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Subelement T3

Radio wave characteristics and signal propagation

 

Electromagnetic is the type of wave that carries radio signals between transmitting and receiving stations. The ionosphere is the part of the atmosphere enables the propagation of radio signals around the world. A radio wave travels through free space at the speed of light. 300,000,000 meters per second is the approximate velocity of a radio wave as it travels through free space. Electric and magnetic fields are the two components of a radio wave. Wavelength is the name for the distance a radio wave travels during one complete cycle. The wavelength gets shorter as the frequency increases. Wavelength in meters equals 300 divided by the frequency in megahertz is the formula for converting frequency to approximate wavelength in meters.

 

The orientation of the electric field is used to describe a radio wave polarization. Horizontal antenna polarization is normally used for long-distance weak-signal CW and SSB contacts using the VHF and UHF bands. Signals could be significantly weaker if the antennas at opposite ends of a VHF or UHF line of sight radio link are not using the same polarization. Either vertically or horizontally polarized antennas may be used for transmission or reception because skip signals refracted from the ionosphere are elliptically polarized.

 

The approximate wavelength is often used to identify the different frequency bands. 30 to 300 MHz are the frequency limits of the VHF spectrum. 300 to 3000 MHz are the frequency limits of the UHF spectrum. 3 to 30 MHz is the frequency range referred to as HF.

 

Direct (not via a repeater) UHF signals are rarely heard from stations outside your local coverage area because the ionosphere does usually not reflect them. UHF signals are often more effective from inside buildings than VHF signals because the shorter wavelength allows them to more easily penetrate the structure of buildings. The distance over which two stations can communicate by direct path is called the radio horizon. VHF and UHF radio signals usually travel somewhat farther than the visual line of sight distance between two stations because the Earth seems less curved to radio waves than to light. Tropospheric scatter is responsible for allowing over-the-horizon VHF and UHF communications to ranges of approximately 300 miles on a regular basis. Temperature inversions in the atmosphere causes tropospheric ducting.

 

If another operator reports that your station’s 2 meter signals were strong just a moment ago, but now they are weak or distorted try moving a few feet or changing the direction of your antenna if possible, as reflections may be causing multi-path distortion. VHF signals received via auroral reflection exhibit rapid fluctuations of strength and often sound distorted. When VHF signals are being received from long distances, signals are being refracted from a sporadic E layer.  Sporadic E is most commonly associated with occasional strong over-the-horizon signals on the 10, 6, and 2-meter bands.

 

Picket fencing is the term commonly used to describe the rapid fluttering sound sometimes heard from mobile stations that are moving while transmitting. Your station can access a distant repeater if buildings or obstructions are blocking the direct line of sight path by using a directional antenna to find a path that reflects signals to the repeater. If data signals propagate over multiple paths the error rates are likely to increase. Random combining of signals arriving via different paths is a likely cause of irregular fading of signals received by ionospheric reflection. Knife-edge diffraction cause radio signals to be heard despite obstructions between the transmitting and receiving stations. 6 meters is best suited for communicating via meteor scatter. Six or ten meters provide long distance communications during the peak of the sunspot cycle. From dawn to shortly after sunset during periods of high sunspot activity is generally the best time for long-distance 10-meter band propagation via the F layer.

 

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SUBELEMENT T3

Radio wave characteristics: properties of radio waves; propagation modes

[3 Exam Questions – 3 Groups]

What should you do if another operator reports that your station’s 2 meter signals were strong just a moment ago, but now they are weak or distorted?

1. Try moving a few feet or changing the direction of your antenna if possible, as reflections may be causing multi-path distortion

Why might the range of VHF and UHF signals be greater in the winter?
B. Less absorption by vegetation

What antenna polarization is normally used for long-distance weak-signal CW and SSB contacts using the VHF and UHF bands?

1. Horizontal

What can happen if the antennas at opposite ends of a VHF or UHF line of sight radio link are not using the same polarization?

1. Signals could be significantly weaker

When using a directional antenna, how might your station be able to access a distant repeater if buildings or obstructions are blocking the direct line of sight path?B. Try to find a path that reflects signals to the repeater

What term is commonly used to describe the rapid fluttering sound sometimes heard from mobile stations that are moving while transmitting?

1. Picket fencing

What type of wave carries radio signals between transmitting and receiving stations?

1. Electromagnetic

Which of the following is a likely cause of irregular fading of signals received by ionospheric reflection?

1. Random combining of signals arriving via different paths

Which of the following results from the fact that skip signals refracted from the ionosphere are elliptically polarized?

1. Either vertically or horizontally polarized antennas may be used for transmission or reception

What may occur if data signals arrive via multiple paths?

1. Error rates are likely to increase

Which part of the atmosphere enables the propagation of radio signals around the world?

1. The ionosphere

How might fog and light rain affect radio range on 10 meters and 6 meters?
B. Fog and light rain will have little effect on these bands

What weather condition would decrease range at microwave frequencies?
C. Precipitation

What is the name for the distance a radio wave travels during one complete cycle?

1. Wavelength

The orientation of the electric field property of a radio wave is used to describe its polarization.

What are the two components of a radio wave?

1. Electric and magnetic fields

How fast does a radio wave travel through free space?

1. At the speed of light

How does the wavelength of a radio wave relate to its frequency?

1. The wavelength gets shorter as the frequency increases

What is the formula for converting frequency to approximate wavelength in meters?

1. Wavelength in meters equals 300 divided by frequency in megahertz

What property of radio waves is often used to identify the different frequency bands?

1. The approximate wavelength is often used to identify the different frequency bands.

What are the frequency limits of the VHF spectrum?

1. 30 to 300 MHz

What are the frequency limits of the UHF spectrum?

1. 300 to 3000 MHz

What frequency range is referred to as HF?

1. 3 to 30 MHz

What is the approximate velocity of a radio wave as it travels through free space?

1. 300,000,000 meters per second

Why are direct (not via a repeater) UHF signals rarely heard from stations outside your local coverage area?

1. UHF signals are usually not reflected by the ionosphere

Which of the following is an advantage of HF vs VHF and higher frequencies?

1. Long distance ionospheric propagation is far more common on HF

What is a characteristic of VHF signals received via auroral reflection?

1. The signals exhibit rapid fluctuations of strength and often sound distorted

Which of the following propagation types is most commonly associated with occasional strong over-the-horizon signals on the 10, 6, and 2 meter bands?

1. Sporadic E

Which of the following effects might cause radio signals to be heard despite obstructions between the transmitting and receiving stations?

1. Knife-edge diffraction

What mode is responsible for allowing over-the-horizon VHF and UHF communications to ranges of approximately 300 miles on a regular basis?

1. Tropospheric scatter

What band is best suited for communicating via meteor scatter?

1. 6 meters

What causes tropospheric ducting?

1. Temperature inversions in the atmosphere

What is generally the best time for long-distance 10 meter band propagation via the F layer?

1. From dawn to shortly after sunset during periods of high sunspot activity

Which of the following bands may provide long distance communications during the peak of the sunspot cycle?

1. Six or ten meters

Why do VHF and UHF radio signals usually travel somewhat farther than the visual line of sight distance between two stations?

1. The Earth seems less curved to radio waves than to light

 

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