How the Tool Estimates an HF Path
This tool can model a radio path in which the antennas at each end have different transmit and receive angles. For example, if the antenna at point A has an angle of 5° and the antenna at point B has an angle of 15°, the calculated reflection height, h, is approximately 112 miles (180 km), which is near the lower portion of the F layer. You can enter the great circle distance and the angles at points A and B, or you can drag the red dot to some location and watch how the angles and other values update as the refraction point is moved.
Examining point C reveals that the geometry is asymmetric. The sub-angle c1 on the left side of point C is not equal to the sub-angle c2 on the right side. This difference results directly from the unequal takeoff and arrival angles at points A and B, demonstrating how the tool can model non-symmetrical propagation paths.
That is contrary to the view that some may have that the angle of arrival at a reflection point is the same as the angle of departure. Such equality may be true for light on a perfectly flat mirror, but not so much for a radio signal bending in the ionosphere. Unlike a perfectly flat mirror, the ionosphere is a refractive medium whose structure varies with altitude, location, and time. As a result, departure and arrival angles are not necessarily equal and may differ substantially.
The ionosphere is a dynamic place. It changes continuously throughout the day, driven by solar radiation.
During daylight hours, ultraviolet light and X-rays ionize the molecules of oxygen (O2), and nitrogen (N2). Normally the oxygen molecule consists of two atoms, including 32 protons and 32 electrons. That’s 16 per atom and two atoms per molecule. Normally, one nitrogen molecule consists of two atoms including 14 protons and 14 electrons. That’s 7 per atom with two atoms per molecule. In their normal state, the oxygen and nitrogen atoms are neutral. When ultraviolet light and X-rays encounter atoms of oxygen and nitrogen, they strip off an electron. The electron doesn’t stay stripped off for long, because the negative electron wants to recombine with the positive molecule. At lower altitudes, the air is thick, and recombining occurs almost instantly. At high altitudes, the air is thin and ions can last for hours.
When a radio signal arrives near the ionized region, the wave interacts with free electrons, causing them to re-radiate the signal. This collective movement changes the effective refractive index of the medium, causing the radio wave to curve back toward Earth. Although the calculator shows a effective point of refraction, the actual refraction takes place as a curve where the incoming and outgoing lines, it would meet at point C if they were extended upward. So, for many purposes, the use of a single point is adequate. Imagine what that must be like when there are tens of millions of radio signals refracting through the region at the same time.
The amount of refraction is a function of frequency and total electron content. While the radio frequency is chosen by the operator, the electron content of the ionosphere continuously changes, both vertically and horizontally, as solar radiation creates and removes ionization. As electron density increases, decreases, and shifts between ionospheric layers, the amount of refraction and the range of frequencies that can be returned to Earth also changes.
As the ionosphere changes, the angle at which a radio wave is refracted can change as well. A signal launched at the same takeoff angle may return to Earth at a different distance or may not return at all, depending on the ionization profile at that moment. The effective height of the refracting region, the amount of bending, and the resulting angle of arrival can vary from hour to hour. Radio operators may find that they completely lose a signal in the middle of a 30-second contact. Consequently, two stations using the same antennas and operating frequency will experience different propagation paths throughout the day.
Although the nature of the ionosphere and the path of a signal may seem to be chaotic, there is a predictable 24-hour cycle that may be classified into four distinct groups.
1. Morning
As the sun rises, ultraviolet and X-ray radiation light up the upper atmosphere.
D and E layers form and the F layer begins to split into two distinct layers: F1 and F2.
The D layer begins to absorb signals, with greatest absorption in the lower frequencies including the 1.8 MHz–7 MHz range. Higher frequencies may pass through the D layer and be refracted off the developing F1 and F2 layers.
2. Midday
Solar radiation reaches a maximum. The D layer is at its thickest, which causes the maximum noticeable solar noise and signal attenuation. The E layer reaches peak density, occasionally allowing short-distance "sporadic E" skips. The F2 layer achieves its highest electron density for the day, but signals must travel through the attenuating D and E layers before reaching the F layers.
3. Evening
As the sun sets, ultraviolet and x-ray radiation diminish and disappear. As a result, the D layer disappears and the E layer diminishes and eventually disappears. The frequencies in 80-, 40-, and 20-meter ranges allow longer distance communication.
4. Nighttime
Without solar ultraviolet and X-ray radiation, ionization levels decrease significantly. Maximum usable Frequency declines, and eventually declines to 20 meters and below 40 meters opens up for possible long-distance communication.
Remember that this tool does not have many guardrails. It is possible to set distance and angle so that the D layer appears to refract a signal. However, the D layer between about 30 and 60 miles altitude, has relatively low electron density but relatively high neutral gas density. Because the air is still fairly dense, the radio-wave energy is converted into heat, causing absorption and noise rather than significant bending.
The D layer is known primarily for generation of noise, which goes along with the absorption of signal absorption and consideratble reduction in signal strength during the day. Technically, the D layer has a refractive index and causes some refraction. However, the amount of refraction is usually small compared to the E and F layers. The E and F regions are responsible for longer distance propagation because their electron densities are higher and because their elevation is much higher.
Following is a little animated simulator. Start the video to see it cycle through a day and repeat. You may also drag the slider, the sun or the moon. Percentages on the right are percentages of the total ionozation.
Everything in the image is an example and is not exact for time or ionospheric condition.
The colors of hayers in the horizontal bars are:
Neutral Layer is essentially inactive/un-ionized, typically at night
Amber/orange Weak, to moderate ionization
Blue Solid, moderate to strong ionization
Red Peak ionization. D layer near solar noon or F2 at the daily maximum