Leveraging Propagation
Two areas of HF signal transmission are sky wave and ground wave propagation. How signals refract across the sky, and over the ground. Sometimes seeing how that might happen can help you leverage propagation. One way to do that is through computer software. The VOACAP software is a free Windows tool for modeling HF propagation.
VOACAP propagation map
HF propagation is affected by the sun and atmospheric conditions. Solar heating changes how the atmospheric layers reflect radio waves. Knowing what band is better for a time of day is an advantage. Higher frequency bands like 10 and 15 meters perform better during the day. That’s because the F layer of the atmosphere gets heated by solar rays.
When darkness increases it affects the MUF, or Maximum Usable Frequency. Even as the MUF drops during the night, there are still ways to make long distance contacts. Switching to a lower frequency HF band like 40 or 80 meters will help.
You can also use transequatorial propagation for an advantage. This is the phenomenon where signals bounce across the equator. Transequatorial propagation is most likely to occur between points separated by 2,000 miles to 3,000 miles over a path perpendicular to the geomagnetic equator. Think of a contact between Atlanta and Ecuador; or New York and Venezuela.
The maximum range for signals using transequatorial propagation is approximately 5,000 miles. Afternoon or early evening is the best time of day for transequatorial propagation. Now you know a good time to get some South American DX contacts!
Illustration of transequatorial propagation
Think about working stations in Europe from the US with a directional antenna. You generally should point it in a north-easterly direction. In the southeast US, it’s around 45 degrees azimuth. That provides a contact distance of about 5,000 miles to central Europe. That’s the “short path.” It’s the shortest distance for your signal to reach the desired location.
So if there is a short path, there must be a long path. To make a long path contact, you go the other way around the globe. That means turning your directional antenna around the complete opposite way to make a contact. For our Europe contact we would turn the antenna toward 225 degrees azimuth. That sends our signal down the “long path.” Sending your signal all the way around the world in the opposite direction! Long-path propagation is most frequent on 40 meters and 20 meters.
Another propagation tip is to follow the grayline. Simply put, grayline is the instability of signals around sunrise and sunset. One type of grayline opening is a long path connection. It would be between two points on the Earth that are simultaneously near sunrise and sunset.
Grayline map
It’s sometimes difficult to make long-distance contacts around 1.8 MHz due to its long wavelength. The best path for long-distance contacts on 160 meters is a path entirely in darkness.
Antenna placement above ground also impacts how you can direct your signal where you want it to go. Changing the antenna’s elevation angle has an effect on HF skip propagation. When you do this, the distance covered by each hop increases ionospheric HF skip. Think NVIS here.
The absence of ground skip is called Chordal-hop propagation. It carries the signal a long distance with no bounces. Chordal-hop propagation has successive ionospheric refractions without an intermediate reflection from the ground. Chordal-hop propagation is desirable. That’s because the signal experiences less loss compared to multi-hop propagation, which uses Earth as a reflector.
Let’s think back on EME and using the Moon as a reflector. How far will your contact go when propagation conditions are optimal for moonbounce? There is a maximum range you can communicate via EME. Separation between two stations on Earth for EME is 12,000 miles, if the Moon is visible by both stations. So for that long EME contact, find a partner station around 12,000 miles away.
The position of the Moon in all axes is important during EME. If the Moon is more tilted on its axis it causes libration fading of an EME signal. That is a fluttery, irregular fading.
The distance between the Earth and Moon changes as the Moon orbits around the Earth. Perigee is when the Moon is closest to Earth. Apogee is when the Moon is furthest away from the Earth. When the Moon is at perigee, is the best time to schedule EME contacts, because the Moon is nearest to Earth. That makes sense because the signal doesn’t have far to travel, leading to lower path loss.
Moon position diagram
Propagation Terms
We’ll close out with a few terms you will be asked about on the exam.
“Extraordinary” and “ordinary” waves are independently propagating, elliptically polarized waves created in the ionosphere.The ordinary wave can travel unchanged, while the extraordinary wave gets refracted.
The radio-path horizon is the line-of-site for a radio signal. In VHF/UHF the radio horizon is different from the geographic horizon. The radio horizon exceeds the geographic horizon by approximately 15 percent of the distance. So, you can transmit line-of-site communications about 15% further than you can actually see.
Amateur radio propagation reporting networks help you understand current solar condition impacts. Tune your radio to WWV or WWVH to hear information like SFI and the A and K-indices broadcast hourly. Take a look at the band you are listening to WWV on when you do that. That will also tell you something about current propagation.
Online you’ll find propagation information in graphic form. This shows you the best bands for propagation. Digital-mode and CW signals can be captured and analyzed to show current propagation. One example of that is the PSK Reporter website. Check that out before you call CQ as another tool to help get more DX contacts in your log.
