Lesson 7


Propagation is the term for how your radio signal moves through the air. It’s the behavior of the waves as they travel. Let’s go back to your Technician level studies and refresh on some terms. An electromagnetic wave travels at a right angle to the electric and magnetic fields. The component fields of an electromagnetic wave are oriented at right angles. The Extra course adds a new component here.  What determines the speed of electromagnetic waves through a medium? It’s the index of refraction or how much signals are bent when entering a material.   

Solar activity is measured using several different scales. The Solar Flux Index, or SFI, measures radio noise at 2800 MHz. A high Solar Flux Index means that there is high solar activity. High solar activity is good for HF propagation.

When reading the SFI index, anything higher than a 90 is considered good. 

Another measure similar to the SFI is the 304A solar parameter. That measures UV emissions at 304 angstroms, correlated to the solar flux index.  That 304A means 304 angstroms, so the answer is right in the question.

The K and A indexes are a measure of magnetic activity. Ham radio operators want to see a low K and A index. That means stable magnetic conditions. A rising A or K index indicates an increasing disturbance of the geomagnetic field. That’s not best for radio because a disturbance can disrupt propagation.  

What kind of signal paths are most likely to experience high levels of absorption when the A-index or K-index is elevated? A through the auroral oval signal path. That means that less of your signal will get through.  

Ham radio operators get excited when they see a high SFI number, and low A and K numbers. This means great conditions for HF!

All of these discussions are of magnetic conditions. That’s because of the magnetic field component of your signal. Another measure of magnetic conditions is Bz (B-sub-Z). Bz’s value represents the North-south strength of the interplanetary magnetic field. Watch for risks of a southward orientation of Bz. It increases the likelihood that incoming particles from the sun will cause disturbed conditions.

Atmospheric Layer Propagation

Different layers of the atmosphere play a role in propagation of radio signals. Each one refracts signals uniquely. Let’s start with the F layer and work our way down to the ground.  

The F layer is the highest layer of the atmosphere. So, it can be used to hop signals distances of 1,500 miles or further.

The E layer of the atmosphere is constantly shifting. Seasons, location, and solar weather all play a role. In June we experience the summer solstice and in December it’s the winter solstice. That’s when Sporadic E propagation is most likely to occur. Around the solstices, especially the summer solstice. Time of day is important too. You are most likely to see Sporadic E between sunrise and sunset.

Also active in the E layer is auroral propagation. The northern lights are a form of auroral activity. That aurora can cause distortion. Severe Geomagnetic storms are most likely to result in auroral propagation. The space weather term “G5” indicates an extreme geomagnetic storm. When those storms happen, it’s time to break out your code key. The best emission mode to use for auroral propagation is CW.  

Aurora Borealis in the night sky

Still in the E layer, we need to consider the impact of meteor strikes. Not the big “Armageddon” kind. These are small groups of meteors entering the Earth’s atmosphere. These meteors are a cloud of pebbles or sand. Meteor strikes can create a linear ionized region in the E layer of the ionosphere. Planning your frequencies for meteor scatter? Use communications in the 28 MHz – 148 MHz frequency range, which is mostly VHF. 

The D layer is created by energy from the sun during daylight, and it disappears at night. The D layer absorbs signals from many HF bands making it difficult to use them during daylight hours. This is why 80 meters doesn’t work well during the day. 

Occasionally, the sun releases intense packets of radiation known as solar flares. Solar flares can be absorbed by the atmosphere, charging up the D layer. A concern over solar flares is that they can cause short-term radio blackouts. Hearing a sudden rise in radio background noise across a large portion of the HF spectrum? That indicates that a solar flare has occurred. All solar flares are classified by a letter. Class X indicates the greatest intensity. 

Illustration of solar flare

Ground-wave propagation is when signals travel just above the Earth’s surface. They propagate parallel to the curve of the Earth. In HF, ground-wave propagation works better at low frequencies. The maximum propagation range decreases when frequency is increased. Vertical polarization is best for ground-wave propagation.

In this low part of the atmosphere you may see tropospheric duct propagation. A microwave signal taking advantage of that can travel 100 miles to 300 miles. Because tropospheric propagation occurs low in the atmosphere, weather conditions are very important. These ducts often form over large bodies of water.  

Want to make some tropospheric duct propagation contacts? Some hams use circularly polarized antennas. This takes advantage of circularly polarized electromagnetic waves. Those are waves with a rotating electric field. That improves how the signal penetrates and bends around obstructions, particularly in Microwave frequencies.  

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