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howto:hambasics:waveinteraction [2019/11/24 20:25] ve7hzfhowto:hambasics:sections:waveinteraction [2021/01/03 08:08] (current) va7fi
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 ====== Principle of Least Time ====== ====== Principle of Least Time ======
  
-Imagine you're on the beach when you suddenly notice a child in distress in the water.  You're a good swimmer but let's say you can run twice as fast as you can swim.  What do you do?+Imagine you're on the beach when you suddenly notice a child in distress in the water.  You're a good swimmer but you can run faster than you can swim.  What do you do?
  
 **Option 1**: You make a B-line for the child because the shortest distance between two points is a straight line. **Option 1**: You make a B-line for the child because the shortest distance between two points is a straight line.
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 {{  refraction_photo_top.png?500  }} {{  refraction_photo_top.png?500  }}
  
-If this last one feels weird to you, imagine this: suppose you're a turtle who can swim twice as fast as you can walk.  It makes sense that you'd want to spend more time in the water and less on the beach:+If this last one feels weird to you, imagine this: suppose you're a turtle who can swim faster than you can walk.  It makes sense that you'd want to spend more time in the water and less on the beach:
 {{  beach4.png  }} {{  beach4.png  }}
  
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   * When going from a "quick" medium to a "slow" medium, light bends away from the surface to spend less time in the slow medium.   * When going from a "quick" medium to a "slow" medium, light bends away from the surface to spend less time in the slow medium.
   * When going from a "slow" medium to a "quick" medium, light does the opposite and bends towards the surface.   * When going from a "slow" medium to a "quick" medium, light does the opposite and bends towards the surface.
 +  * Whatever it does, light always wants to spend less time in a "slow" medium and more time in a "fast" medium because that's the overall quickest way to get from A to B.
  
  
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 This behaviour is a bit hard to explain without going into the math, but here's an animation that allows you to explore it: This behaviour is a bit hard to explain without going into the math, but here's an animation that allows you to explore it:
-  * You can move four points around to see how the refracted ray changes: "//n//<sub>1</sub>", "//n//<sub>2</sub>", "Laser", and "Entry point".+  * You can move four points around to see how the refracted ray changes: "\$n_1\$", "\$n_2\$", "Laser", and "Entry point".
   * Note though that this particular animation only works if the laser is below the horizontal line.   * Note though that this particular animation only works if the laser is below the horizontal line.
  
-<html>+{{ggb>/howto/hambasics/sections/snells2.ggb 705,403}}
  
-<script type="text/javascript" language="javascript" src=" +\$n_1\$ and \$n_2\$ are the [[wp>Refractive_index |Refractive Indices]] of the media.  They are defined as the ratio of the speed of light in vacuum to the speed of light in the media \$\left(n = \frac{c}{v}\right)\$.  For example, if \$n = 2\$, then the speed of light is twice as //slow// in the medium as it is in vacuum.  The bigger \$n\$ is, the slower the speed.  \$n = 1\$ means that the speed is the same as the speed of light in a vacuum.
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-//n//<sub>1</sub> and //n//<sub>2</sub> are the [[wp>Refractive_index |Refractive Indices]] of the media.  They are defined as the ratio of the speed of light in vacuum to the speed of light in the media $\left(n = \frac{c}{v}\right)$.  For example, if //n// = 2, then the speed of light is twice as //slow// in the medium as it is in vacuum.  The bigger //n// is, the slower the speed.  //n//=1 means that the speed is the same as the speed of light in a vacuum.+
  
 A few things to try: A few things to try:
-  * Set //n//<sub>1</sub> = 1 and Set //n//<sub>2</sub> = 2 and move the Laser and the Entry Point around.  These are the paths when you can run twice as fast as you can swim.  Notice that if you set //n//<sub>1</sub> = 2 and //n//<sub>2</sub> = 4, or //n//<sub>1</sub> = 2.5 and //n//<sub>2</sub> = 5, it shouldn't matter.  What really matters is the relative speeds between the two media. +  * Set \$n_1 = 1\$ and Set \$n_2 = 2\$ and move the Laser and the Entry Point around.  These are the paths when you can run twice as fast as you can swim.  Notice that if you set \$n_1 = 2\$ and \$n_2 = 4\$, or \$n_1 = 2.5\$ and \$n_2 = 5\$, it shouldn't matter.  What really matters is the relative speeds between the two media. 
-  * Now move the laser in a straight line so that the angle //θ//<sub>1</sub> doesn't change.  The refracted ray shouldn't change either.  So it doesn't matter how far the laser is from the surface.  What matters is the angle at which the beam hits the surface.+  * Now move the laser in a straight line so that the angle \$\theta_1\$ doesn't change.  The refracted ray shouldn't change either.  So it doesn't matter how far the laser is from the surface.  What matters is the angle at which the beam hits the surface.
   * Now move the laser back and forth in a semi circle around the Entry Point.  Although the laser is the same distance away from the Entry Point, the angle of incidence changes so the refracted ray changes.   * Now move the laser back and forth in a semi circle around the Entry Point.  Although the laser is the same distance away from the Entry Point, the angle of incidence changes so the refracted ray changes.
-  * Now set //n//<sub>1</sub> = 1.5 and Set //n//<sub>2</sub> = 1 and play with the laser to change its angle of incidence (<fc #ff0000>important</fc>).  At what angle do you notice that the refracted ray goes parallel to the surface?  This is called the critical angle.  Passed that angle, the ray can't go through and gets reflected instead.+  * Now set \$n_1\$ = 1.5 and Set \$n_2\$ = 1 and play with the laser to change its angle of incidence (<fc #ff0000>important</fc>).  At what angle do you notice that the refracted ray goes parallel to the surface?  This is called the critical angle.  Passed that angle, the ray can't go through and gets reflected instead.
  
  
 ===== Example ===== ===== Example =====
  
-Here's an underwater picture VE7HZF took in a lake with a waterproof camera.  The camera is completely submerged under water looking up toward the surface.  Above a certain angle, it's possible to see the beach, trees, and the sky.  But below a certain angle, we see the reflection of his wetsuit.+Here's an underwater picture that VA7FI took in a lake with a waterproof camera.  The camera is completely submerged under water looking up toward the surface.  Above a certain angle, it's possible to see the beach, trees, and the sky.  But below that angle, we see the reflection of his wetsuit.
 {{  laketir.jpg  }} {{  laketir.jpg  }}
  
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 {{  tirsketch.png  }} {{  tirsketch.png  }}
  
-The other cool thing about that picture is that if you zoom in on the beach, you'll see the colours separate (as if through a prism).  This indicates that the index of refraction, //n//, depends on the frequency.This will be important when we relate all of this back to radio waves. +The other cool thing about that picture is that if you zoom in on the beach, you'll see the colours separate (as if through a prism).  This indicates that the index of refraction, //n//, depends on the frequency.  This will be important when we relate all of this back to radio waves. 
-{{  laketirZoom.jpg?650  }}+{{  laketirzoom.jpg?650  }}
  
  
  
 ===== Snell's Law (Optional) ===== ===== Snell's Law (Optional) =====
 +<hidden>
 Snell's law gives the relationship between the angle of incidence and refraction depending on the refraction indices: Snell's law gives the relationship between the angle of incidence and refraction depending on the refraction indices:
-$$ n_1 \sin(\theta_1) = n_2 \sin(\theta_2) $$+\$$ n_1 \sin(\theta_1) = n_2 \sin(\theta_2) \$$
  
 {{ refractionreflextion.png?600 }} {{ refractionreflextion.png?600 }}
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 There are four interesting cases here: There are four interesting cases here:
  
-  * If $n_1 < n_2$ (high speed to low speed), then the left hand side of the equation is in danger of being less than the right hand side.  To maintain the equality, $\theta_1 > \theta_2$, which means that the path curves away from the surface. +  * If \$n_1 < n_2\$ (high speed to low speed), then the left hand side of the equation is in danger of being less than the right hand side.  To maintain the equality, \$\theta_1 > \theta_2\$, which means that the path curves away from the surface. 
-  * If $n_1 > n_2$ (low speed to high speed), then the right hand side of the equation is in danger of being less than the left hand side.  To maintain the equality, $\theta_1 < \theta_2$, which means that the path curves away from the surface. +  * If \$n_1 > n_2\$ (low speed to high speed), then the right hand side of the equation is in danger of being less than the left hand side.  To maintain the equality, \$\theta_1 < \theta_2\$, which means that the path curves away from the surface. 
-  * If we keep increasing  $n_1$ compared to $n_2$, then $\theta_2$ can increase to the point where it's going parallel to the surface ($\theta_2 = 90^\circ$), which means that: $\frac{n_1}{n_2} \sin(\theta_1) = 1 $.  At this point, we call $\theta_1$ the critical angle. +  * If we keep increasing  \$n_1\$ compared to \$n_2\$, then \$\theta_2\$ can increase to the point where it's going parallel to the surface (\$\theta_2 = 90^\circ\$), which means that: \$\frac{n_1}{n_2} \sin(\theta_1) = 1 \$.  At this point, we call \$\theta_1\$ the critical angle. 
-  * If we keep increasing $n_1$ even further, then  $\frac{n_1}{n_2} \sin(\theta_1) >1 $, which means that it's impossible for $\theta_2$ to keep up since $\sin(\theta_2) \leq 1$.  This is when Total Internal Reflection occurs, which is what we use to "bounce" radio waves off the ionosphere (more on that next). +  * If we keep increasing \$n_1\$ even further, then  \$\frac{n_1}{n_2} \sin(\theta_1) >1 \$, which means that it's impossible for \$\theta_2\$ to keep up since \$\sin(\theta_2) \leq 1\$.  This is when Total Internal Reflection occurs, which is what we use to "bounce" radio waves off the ionosphere (more on that next). 
- +</hidden>
- +
-====== Polarization ====== +
- +
-===== How To Make A Radio Wave ===== +
-Back on the [[intro#hz |Intro Page]], we introduced to the idea of frequency and saw that +
- +
->A Hertz (Hz) is a measure of how fast something vibrates [...] +
-+
->Just seeing “Hz” doesn't tell you anything about what it is that's oscillating in the same way that seeing “°C” doesn't tell you anything about what it is that has temperature. “Hz” is a unit of measure, not a thing itself. +
-+
->Without going into too much detail (yet), radio waves are created by oscillating electric currents. How many times this current oscillates per second is called the frequency, which is measured in Hz (or kHz, MHz, GHz).  +
- +
-It's now time to add a few more details.  Here is a basic recipe for making a radio wave: +
-  - Get a length of conducting wire and lay it in a straight line. +
-  - Cut it in half right in the middle and bend both ends at right angle. +
-  - Connect the two middle ends to each side of an alternating current generator. +
- +
-{{dipole.gif}}{{radiationpatternh.jpg}} +
- +
- +
-Voila! Assuming that the length of the antenna (the two pieces of wires) match the frequency of the current generator (more of this later), and that the antenna is high enough above the ground, you've created a radio wave.((GIF from [[wp>Dipole_antenna |Wikipedia Dipole Antenna]]))  As electrons move up and down the length of the wires, they create varying electric and magnetic fields that couple together according to [[wp>Maxwell's_equations |Maxwell's Equations]] and propagate outward in a doughnut shape.((Picture modified from [[wp>Dipole_antenna |Wikipedia Dipole Antenna]])) +
- +
- +
-===== Horizontal vs Vertical Polarization ===== +
- +
-{{  polarization.jpg}} +
-Here's the critical part though:  In the same way that an alternating current through an antenna creates a radio wave, a radio wave hitting an antenna induces an alternating current through it **if the radio wave hitting the antenna is in the same "direction" as the antenna**. +
- +
-This "direction" is called polarization. +
- +
- +
-===== Effect on Communication ===== +
-In practice, polarization is more important for VHF and UHF communication because signals go directly from the transmitting station to the receiving one.  For skywave HF communications, the ionosphere can change the polarization of the signal from moment to moment as the radio wave refracts, reflects, or goes through magnetic fields in the atmosphere.  As such polarization of the antennas on HF frequency doesn't matter much. +
- +
  
 ====== Scattering ====== ====== Scattering ======
  
-{{:howto:hambasics:scattering.png?85  }}+{{howto:hambasics:sections:scattering.png?85  }}
 Scattering occurs when an EM wave hits a bunch of "small particles" that in turn re-radiate the wave in all direction.  Note that the "small particles" can be single atoms, molecules, dust, or pockets of gas with a different index of refraction.  They can also be bigger objects like meteors or small planes!  The size of the "particle" is always relative to the wavelength of the EM wave.  To a 160m radio wave, a meteor is small, but to a laser beam (≈500nm), a dust particle is very big. Scattering occurs when an EM wave hits a bunch of "small particles" that in turn re-radiate the wave in all direction.  Note that the "small particles" can be single atoms, molecules, dust, or pockets of gas with a different index of refraction.  They can also be bigger objects like meteors or small planes!  The size of the "particle" is always relative to the wavelength of the EM wave.  To a 160m radio wave, a meteor is small, but to a laser beam (≈500nm), a dust particle is very big.
  
-Here are a few examples((The laser pictures were taken by Patrick, VE7HZF with help from Justine.  The picture of the forest is from: [[https://www.souvenirpixels.com/Photo-blog/i-cZgCHvZ]])) in the visible light spectrum:+Here are a few examples((The laser pictures were taken by Patrick, VA7FI with help from Justine.  The picture of the forest is from: [[https://www.souvenirpixels.com/Photo-blog/i-cZgCHvZ]])) in the visible light spectrum:
  
 The first picture shows a laser beam shinning at the wall. The first picture shows a laser beam shinning at the wall.
-{{ :howto:hambasics:laser1.jpg }}+{{ howto:hambasics:sections:laser1.jpg }}
  
 In the second picture, water is sprayed into the path of the laser beam. In the second picture, water is sprayed into the path of the laser beam.
-{{ :howto:hambasics:laser2.jpg }}+{{ howto:hambasics:sections:laser2.jpg }}
  
-{{  :howto:hambasics:lightscattering.jpg}}The reason the beam is invisible in the first picture is that all the light from the laser travels toward the wall (and none toward the camera).  But in the second picture, the water vapour scatters some of that light in random directions, allowing some of it to reach the camera.  There's a subtle point here: light from a regular light bulb also does this.  What I mean is this:+{{  howto:hambasics:sections:lightscattering.jpg}}The reason the beam is invisible in the first picture is that all the light from the laser travels toward the wall (and none toward the camera).  But in the second picture, the water vapour scatters some of that light in random directions, allowing some of it to reach the camera.  There's a subtle point here: light from a regular light bulb also does this.  What I mean is this:
   * Look at the light bulb in the room you're in.   * Look at the light bulb in the room you're in.
   * Now look at an object that the light bulb illuminates.   * Now look at an object that the light bulb illuminates.
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 ====== Diffraction ====== ====== Diffraction ======
  
-{{:howto:hambasics:diffraction.png }}+{{howto:hambasics:sections:diffraction.png }}
 Diffraction is the bending of waves around the corners of an obstacle or through an aperture.  The diffracting object or aperture effectively becomes a secondary source of the propagating wave, which in turns can interact with the main wave or other diffracted waves. Diffraction is the bending of waves around the corners of an obstacle or through an aperture.  The diffracting object or aperture effectively becomes a secondary source of the propagating wave, which in turns can interact with the main wave or other diffracted waves.
  
 All waves do this to an extent, but the phenomena is most pronounced when the the wavelength is of the same order as the size of the diffracting object.((The two pictures of diffraction were adapted from [[http://www.saburchill.com/physics/chapters2/0008.html]])) All waves do this to an extent, but the phenomena is most pronounced when the the wavelength is of the same order as the size of the diffracting object.((The two pictures of diffraction were adapted from [[http://www.saburchill.com/physics/chapters2/0008.html]]))
-{{ :howto:hambasics:diffraction_01.jpg }}+{{ howto:hambasics:sections:diffraction_01.jpg }}
  
  
-====== Effect on Communications ======+===== Effect on Communications =====
  
-{{ :howto:hambasics:wave-diffraction-radio.gif}}+{{ howto:hambasics:sections:wave-diffraction-radio.gif}}
 Since radio waves can bend around obstacles that are similar in size to the wavelength of the signal, lower frequencies can band over hills and travel beyond the horizon as ground waves because of diffraction (more on this later).((Image of the radio tower and mountain is from [[https://kistodaynews.com/2017/09/12/scitech-magazine-waves/]])) Since radio waves can bend around obstacles that are similar in size to the wavelength of the signal, lower frequencies can band over hills and travel beyond the horizon as ground waves because of diffraction (more on this later).((Image of the radio tower and mountain is from [[https://kistodaynews.com/2017/09/12/scitech-magazine-waves/]]))
  
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 ====== Interference ====== ====== Interference ======
  
-An important property of waves (radio, sound, water, quantum mechanical!, or otherwise) is that they can interfere with one another.  Here's a //Veritasium// video showing how light going through two slits can interfere: In some places, the waves add up, in other places, they cancel out.  Although not directly about radio waves, we saw in the [[intro#electromagnetic_spectrum |intro]] that light and radio waves are in fact on the same electromagnetic spectrum.+An important property of waves (radio, sound, water, quantum mechanical!, or otherwise) is that they can interfere with one another.  Here's a //Veritasium// video showing how light going through two slits can interfere: In some places, the waves add up, in other places, they cancel out.  Although not directly about radio waves, we saw in the [[intro#electromagnetic_spectrum|intro]] that light and radio waves are in fact on the same electromagnetic spectrum.
  
 {{ youtube>Iuv6hY6zsd0 }} {{ youtube>Iuv6hY6zsd0 }}
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 In terms of radio signals, every time you have more than one source (either because of reflection or because of another radio or antenna), you'll have regions where the signal fades and regions where it increases.  Here's why... In terms of radio signals, every time you have more than one source (either because of reflection or because of another radio or antenna), you'll have regions where the signal fades and regions where it increases.  Here's why...
  
- 
-===== Wave Addition ===== 
- 
-Two waves add up together at every point.  Here, the <fc #4682b4>blue</fc> and <fc #008000>green</fc> waves are generated and add up together to form the <fc #ff0000>red</fc> wave.  You can move the blue and green waves and see the result.  To convince yourself that the red wave is really the sum of the blue and green waves, look at points <fc #4682b4>A</fc>, <fc #008000>B</fc>, and <fc #ff0000>C</fc> You  can move the blue or green waves by sliding their phase (<fc #4682b4>φ</fc> and <fc #008000>Φ</fc>) around.  You'll see that point <fc #ff0000>C</fc> is always the sum of <fc #4682b4>A</fc> and <fc #008000>B</fc>. 
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-Where do the blue and green waves need to be so that... 
-  * the red wave is the biggest? 
-  * the red wave is cancelled out?((Fun fact: This is how [[wp>Active_noise_control |noise cancelling headphones]] work.  The headset has a microphone that picks up the noise, inverts the waves, and plays them back in the ear piece.  The combination of the real life noise and the inverted noise being played in the speaker cancel out (somewhat).)) 
- 
-If you press the play button on the bottom left corner, you'll see the blue wave travel to the right and the green wave travel to the left.  The red wave oscillates up and down but doesn't travel anywhere.  This is called a //standing wave//, which we'll see again later when we discuss SWR. 
  
  
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 The first thing to notice is that when a wave reflects off a surface, it suffers a half-wavelength phase shift.  This means that if the receiver is right next to the "mirror", the signal will cancel out. The first thing to notice is that when a wave reflects off a surface, it suffers a half-wavelength phase shift.  This means that if the receiver is right next to the "mirror", the signal will cancel out.
  
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 If the receiver then moves away from the "mirror", the <fc #008000>reflected signal</fc> has to travel over a longer distance than the <fc #4682b4>direct signal</fc> before reaching the receiver.  This means that phase between the two waves will change, sometimes cancelling each other, sometimes reinforcing each other.  When the path difference (Δ) between the reflected and direct waves is a whole number of the wave length, the two waves cancel each other because of the half-wavelength difference from the reflection.  But when the difference is a multiple of a half wavelength, the two waves add up constructively and the resulting signal is stronger. If the receiver then moves away from the "mirror", the <fc #008000>reflected signal</fc> has to travel over a longer distance than the <fc #4682b4>direct signal</fc> before reaching the receiver.  This means that phase between the two waves will change, sometimes cancelling each other, sometimes reinforcing each other.  When the path difference (Δ) between the reflected and direct waves is a whole number of the wave length, the two waves cancel each other because of the half-wavelength difference from the reflection.  But when the difference is a multiple of a half wavelength, the two waves add up constructively and the resulting signal is stronger.
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   * B-007-008-002 -> B-007-008-005   * B-007-008-002 -> B-007-008-005
  
-[[sections |{{/back.png }}]] [[propagation |{{  /next.png}}]]+[[polarization |{{/back.png }}]] [[propagation |{{  /next.png}}]]
    
  
howto/hambasics/sections/waveinteraction.1574655933.txt.gz · Last modified: 2019/11/24 20:25 by ve7hzf