Technician Class Series – Weak Signal Propagation

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Hello everybody and welcome back to Everything Hamradio! Today we are going to continue our series on the Technician Class License Question Pool! Today we will be talking about Weak Signal Propagation.

As always, the correct answers will be in bold. I also recommend that you only read the correct answers when studying for your test. If you do this, when you take your test and you see a question, there will be a better chance that the correct answer will jump out at you easier. Some say that this approach is kind of like cheating, but the way that I see it, you will always be learning something in this hobby and you don’t have to know everything there is to know about the hobby to get your license.

If you would like to purchase a copy of the Technician Class study book, written by Gordon West, WB6NOA with Eric P. Nichols, KL7AJ, that this series is loosely based around, below is a link to it.

Let’s move on to the good stuff, shall we?


 

T5C07 What is a usual name for electromagnetic waves that travel through space?
  1. Gravity waves
  2. Sound waves
  3. Radio waves
  4. Pressure waves
Lets talk about the ones that are not right, first off, Gravity doesn’t come in waves, its a constant, and Pressure waves have nothing electrical or magnetic about them. That leaves us with Sound and Radio. Sound waves are auditory so there isn’t any electrical or magnetic properties to them either. That leaves us with Radio waves. Radio waves have both electrical AND magnetic fields and since the question is asking about electromagnetic waves, Radio waves is the correct answer. 

T3C10 What is the radio horizon?
  1. The distance over which two stations can communicate by direct path
  2. The distance from the ground to a horizontally mounted antenna
  3. The farthest point you can see when standing at the base of your antenna tower
  4. The shortest distance between two points on the Earth’s surface
Radio Horizon
The approximate distance (in miles) to the radio horizon can be calculated by multiplying the square root of the antenna height (in feet) by 1.415 times. For example, the distance to the radio horizon for an antenna 1,000 feet above the ground is just under 45 miles.
Much like looking at the horizon with your eyes, you can only see so far and then all your see is a curved line. Since you are basically seeing light waves, it is the same for radio waves. In a direct line-of-sight, which is what most vhf/uhf signals travel on, it can only go so far before it runs into the planet surface and wont go any further. If you look at the picture to the right, you will see a picture explaining this a little better and even an example as how far the radio horizon is.

T3C11 Why do VHF and UHF radio signals usually travel somewhat farther than the visual line of sight distance between two stations?
  1. Radio signals move somewhat faster than the speed of light
  2. Radio waves are not blocked by dust particles
  3. The Earth seems less curved to radio waves than to light
  4. Radio waves are blocked by dust particles
Because of the way that radio waves travel through the air and get refracted more than light waves do, you will be able to talk on the radio for a little bit longer than you can see. Radio waves travel approximately 15% farther than light waves do. For example, if you and a friend start walking away from each other on a flat surface, you would loose sight of each other at about 5 miles apart but you will still be able to talk to each other for another 30% since both you and your friend have an extra 15% each.

 


T3C01 Why are direct (not via a repeater) UHF signals rarely heard from stations outside your local coverage area?
  1. They are too weak to go very far
  2. FCC regulations prohibit them from going more than 50 miles
  3. UHF signals are usually not reflected by the ionosphere
  4. They collide with trees and shrubbery and fade out
UHF signals rarely will bounce off the ionosphere like the VHF and HF signals do because of their small size. Here is a practical example of this. Lets say that a small rock is a UHF frequency and a large rock is a VHF or HF frequency. Both rocks are thrown with the same amount of force at a wall. Because of the size of the smaller rock, it will have less friction when it encounters the wall so it will have a better chance of going through the wall. As opposed to the large rock, because it is heavier and had more surface, it will have more contact with the surface of the wall and will be more likely to bounce back.

T3C05 Which of the following effects might cause radio signals to be heard despite obstructions between the transmitting and receiving stations?
  1. Knife-edge diffraction
  2. Faraday rotation
  3. Quantum tunneling
  4. Doppler shift
Much like signals being reflected off the ionosphere, sometimes a VHF signal can be heard in places that are not direct line of sight from each other. If two people live on opposites side of a ragged hill, they may still be able to talk to each other and here is why. VHF signals are transmitted in a vertical polarization, so when those vertical signal strike a sharp edge of a distant  object the bottom half of the way will drag along the ground and turn the wave downward. Once the wave gets a clear light it will again travel in a straight line and since it was turned downward, it will travel down the backside of the hill, allowing you to talk to your friend. This is called Knife-edge diffraction.  

T3C06 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
  2. D layer refraction
  3. F2 layer refraction
  4. Faraday rotation
This occurs quite frequently over water where there is a layer of cool air along the surface level, and cool air in the upper ionospher, but a layer of warm, moist air is held in between the two in the troposphere.  Tropospheric scatter, or Tropospheric ducting as it is more commonly called, allows your signal to be caught in that layer of warm air and travel many miles before coming out of that layer and return back to the surface for reception.

T3C08 What causes tropospheric ducting?
  1. Discharges of lightning during electrical storms
  2. Sunspots and solar flares
  3. Updrafts from hurricanes and tornadoes
  4. Temperature inversions in the atmosphere
 When a layer of warm air traps a layer of colder air below it, this is called a Temperature Inversion. This allows us to have the tropospheric ducting to occur and we can talk much further than normal direct line-of-sight.

T3C03 What is a characteristic of VHF signals received via auroral reflection?
  1. Signals from distances of 10,000 or more miles are common
  2. The signals exhibit rapid fluctuations of strength and often sound distorted
  3. These types of signals occur only during winter nighttime hours
  4. These types of signals are generally strongest when your antenna is aimed
Sometimes when a VHF signal encounters an auroral curtain, some of the waves can bounce back the other direction, much like the signal striking a metal building. The entire signal doesn’t get bounced back, but parts of it do. This causes the receiving person to her rapid fluctuations of strength and distortion. 

T3C07 What band is best suited for communicating via meteor scatter?
  1. 10 meters
  2. 6 meters
  3. 2 meters
  4. 70 cm
As a technician class licensee, you can use the entire 6 meter band with up to 1500 watts of power. This is more than enough power to be able to bounce a signal off a meteor tail or even the moon! 

T3A11 Which part of the atmosphere enables the propagation of radio signals around the world?
  1. The stratosphere
  2. The troposphere
  3. The ionosphere
  4. The magnetosphere
Different parts of the ionosphere  reflect different frequencies, the higher the frequency the lower the distance you are able to talk and the lower it travels up into the atmosphere. The sun’s ultraviolet radiation ionizes the atmosphere and different layers of the the ionosphere are charges at different frequencies. If the frequency you are talking on is higher than the frequency of a layer of the ionosphere it will continue through to the next layer, if not it is reflected back to the earth.

T3C02 Which of the following might be happening when VHF signals are being received from long distances?
  1. Signals are being reflected from outer space
  2. Signals are arriving by sub-surface ducting
  3. Signals are being reflected by lightning storms in your area
  4. Signals are being refracted from a sporadic E layer
Sometimes during the summer and normally during the early morning and late afternoon, the E layer will ionize at a frequency that will reflect your signal a much greater distance than normal. Because this isn’t a common occurrence and is more sporadic in nature, it is called Sporadic E Layer Refraction.  

T3C09 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
  2. From shortly after sunset to dawn during periods of high sunspot activity
  3. From dawn to shortly after sunset during periods of low sunspot activity
  4. From shortly after sunset to dawn during periods of low sunspot activity
During times of high sunspot activity, you will be able to talk a lot further than you normally would be able to do on 6 and 10 meters. During the day time hours during high sunspot activity is when you have the best time because during the night time hours, the F1 and F2 layers combine to form just the F layer. So the answer is from dawn to shortly after sunset during periods of high sunspot activity is the correct answer. 

T3A09 Which of the following results from the fact that skip signals refracted from the ionosphere are elliptically polarized?
  1. Digital modes are unusable
  2. Either vertically or horizontally polarized antennas may be used for transmission or reception
  3. FM voice is unusable
  4. Both the transmitting and receiving antennas must be of the same polarization
Normally, if you have an antenna that is horizontally polarized you can not receive a signal from someone transmitting with a vertically polarized antenna, the waves are just not right. However, when a signal is refracted from the ionosphere you can receive on either vertical or horizontally polarized antenna. This is because the wave is no longer vertical or horizontal, it is elliptical.

T3C04 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. Backscatter
  2. Sporadic E
  3. D layer absorption
  4. Gray-line propagation
Occationally, you and a friend will be talking on 10, 6 or 2 meters and all the sudden someone will break in and say that they are 1,500 miles away! You are able to talk to them and they are able to talk to you. It may last 5 minutes, it may last 5 hours. It is kind of like someone places a mirror up in the atmosphere for a while, but then gets tired of holding it there and takes it down again. This type of signal is called sporadic E

T3C12 Which of the following bands may provide long distance communications during the peak of the sunspot cycle?
  1. Six or ten meters
  2. 23 centimeters
  3. 70 centimeters or 1.25 meters
  4. All of these choices are correct
 As a Technician Class Operator the two bands that you will have voice communications on that will give you the best distance is the 6 and 10 meter bands. 

T3B02 What property of a radio wave is used to describe its polarization?
  1. The orientation of the electric field
  2. The orientation of the magnetic field
  3. The ratio of the energy in the magnetic field to the energy in the electric field
  4. The ratio of the velocity to the wavelength
I’m sure that you have seen pictures that show how the magnetic field of the earth is sometime in your life. Antennas are basically the same way. Vertical or horizontal antennas will transmit along the long end of themselves. Maybe you remember KITT from the TV show Knight Rider. Imagine that red light on the front of the car as an antenna. The light going back and forth are the waves coming off the antenna. The light that comes from that light bar shines off the front of it and not the ends.  

T3A08 Which of the following is a likely cause of irregular fading of signals received by ionospheric reflection?
  1. Frequency shift due to Faraday rotation
  2. Interference from thunderstorms
  3. Random combining of signals arriving via different paths
  4. Intermodulation distortion 
 Unlike on HF, on VHF signal may go in several directions and end up in the same place due to ionospheric reflection. One time you are talking to someone, and you have a beam antenna, you may receive the sending station from due east. When the signal fades on you, you can do one of two things, you can give it a few seconds and try it again or you can adjust your antenna a few degrees to the left or right and you might hear them again


So that brings us to the end of this section. Next week we will be talking about Talking to Outer Space! Please share my blog with your friends and if you have not done so already, please subscribe to my email list to get the latest and quickest notifications of any new post that I publish. You can sign up in the box on the sidebar. If you would rather, you can also sign up for my RSS feed. Please Like me on Facebook, and follow me on Twitter, Google+, LinkedIn and StumbledUpon. Links to all of these can be found under social on the menu.

Thanks for stopping by. If you have any questions or comments about today’s post, please leave them in the comments below or shoot me an email a k5clm@everythinghamradio.com.

Until next time…

73 de Curtis, K5CLM

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  • Andy Brittain G0HXT

    Small typo in the supporting text of T3C03 “her rapid”? Like the series and would like to get your emails.

    • Thanks Andy, good catch! It’s good to hear that you are liking the series, if you were the only one it would be worth doing. To subscribe to get emails go to the subscribe page on the menu and fill out the form. Once you click submit, you will get an email to confirm it then, viola you’re done…thanks for reading