness. This marks the peak of the DX window on the lower shortwave bands and the mediumwave band. With very short daylight periods, the maximum usable frequencies are generally lower across propagation paths over areas in the Northern Hemisphere, making for quiet higher shortwave bands. At the same time, shortwave propagation on the mid-shortwave bands, like 31 meters, often stays active around the clock.
Last month, we looked at how different
shortwave propagation is during the fall and winter as compared to spring
and summer. This month, let’s look at why DX in the Northern Hemisphere is
so hot on the mediumwave frequencies during the late fall Mediumwave Propagation The frequencies between 300 and 3000 kHz are known as the medium waves (MW). In North America, the AM broadcast band, from 530 to 1700 kHz, falls right at the bottom third of the mediumwave spectrum. Many other countries have stations using these frequencies as well. One of the reasons MW DXers enjoy catching these domestic and foreign stations is because of the challenge. Most MW signals never make it past 800 to 1,000 miles, first because of ground wave signal loss and second because of the D-layer absorption. Occasionally, however, exciting but often short-lived openings of over 3,000 miles occur. During the late fall, winter, and early spring months these openings increase. Shorter paths also become more stable and last longer. During daylight hours, the sun’s radiation causes the D-layer to become highly ionized. The long wavelengths of MW radio signals never make it through this dense ionospheric layer, and so cannot reach the E- and F-layers for refraction back to earth. Most of the MW signal is lost, so those transmissions can only be heard via groundwave propagation in a localized region. When night falls, the direct influence of solar radiation decreases, allowing the D-layer to calm down and recombine. Some experts believe that the D-layer completely disappears, but evidence now suggests that some ionization still lingers through the hours of darkness, sometimes even intensifying under certain conditions.
|
|
||