HF Antennas and Propagation
An Introduction for QRPers
Part II
Maximum and Minimum Usable Frequencies
On the previous page we looked at the refractive ability of the E and F layers, and the absorption caused by the D region. During daylight hours, D region ionization sets the lowest frequency that can penetrate D and reach either E or F. This is called the minimum usable frequency. Toward the upper limit, minimum usable frequency depends on the wave angle and transmitter power.
Generally, 40 M is useful during daylight only for short-range contacts where the wave angle is nearly vertical. The reason is easy to understand: The wave has a shorter path through the D region, and is subject to less absorption. Because the D region is closest to earth, as sunset approaches it is the first part of the ionosphere to go into darkness. Given sufficient transmitter power and the right wave angle, it's possible to make 40-M contacts in an easterly direction two or more hours before sunset.
Assuming a signal can penetrate the D region, it still must be refracted by either the E or F layer. While the E layer sometimes propagates one hop on a path, maximum usable frequency (MUF) is generally determined by the F layer. Here again, wave angle plays a big role. Waves striking either E or F are more likely to be refracted back to earth if the angle is shallow. Propagation-prediction programs that calculate MUF base their results on a wave angle supplied by the user. Steeper wave angles lower the MUF, shallower angles increase it. For a given antenna system then, wave angle, plays a critical part in determining the upper and lower frequencies that can be propagated via the ionosphere.
How wave angle affects Maximum Usable Frequency. Waves at high angles are not refracted back to Earth.
Of course, MUF also depends on the direction the wave travels. In the morning, when the ionosphere to your east has been illuminated for several hours, but is still in darkness to your west, you'd expect a higher MUF looking east, but also a higher miniumum usable frequency.
The Grayline
The Grayline is an area along the sunrise and sunset terminators. Earth's shadow shades the D region first at sunset, and delays D region illumination at sunrise. Being higher in altitude, the F layer is illuminated first in the morning, and remains illuminated later in the day. Because Earth wobbles on its orbit, the grayline angle varies throughout the year, as does the width of the gray zone.
The terminator or grayline as seen from the International Space Station. (NASA)
Propagation on paths along the grayline is often greatly enhanced, though the enhancement may last only a minute. The grayline phenomenon is exploited mostly by DXers on 160, 80 and 40 M.
Long Path Propagation
As we've seen, long-distance propagation on 160, 80 and 40 M requires a path in darkness. Otherwise, signals are absorbed by the D region. Signals on 20 M and higher frequencies generally propagate best through daylight areas where F-layer ionization is highest. The 30-M band falls in between, showing characteristics of both 40 and 20 M. Sometimes the path between two widely separated locations doesn't meet our criteria, but yet we can hear signals from those places. What's going on?
Looking at a map that shows areas of darkness and daylight, we may see that the longer path between those locations does support propagation.
The short path between KR1S and YB1A is entirely in daylight. (Image from DX Atlas.)
In the image above, a screenshot from DX Atlas, the red line shows the short path from my location to YB1A in Indonesia, at 2330 UTC on March 6. The entire path is in daylight, so we wouldn't expect to work Halim on 40 M. Even the long path, looking south from both locations, is largely in daylight. Imagine my surprise, then, when YB1A, who was ragchewing with someone in his part of the world, suddenly popped up on 40 M! While I didn't work him with QRP, his signal was so strong I think it would have been possible with a good antenna.
Long-path propagation occurs on all HF bands, probably more often than we realize. Hams with rotatable directional antennas tend to point them in directions where they expect to hear signals. During periods of high solar activity, even they can't help but notice long-path openings. In peak years, 10 M opens long-path to Japan from the U.S. East Coast almost every morning. Being, well, longer, long paths are a little harder to cover with QRP power. But because not as many DXers are looking on those paths, you face less competition from QRO stations.
Summary
This wraps up our investigation of the ionosphere. We've looked at its components, why it exists, and the sun's varying influence on it. Along the way we've briefly touched on the importance of wave angle for long-distance propagation. The density of free electrons along the path determines the maximum wave angle that will be returned to earth. In the next section we'll use a couple of popular propagation-prediction programs to help us determine those wave angles. Then we'll study antennas to see which ones do the best job of emitting waves at the angles we need to make more contacts.