<< radio home
by disinfoniacs #69 & #1br>
The ionosphere is a layer of the Earth's atmosphere extending from about 50 to 600 km in altitude. It is ionized by solar radiation, which strips electrons from the atoms and molecules that make up the air. This ionization creates a plasma, which can reflect radio waves.
The ionosphere is made up of several layers, each with its own properties. The D layer is the lowest layer, at an altitude of about 50 to 90 km. It is most active during the daytime and absorbs most radio signals with frequencies below about 10 MHz. The E layer is next, at an altitude of about 90 to 160 km. It is most active during the daytime and reflects radio waves with frequencies up to about 10 MHz. The F layer is the highest and most complex layer, extending from about 160 to 600 km. It is divided into two sub-layers, F1 and F2. The F1 layer is most active during the daytime and reflects radio waves with frequencies up to about 15 MHz, while the F2 layer is most active at night and reflects radio waves with frequencies up to about 30 MHz.
Long-distance ionospheric propagation is significantly more prevalent on HF frequencies, which range from 3 to 30 MHz. This is because HF waves can penetrate the ionosphere and bounce back to Earth, allowing them to travel long distances. In contrast, VHF (very high frequency) and higher frequencies cannot penetrate the ionosphere and are limited to line-of-sight communication.
Under specific atmospheric conditions, a phenomenon known as "sporadic E layer" may form, which holds special significance for ham radio. This layer can occur at altitudes between about 90 to 150 km, and it is characterized by a high concentration of ionized particles. Sporadic E propagation, also known as "band openings," frequently results in occasional strong over-the-horizon signals on the 10-meter HF band, as well as the 6- and 2-meter VHF bands. Many hams attempt to capitalize on these openings by pursuing the VUCC (VHF/UHF Century Club) award, tracking and confirming contacts in unique grid squares.
Direct UHF signals are rarely detected from stations outside their local coverage area as HF signals are not typically reflected by the ionosphere. Instead, UHF signals rely on line-of-sight communication and are limited to a range of about 50 miles. However, under certain atmospheric conditions, such as tropospheric ducting, UHF signals can be ducted over long distances.
From an antenna perspective, the ionosphere makes it easy to skip signals. As the ionosphere elliptically polarizes skip signals, either vertically or horizontally polarized antennas may be used for transmission or reception. However, the choice of polarization depends on the angle of incidence and the location of the receiver or transmitter. Vertical polarization is typically used for long-distance communication, while horizontal polarization is preferred for short-range communication.
The troposphere is the lowest layer of Earth's atmosphere, extending from the Earth's surface up to about 7 to 20 kilometers, depending on the latitude and season. The troposphere is characterized by a decrease in temperature with increasing altitude, and is where weather occurs, with most of the Earth's clouds and precipitation forming within it.
One fascinating effect that can occur within the troposphere is known as tropospheric ducting, which can be harnessed to facilitate long-distance communication on both VHF (Very High Frequency) and UHF (Ultra High Frequency) frequencies. Tropospheric ducting is a phenomenon in which the normal vertical temperature gradient is inverted, allowing a layer of warm air to be trapped between two layers of cooler air. This inversion layer acts as a waveguide, trapping VHF or UHF signals and allowing them to travel much farther than they would be able to through normal line-of-sight propagation.
This effect is particularly useful for ham radio operators who wish to communicate over longer distances on these frequencies. Under ideal conditions, tropospheric ducting can enable communication up to 300 miles or more. However, the effect is highly dependent on weather conditions and atmospheric stability, and is not always present or reliable.
Sunspots are temporary phenomena on the Sun's photosphere that appear as spots darker than the surrounding areas. They exhibit an 11-year cycle of activity, with a peak predicted in 2025. The number of sunspots increases during periods of high activity, which is known as the solar maximum.
During high sunspot activity, the increased radiation and charged particles emitted by the Sun can significantly impact the Earth's ionosphere, causing it to become more ionized and increasing the number of ionospheric layers. This can lead to enhanced propagation of radio waves at certain frequencies, particularly the 6- and 10-meter bands. The optimal time for long-distance 10-meter band propagation via the F layer is from dawn to shortly after sunset during periods of high sunspot activity. This is because the ionosphere is most ionized during the day and the F layer is at its highest altitude, allowing for longer range communication.
Meteors, on the other hand, are small pieces of space debris that burn up upon entry into the Earth's atmosphere. When a meteor burns up, it ionizes the surrounding air, creating a trail of ionized particles that can reflect radio waves. This effect is known as meteor scatter and can be utilized for long-distance communication on the 6-meter band. Meteor scatter communication typically involves transmitting short bursts of data during meteor showers when the ionized trails are most prevalent.
To conclude, let's delve into a couple of downsides of the reflection concepts discussed earlier that can lead to signal distortion.
When receiving a signal reflecting off multiple buildings or objects simultaneously, you may experience "picket fencing" or rapid flutter on mobile signals, a result of multipath propagation. Similarly, multipath propagation can cancel or reinforce signals in VHF bands, causing signal strength to fluctuate greatly when the antenna is moved just a few feet.
While auroras may be aesthetically pleasing to behold, they can also reflect VHF signals, resulting in rapid fluctuations in strength and distorted sound.
Lastly, ionospheric reflection can lead to irregular fading of signals received due to the random combination of signals arriving via different paths.
Review section T3 on hamstudy.org and take the practice quiz for that chapter until you can consistently hit 85%+
<< previous lesson | next lesson >>
<< radio home