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This covers further aspects of HF propagation, with just a single section of 11 questions.
Often called layers, these are now "regions", according to the examiner. I expect this may be because the change in density of the region is more gradual than the quite defined cloud base which might be around 830 metres in Canberra, frequently placing the all but the base Telstra Tower in cloud during winter.
The layers are the D, E, F1, and F2 layers, from lowest to highest. These extend to several hundred kilometres above the Earth's surface, depending on the time of day.
The E, F1, and F2 regions act to refract signals back to earth, allowing HF propagation over great distances around the globe.
However, during the day the D layer absorbs signals from 160 to 40 metres, and in the newer 630 metre band. Thus these bands, and MF and low SW broadcast bands only provide local ground-wave coverage during the day. Higher frequency signals can penetrate this layer, both mid to upper HF frequencies which may be then refracted back to earth by higher layers. VHF, UHF, and microwave frequencies can reach satellites and more distant space vehicles. Likewise, these signal can come from objects in space to Earth, including GNSS (GPS, etc) signals; and signals for interplanetary spacecraft, such as Voyager 1 & 2.
Natural signals from objects in space can be received on earth, in spectrum from 13 MHz to 31.5 GHz: Wikipedia: Radio astronomy
In the evening the D-layer dissipates, and MF and lower HF bands provide good regional coverage. Certainly, when I was younger Radio Moscow used to broadcast into western Europe in the AM (MW) broadcast band at night, as well as Shortwave, while BBC Radio Scotland on 810 kHz is audible in Norway. 2ZB from New Zealand on 1035 kHz can be heard in eastern Australia if you are away from the SBS Radio transmitter in Wollongong.
A rule-of-thumb used by sailors, applicable to all, is "The higher the sun, the higher the frequency".
No A, B, or C layers? Initially the E-layer inherited its name, as it was the layer from which the "Electric waves" were returned. The one below it was named the D-layer, based on an assumption that lower layers may be discovered, but these were not.
It exists in the three layers above the stratosphere.
There is an angle above which signals hitting the ionosphere will not refract. This is the critical angle, and it means that there is a zone, called the "skip zone", where signal are not returned to earth. This is varies, by time and frequency. It is maybe a bit like skipping a stone off water, where a shallow angle provides a bounce, while a greater angle means that it penetrates the surface, and sinks.
However, as with VHF signals and the troposphere, HF signals can be randomly scattered back to earth by small dense areas in the ionosphere. These signals can suffer flutter and distortion.
Way off topic, the Soviets discovered that it was possible to negatively affect the ionosphere by focusing too much energy onto one part of it. A very large array of towers was formed into a parabolic reflector in an attempt to maximise the signal received on the US east coast, meaning a basic domestic radio or radio-cassette player with SW band(s) could receive their broadcasts of news and culture. Instead, the signal in NYC was reduced, compared to existing antennas.
This is the highest frequency at which signals are refracted to earth.
It depends on the density of electrons in the ionosphere, which varies by time of day. This can relate to NVIS (below), or more often to F layers, especially F2. This can be enhanced or depressed by solar activity, with the Coronal Mass Ejection in May 2024 depressing it by 40% over what is expected during this time of year and latent solar conditions. This also causes a major aurora event.
More: Wikipedia: Critical frequency
This term appears as a distractor in a question where "MUF" is the correct answer. "Critical Frequency" appears in a question, not as correct answer. The term is also used in place of Nyquist frequency, in digitalisation of voice or similar signals.
In Australia bush search and rescue groups use small Q-MAC HF radios with dipole antennas which can be thrown out across the brush, or even on dry ground, and by transmitting on frequencies such as 3.8 to 4 MHz, and around 5 MHz, they can report their position, and their intention (where the will search next), or receive instructions.
The same effect can be used on 80, 60, or 40 metres to operate emergency and radio club nets, where the desired coverage is several kilometres up to a few hundred km, rather then the many thousands of km DXers may desire. Low dipoles work best.
For stationary or mobile use from a vehicle, a flexible antenna pulled to a roughly horizontal position by means of a bungee cord or similar will help to provide NVIS propagation, as will using an adjustable mount.
As always, these are actual questions from the General exam pool.
G3C01
Which ionospheric region is closest to the surface of the Earth?
A. The D region
B. The E region
C. The F1 region
D. The F2 region
Layers are named from the surface up, so it is the D layer, answer A.
G3C02
What is meant by the term "critical frequency" at a given incidence angle?
A. The highest frequency which is refracted back to Earth
B. The lowest frequency which is refracted back to Earth
C. The frequency at which the signal-to-noise ratio approaches unity
D. The frequency at which the signal-to-noise ratio is 6 dB
This is the highest frequency which is refacted back to earth, answer A.
G3C03
Why is skip propagation via the F2 region longer than that via the other ionospheric regions?
A. Because it is the densest
B. Because of the Doppler effect
C. Because it is the highest
D. Because of temperature inversions
It is because it is the highest layer, so signals travel further before they are returned to Earth, and they also land further away, answer C.
If you had an air-cannon, with a fixed angle, and shot tennis balls at the ceiling of a gymnasium, the higher the ceiling, the further the point they bounced to the floor would be.
G3C04
What does the term "critical angle" mean as used in radio wave propagation?
A. The long path azimuth of a distant station
B. The short path azimuth of a distant station
C. The lowest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions
D. The highest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions
This is the highest take-off angle that will be returned under particular conditions, answer D.
This means that there is often a zone between where the ground-wave peters out, and where skywave coverage commences, where the signal is not returned, this being the "skip zone", as signals skip over it.
G3C05
Why is long distance communication on the 40-, 60-, 80-, and 160-meter bands more difficult during the day?
A. The F region absorbs signals at these frequencies during daylight hours
B. The F region is unstable during daylight hours
C. The D region absorbs signals at these frequencies during daylight hours
D. The E region is unstable during daylight hours
The D layer absorbs them signal, answer C.
You may have noticed that MW (AM) signals are local only during the day, but travel hundreds of km, or more, at night. This is due to the behaviour of the D layer mentioned above.
G3C06
What is a characteristic of HF scatter?
A. Phone signals have high intelligibility
B. Signals have a fluttering sound
C. There are very large, sudden swings in signal strength
D. Scatter propagation occurs only at night
These have a fluttering sound, answer B.
G3C07
What makes HF scatter signals often sound distorted?
A. The ionospheric region involved is unstable
B. Ground waves are absorbing much of the signal
C. The E region is not present
D. Energy is scattered into the skip zone through several different paths
The scatter is from different regions in the ionosphere which are randomly more dense than the rest, and these varying paths cause distortion, answer D.
G3C08
Why are HF scatter signals in the skip zone usually weak?
A. Only a small part of the signal energy is scattered into the skip zone
B. Signals are scattered from the magnetosphere which is not a good reflector
C. Propagation is via ground waves, which absorb most of the signal energy
D. Propagation is via ducts in the F region, which absorb most of the energy
Only a small amount of the signal is bounced down to the station in the skip zone, answer A.
G3C09
What type of propagation allows signals to be heard in the transmitting station's skip zone?
A. Faraday rotation
B. Scatter
C. Chordal hop
D. Short-path
Signals can be scattered by the ionosphere, answer B.
These will typically be weak and variable.
G3C10
What is near vertical incidence skywave (NVIS) propagation?
A. Propagation near the MUF
B. Short distance MF or HF propagation at high elevation angles
C. Long path HF propagation at sunrise and sunset
D. Double hop propagation near the LUF
NVIS provides short distance MF or HF propagation, with signals refracting at high elevation angles, answer B.
Near Vertical and high elevation sounds like the same thing, so yes, it is shorter range propagation in which signals travel up, and are returned over a distance from zero to several hundred kilometres.
G3C11
Which ionospheric region is the most absorbent of signals below 10 MHz during daylight hours?
A. The F2 region
B. The F1 region
C. The E region
D. The D region
The D region absorbs low HF and MF signals during the day, answer D.
That said, 10 MHz (30 metres) itself can perform well during the day, at times covering the 4000 km between Sydney and Perth.
Perhaps we can think of the E region, and the two F regions as enabling, and facilitating propagation; and the D region as diminishing it?
If you want to nerd-out fully on this, perhaps after your exam, there are plenty of articles on the subject. It is also a section of the Extra paper, with more theoretical content. This includes the Australian government's Space Weather site: Educational material.
Another is: SolarHam by VE3EN.
On to: Amateur Practices 1 - Station Set-up and Test Equipment
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, May 2024.
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