# Beginner’s guide to HF radio propagation

A long long time ago, as a kid, I had the chance to experiment with community band (CB) radios. On some occasions, especially during the summer season, we would receive signals from far far away and occasionally established two way contacts with some of these distant stations. At that time, I was told this was called “Skip” and I didn’t bother looking in to the phenomenon any further, we just enjoyed it while it lasted.

Many years later, after working intensively at building internet networks, I renewed my interest in radio communications and one thing I was determined to invest time on, was this phenomenon allowing a radio signal to have the capacity to travel great distances. As a kid, we learned that the further you moved away from a transmitter, the weaker the signal became and once you reached a certain point, communications were no longer possible. That is where signal propagation concepts come into play. Before we go any further, let’s  lay down a few definitions necessary to better understand radio signal propagation.

Radio signal: consist of electromagnetic pulses transmitted using a radio transceiver and an antenna. The rhythm at which these pulses are transmitted is called frequency. One (1) Mhz frequency is 1 millions electromagnetic pulses per second. HF frequencies are frequencies below 30 MHz and those are the frequencies who have propagation properties when interacting with our sky.

Atmosphere: is a envelop of gases that is approximately 10 kilometres thick and surrounds our planet. Troposphere: The layer at which most of our atmosphere is located. It can be found at altitudes between 5 and 20 kilometres above ground. Stratosphere: The layer found between 20 and 50 kilometres. Mesosphere: The layer found between 50 and 85 kilometres above ground. Thermosphere: The layer found between 85 and 700 kilometres. Exosphere: The layer found between 700 and 10,000 kilometres above ground. Aurora: gaseous plasma occurring at approximately 100 kilometres above ground.

Ionosphere (The culprit in HF propagation): It is an area which includes the Thermosphere and Exophere layers, which is located between 75 and 700 kilometres above the earth. The ionosphere is split into several layers called D,E (E1,E2,E3) and F (F1,F2) etc. These sub-layers contains gases, charged particles, electromagnetic fields, water particles, all of which are necessary to create the conditions needed for HF radio signal propagation. These layers separate and merge back together, drop or rises in altitude constantly. This is caused by the presence or absence of Sun rays stimulating these layer.

Amateur radio operators have terms such as “The band is opened !’. This means that the ionosphere layer interacted in such a way that they created a temporary radio wave reflector that will refract radio waves back to earth on or around 21 MHz.  Refraction:  It’s the change in direction of a wave, due to changes of the medium its travelling thru. Now that we have given some definitions what is necessary to understand radio signal propagation, let’s look at how it all works.

I have found that waves can travel is one of there ways: they can go in a straight line, change direction by curving around objects or change directions radically by bouncing on things.

The first kind of radio signal propagation happens at ground level. They are called space waves and ground (surface) waves. Space waves can be defined as waves traveling in a straight line in gaseous elements, allowing communication when line of sight is established. Point “A” to point “B”, in a straight line. That’s pretty much how most people understand how a radio signal is being transmitted and received. As long as there is nothing in between to attenuate and/or absorb the signal, you are in business.

Ground waves behave a bit differently. They follow the curvature of its environment, hence, the curvature of the earth and its terrain That would explain why I could, as a kid, establish two-way CB radio communication with stations that are below the earth’s horizon. This type of propagation has limited range and is mostly effective at night when the weather and temperature are more stable. Since a wave travels in all directions at ounce, when the horizon drops because of the curvature of the earth, the wave will want to go down with it, making the wave follow the terrain.

Some frequencies (bands) will have better results than others as ground waves are concerned. When using ground wave propagation, some bands will achieve a few hundred kilometres, other will reach several times that distance, but all in all, ground wave propagation is somewhat static and no extraordinary great distances can be achieved.

Next kind of radio signal propagation is called “Inversion“. That’s when the temperature difference in troposphere layers allow for radio signals to bounce back to earth, or get refracted away from the radio horizon. The troposphere is pretty much the section of the sky where you’ll find clouds and planes and most of our breathable air. It’s not very high in altitude, but does allow certain frequencies (VHF & UHF for example) to travel much greater distances than just with space (line of sight) or ground waves.

These temperature inversions happen most frequently after sunset and before sunrise. Amateur radio repeaters often fall victim to temperature inversion propagation where two distinct repeaters using the same frequency, would in normal conditions, not be able to hear one another, suddenly begin interfering with each other when their signals are subjected to inversion propagation. On the other hand, this allows amateur radio operators to reach repeaters they would normally not be able to.

Finally, HF propagation is the mother of all type of propagations and by far the most interesting phenomenon to try to understand because it is directly responsible for being able to achieve mind boggling distances in Amateur Radio transmissions.

As mentioned before, properties of radio signals below 30 MHz can have their path altered by layers  found in our sky. The angle at which these layers are reached by the signal is critical, as does the frequency of the signal. Some frequencies propagate better than others during the day, other have better propagation at night. Sometimes, propagation isn’t there at all. So why is this phenomenon so dynamic and unpredictable? Simply because the layers of gases in our sky are unstable and constantly changing. Each time a signal leaves earth, hits the sky and comes back to earth, it’s called a “hop”. Each time this happens, a bit of energy is lost, just like our pool ball hitting the side of the pool table. Each time a signal does a “Hop”, it can easily travel hundreds to thousands of kilometres.

A radio signal can achieve several hops by continuously bouncing between the earth and the ionosphere, each time, loosing a bit of strength, each time going a bit further. In a nutshell, that’s how Amateur radio operators use this phenomenon to make radio contacts with other amateur radio operators around the globe.

The layer responsible for radio signals being refracted back to earth is called the ionosphere. The ionosphere the area composed of the thermosphere and exosphere. The ionosphere is split in several sub-layers called D, E, and F, these are also split into sub-layer, F2,F2 and so on. What is important to know is that each layers will refract back different frequencies, when hit at certain angles and in certain times of the day or night, and with certain conditions.

I discovered by reading that the ionosphere is extremely dynamic, meaning, changing constantly. It changes in altitude, density, its layers merge and separate with one another, get charge and discharged of electric particles, water particles, get bent out of shape by electro magnetic forces and finally get metamorphosed by our Sun’s solar storm activities. The ionosphere is therefor a very thick (75 to 700 kilometres) radio wave filter that either lets a radio signal thru, absorbs it, or refracts it back to earth. This said, one thing you quickly discover when playing with HF propagation is that this phenomenon is very unpredictable. One day I might have a great communication corridor with Florida on 15 meter band, the next day with Brazil on 17 meter. On the other hand, you might go for days and weeks without any long distance HF propagations at all.

Many other factors will help achieve good HF signal propagation, the polarization of your antenna, the takeoff angle of your signal lobes, the hight and gain of your antenna, your station’s power output, the hemisphere on the planet you are located in, the latitude and longitude of your location, the seasons, the weather and finally, the sun solar cycle.

I found that the MOST important ingredient which affects how HF propagation behaves is our Sun. Our Sun’s radiations will ionized the ionosphere’s layer, creating areas where radio signals can refraction. Needless to says that this complex interaction is between our Sun and layers of gases found in our sky is a bit predictable. For example. I learned that I have greater changes of making great radio contact at our around sunsets and sunrises. Therefor, I plan my hobby activities around these time of the day. During the day, or at night, I also know that propagation will behave in a certain way on some frequencies, therefor will also plan accordingly. For example, 80 meter band (3.5-4.0 MHz) will reach best propagation after sun set. That’s when most HF operators gather around to chat and run radio nets. I have found that two bands offer some propagation almost around the clock, that would be 40 meter and 20 meter bands. On 40m, propagation increase in distance after sunset while 20m will increase in distance after sun rise.

If you would like to learn more about how the Sun affects our ionosphere, here’s a little article I wrote about solar mechanics and Amateur Radio. It can be viewed by clicking here.

In the world of Amateur Radio, propagation is not always limited to frequencies below 30 MHz. Often, you will hear people talk about Sporadic E propagation. It’s essentially a condition where thin layers of ionized particles are formed when strong magnetic storms hit our earth. These layer can refract frequencies ranging from 30 MHz to 100 MHz. That is why 6m bands (50-54 MHz) is popular amongst amateurs chasing radio contacts when we have Aurora activities.

There are so many other things that will influence radio signal propagation, too many to discuss them all here, but some of the most important ones are hemispheric angles, where the northern hemisphere is tilted towards the sun during summer, and away from the sun during winter.

This will determine which bands have propagation and  how efficient this propagation will become. The opposite effects will happen in the southern hemisphere. If you didn’t figured it out by now, operating an Amateur Radio station in our around the equator gives that station many advantages over stations, having propagation benefits from both hemispheres. Location, location, location is what dictates how propagation will help you enjoy Amateur Radio hobby or not. I often hear stations near the equator making contacts all over the planet, while not being able to hear a single one of those stations. That has everything to do with the lousy location, I’m in, Canada, which is not the better place on the globe to take advantage of radio signal propagation, but I’ll enjoy what I can get.

In conclusion, HF radio signal propagation is not an exact science and is by far unpredictable. Fortunately, there are tools and websites that can help you view and plan your radio activities. Propagation stations all over the world send out test beacons and when this beacon is received by other stations, this data is used to plot maps of propagation. These tools and maps really help me determine if and when I will sit down and turn on my transceiver or go enjoy another hobby.

Reference:

Definition of radio propagation: Radio propagation refers to the way in which radio waves travel through the atmosphere from a transmitter to a receiver. Radio waves are a type of electromagnetic radiation that travel at the speed of light and can be used to transmit information over long distances.

When radio waves are transmitted from a transmitter, they propagate outward in all directions, but their strength decreases with distance due to absorption, reflection, and scattering in the atmosphere. The behavior of radio waves during propagation depends on several factors such as the frequency of the waves, the time of day, the season, and the location of the transmitter and receiver.

The ionosphere, a layer of ionized gas in the upper atmosphere, plays a crucial role in radio propagation. The ionosphere can reflect radio waves back to the earth’s surface, allowing long-distance communication over the horizon. However, the ionosphere is not a perfect reflector and can absorb, refract, and scatter radio waves, leading to attenuation and distortion of the signal.

Radio waves can also propagate through the earth’s surface and atmosphere using other methods, such as ground wave propagation and line-of-sight propagation. Ground wave propagation is used for short-distance communication and relies on radio waves traveling along the earth’s surface. Line-of-sight propagation is used for communication between two points that have a clear line of sight, such as between two mountain peaks or between a satellite and a ground station.

In summary, radio propagation is the way in which radio waves travel through the atmosphere and the earth’s surface from a transmitter to a receiver. It is affected by various factors such as the frequency of the waves, the time of day, the season, and the location of the transmitter and receiver. Understanding radio propagation is important for designing and optimizing radio communication systems for efficient and reliable communication.

73′s – VE2XIP

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