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Subelement T3
Radio wave characteristics: properties of radio waves; propagation modes
Section T3C
Propagation modes: line of sight; sporadic E; meteor and auroral scatter and reflections; tropospheric ducting; F layer skip; radio horizon
Why are direct (not via a repeater) UHF signals rarely heard from stations outside your local coverage area?
  • They are too weak to go very far
  • FCC regulations prohibit them from going more than 50 miles
  • Correct Answer
    UHF signals are usually not reflected by the ionosphere
  • UHF signals are absorbed by the ionospheric D layer

Any time you hear stations directly (not using a repeater system) from far away, you can assume some sort of atmospheric condition is helping the signal to travel. Most long-distance radio waves bounce off the ionosphere. However, signals in the UHF spectrum have such a short wavelength that they don't bounce off the ionosphere at all -- they pass right through it into outer space. (This is why higher frequencies are ideal for communicating with satellites). Thus, if you hear a signal in the UHF band, it's safe to assume the source of that signal is nearby.

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Tags: uhf propagation radio waves ionosphere arrl chapter 4 arrl module 7

Which of the following is an advantage of HF vs VHF and higher frequencies?
  • HF antennas are generally smaller
  • HF accommodates wider bandwidth signals
  • Correct Answer
    Long distance ionospheric propagation is far more common on HF
  • There is less atmospheric interference (static) on HF

Think of VHF as very sharp waves that poke right through the atmosphere. Whereas, HF has very dull waves that bounce off the atmosphere. So VHF is mostly line of sight and HF is long distance. In this answer the only true option is long distance communications.

HF antennas are much bigger, tend to have smaller bandwidth, and there is generally a lot more noise / static.

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Tags: arrl chapter 4 arrl module 7

What is a characteristic of VHF signals received via auroral reflection?
  • Signals from distances of 10,000 or more miles are common
  • Correct Answer
    The signals exhibit rapid fluctuations of strength and often sound distorted
  • These types of signals occur only during winter nighttime hours
  • These types of signals are generally strongest when your antenna is aimed west

There is a lot going on here for a quick explanation but the best way to understand what is happening is to think of throwing a rock in a small pond and then shining a flashlight on the water. The beam is dancing around on the ripples and waves. The radio waves are doing the same thing in the atmosphere that being excited by the energy causing the auroral phenomenon and just about as quickly.

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Tags: vhf propagation ionosphere arrl chapter 4 arrl module 7

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?
  • Backscatter
  • Correct Answer
    Sporadic E
  • D layer absorption
  • Gray-line propagation

Every now and then a type of propagation occurs that carries the RF energy within a particular range of frequencies quite a long distance, refracting it in just the right way over and over. This type of propagation is known as Sporadic E. It occurs when clouds of intensely ionized gas form in the E region of the earth's ionosphere typically between 90 and 120 km in altitude. The mechanisms behind the formation of the ionized gas clouds are beyond the scope of this text.

Backscatter generally scatters a signal back towards its source, which would not result in strong over-the-horizon signals. The operative word in D layer absorption is absorption where the RF signal is attenuated, not refracted, in the ionospheric layer closest to the ground. Gray-line propagation is a reference to a 45 - 60 minute period around twilight when D layer absorption is diminished but some refraction of signals on the 10 and 15-meter bands can occur before the solar ionization in the E and F layers is diminished with nightfall.

For more information see: "Sporadic E, Es Propagation" on Electronics-Notes.com.

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Tags: propagation vhf uhf 2 meter 10 meter 6 meter arrl chapter 4 arrl module 7

Which of the following effects might cause radio signals to be heard despite obstructions between the transmitting and receiving stations?
  • Correct Answer
    Knife-edge diffraction
  • Faraday rotation
  • Quantum tunneling
  • Doppler shift

In general, radio signals don't penetrate dirt or rock very well at all. So if you're hearing a signal on the other side of a mountain, it's likely due to knife-edge diffraction, a physical phenomenon that occurs when waves hit a sharp edge.

Faraday rotation is way too complex to be explained or even included on a Technician Class license exam.

Quantum tunneling has to do with devices like Tunnel Diodes, which aren't discussed in the Technician Question Pool, and certainly have nothing to do with radio waves.

Doppler Shift, although a topic that does appear in the Question Pool, has to do with the source of the signal moving toward or away from you, and has nothing to do with hearing a signal despite obstructions.

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Tags: arrl chapter 4 arrl module 7

What mode is responsible for allowing over-the-horizon VHF and UHF communications to ranges of approximately 300 miles on a regular basis?
  • Correct Answer
    Tropospheric ducting
  • D layer refraction
  • F2 layer refraction
  • Faraday rotation

There are several modes that can allow communication that ranges "over-the-horizon" or beyond line-of-sight such as Ducting and Troposheric scatter. The key to differentiate these two in this question is the mentioning of "VHF" frequencies.

Tropospheric scatter is where the signals are bent or reflected back to earth in a somewhat random manner to station a significant distance away on a regular basis. But it works in the UHF and microwave frequencies, and is best around 2 GHz.

Tropospheric ducting happens when a large mass of cold air is overrun by warm air causing a temperature inversion, it is relatively common during summer and autumn months and can work as low as 40 MHz, and most commonly works above 90 MHz which covers most the VHF bands.

More information can be found here.

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Tags: propagation ionosphere vhf uhf arrl chapter 4 arrl module 7

What band is best suited for communicating via meteor scatter?
  • 10 meter band
  • Correct Answer
    6 meter band
  • 2 meter band
  • 70 centimeter band

Meteor scatter communication is done by reflecting radio waves off ionized particles in the ionosphere that were caused by meteors passing through. The 6-meter band is excellent for meteor scatter due to its wavelength, and because it is a quiet band. Wavelengths longer than 6 meters are not effectively reflected by meteor scatter; shorter wavelength bands, such as the 2-meter band, are not as quiet which makes it difficult to hear these weak signals from 500 to 1500 miles away.

Here is a memory aid: The "6" in 6 meters looks like a meteor with a curved tail

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Tags: 6 meter propagation arrl chapter 4 arrl module 7

What causes tropospheric ducting?
  • Discharges of lightning during electrical storms
  • Sunspots and solar flares
  • Updrafts from hurricanes and tornadoes
  • Correct Answer
    Temperature inversions in the atmosphere

To remember this answer, think of heating ducts in a building that carry different-temperature air long distances from a central unit, just like the temperature inversions that cause tropospheric ducting.

Tropospheric ducting is an atmospheric effect caused by a differential temperature layer that causes reflection or refraction of radio wave. These reflective layers can form a radio wave "duct", much like the ducts that are used to duct warm or cool air through our homes. These ducts are often caused by thermal inversions and other weather phenomena. Radio wave propagation can extend from 300 to 500 miles, sometimes as far as 1000 miles, through tropospheric ducting.

The troposphere is the lowest level of the atmosphere and is where temperature inversions occur; understanding this relationship will help you choose the correct answer.

Further information can be found at http://en.wikipedia.org/wiki/Tropospheric_propagation#Tropospheric_ducting

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Tags: propagation ionosphere arrl chapter 4 arrl module 7

What is generally the best time for long-distance 10 meter band propagation via the F layer?
  • Correct Answer
    From dawn to shortly after sunset during periods of high sunspot activity
  • From shortly after sunset to dawn during periods of high sunspot activity
  • From dawn to shortly after sunset during periods of low sunspot activity
  • From shortly after sunset to dawn during periods of low sunspot activity

Remember that 10 meters follows the sun and thus is best in daylight hours.

The 10 meter band is best during daylight hours due to the nature of this wavelength and how it refracts through or reflects off of the F2 layer of the ionosphere.

During periods of increased sunspot activity, band openings may begin well before sunrise and continue into the night.

In areas near the equator, 10 meters is effective even during periods of low solar activity. This is demonstrated by good propagation between areas in Africa to the Caribbean.

More information is found here.

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Tags: 10 meter propagation arrl chapter 4 arrl module 7

Which of the following bands may provide long distance communications during the peak of the sunspot cycle?
  • Correct Answer
    6 or 10 meter bands
  • 23 centimeter band
  • 70 centimeter or 1.25 meter bands
  • All of these choices are correct

Since they're talking about sunspot cycle, they're talking about ionospheric refraction. But 23 centimeters and 70 centimeters have wavelengths that are too short to be reflected or refracted by the ionosphere - they pass right through without enough bending to make it back to earth.

Six and ten meters are refracted somewhat, but only when we have high sunspot activity is there enough ultraviolet radiation to bend a signal all the way back down to the earth - without the high ultraviolet, the signal bends, but not enough to get it back to earth.

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Why do VHF and UHF radio signals usually travel somewhat farther than the visual line of sight distance between two stations?
  • Radio signals move somewhat faster than the speed of light
  • Radio waves are not blocked by dust particles
  • Correct Answer
    The Earth seems less curved to radio waves than to light
  • Radio waves are blocked by dust particles

When we talk about radio wave propagation we often say that it is "line of sight". This may cause you to think that radio signals will only travel to where you can see with your eyes. But, this is not always the case depending on the frequency band and atmospheric conditions that may make radio waves go beyond the horizon that we can see. VHF and UHF can bend somewhat around the curvature of the Earth and thus travel further than we can see. This assumes that there are not other significant obstacles that may block the signal such as buildings, trees and hills. This is also called the "radio horizon", or the distance where the radio signal between two points is blocked by the curvature of the Earth.

The key to remembering this question and answer pair is that VHF and UHF signals can bend (or "curve") around Earth.

Learn more at http://en.wikipedia.org/wiki/Radio_horizon.

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Tags: vhf uhf propagation arrl chapter 4 arrl module 7

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