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howto:hambasics:waveinteraction [2019/11/20 17:53] – [Effect on Communication] ve7hzfhowto:hambasics:sections:waveinteraction [2021/01/03 08:08] (current) va7fi
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-<box orange |**Under Construction**> 
-VE7HZF is editing this section, please do not edit it until this notice is taken down. 
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 ====== Wave Interaction ====== ====== Wave Interaction ======
 When an electromagnetic wave (radio, light, etc) hits a surface, it can do one or a mix of six things((Picture from [[http://www.mrwaynesclass.com/lightOptics/reading/index02.html]])): When an electromagnetic wave (radio, light, etc) hits a surface, it can do one or a mix of six things((Picture from [[http://www.mrwaynesclass.com/lightOptics/reading/index02.html]])):
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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>
  
 +====== Scattering ======
  
-====== Polarization ======+{{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.
  
-===== How To Make A Radio Wave ===== +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:
-Back on the [[intro#hz |Intro Page]], we introduced to the idea of frequency and saw that+
  
->A Hertz (Hz) is measure of how fast something vibrates [...] +The first picture shows laser beam shinning at the wall
-+{{ howto:hambasics:sections:laser1.jpg }}
->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: +In the second picture, water is sprayed into the path of the laser beam
-  - Get a length of conducting wire and lay it in a straight line. +{{ howto:hambasics:sections:laser2.jpg }}
-  - 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}}+{{  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. 
 +  * Now look at an object that the light bulb illuminates. 
 +  * Now imagine a straight line between the light bulb and that object. 
 +Just as with the laser in the first picture, you don't see any light along //that// line.  If you did, the entire room would be glowing white from all the different light rays that the light bulb emits.  In fact, you can see this on a foggy day when it's also sunny.  Or on a foggy evening when you're driving with the high beam on. 
  
 +===== Effect on Communications =====
 +We'll talk about the effects of scattering on communications in more detail later because we need to see a few more basics first.  But for now, we'll just say that a radio signal received through scattering will generally be weak, and suffer from rapid flutter or hollow sounding distortion.
  
-Voila! Assuming that the length of the antenna (the two pieces of wiresmatch the frequency of the current generator (more of this later), and that the antenna is high enough above the groundyou've created 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 doughnut shape.((Picture modified from [[wp>Dipole_antenna |Wikipedia Dipole Antenna]]))+It'll be weak because only a small portion of the energy reaches you (think of how much weaker the scattered light from the laser beam is compared to the what reaches the wall directly).  And it'll be distorted because your antenna will be receiving the signal from multiple directions (radio wave-paths) at once (think of how you can see the green laser scattered by the mist as an extended lineinstead of single point).  As we'll next, when a signal splits and takes different path (of different lengths), they recombined with sort of "echo" can cause distortion 
  
  
-===== Horizontal vs Vertical Polarization =====+====== Diffraction ======
  
-{{  polarization.jpg}} +{{howto:hambasics:sections:diffraction.png }} 
-Here'the critical part though:  In the same way that an alternating current through an antenna creates 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**.+Diffraction is the bending of waves around the corners of an obstacle or through an aperture.  The diffracting object or aperture effectively becomes secondary source of the propagating wave, which in turns can interact with the main wave or other diffracted waves.
  
-This "direction" is called polarization.+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:sections:diffraction_01.jpg }}
  
  
-===== Effect on Communication ===== +===== Effect on Communications =====
-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.+
  
 +{{ 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/]]))
  
 +And as we'll see in the next section, diffracted waves can also recombine with the original signal and create multi-path interference.
  
-====== Scattering ======+====== Interference ======
  
-{{:howto:hambasics:scattering.png?85  }} +An important property of waves (radiosoundwaterquantum mechanical!, or otherwise) is that they can interfere with one another.  Here'//Veritasium// video showing how light going through two slits can interfere: In some places, the waves add up, in other placesthey 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.
-Scattering occurs when an EM wave hits a bunch of "small particles"((The "small particles" can be single atomsmoleculesdustor 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 wavea meteor is small, but to a laser beam (≈500nm), a dust particle is very big.)) that in turn re-radiate the wave in all direction.  Here are few examples((The laser pictures were taken by PatrickVE7HZF 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.  In the second picture, water is sprayed into the path of the laser beam. +{{ youtube>Iuv6hY6zsd0 }}
-{{ :howto:hambasics:laser1.jpg }} +
-{{ :howto:hambasics:laser2.jpg }}+
  
-The reason the beam is invisible in the first picture is that all the light from the laser travels toward the wall (an 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 for a regular light bulb also does this to some extent.  What I mean is this: 
-  * Look at the light bulb in the room you're in. 
-  * Now look at an object that the light bulb illuminates. 
-  * Now imagine a straight line between the light bulb and that object. 
-Just as with the laser in the first picture, you don't see any light along //that// line.  If you did, the entire room would be glowing white from all the different light rays that the light bulb emits.  In fact, you can see this on a foggy day when it's also sunny.  Or on a foggy evening when you're driving with the high beam on. {{ :howto:hambasics:lightscattering.jpg  }} 
  
-Now back to radio waves... +Here's a computer animation from [[wp>Wave_interference#Mechanisms |Wikipedia]] showing the same principle: 
-====== Troposphere ======+{{ two_sources_interference.gif }} 
 + 
 +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... 
  
-On the previous page, we discussed how the Ionosphere (the region of our atmosphere between 50km and 400km altitude) can, reflect and refract radio waves, let them pass straight through, or absorbed them completely mostly due to the sun's ionization of the gas in these layers. 
  
-Here we discuss how the [[wp>Troposphere |troposphere]] (the lowest region of our atmosphere below 20km altitude) can also affect radio waves.+===== Wave Reflection and Multipath =====
  
 +More commonly, radio waves often suffer from //multipath// interference caused by some sort of reflection, refraction, or diffraction (from mountains, the ground, buildings, the ionosphere, ...)  This leads to fading (//QSB//) as either the transmitter, the receiver, or the reflective surface moves.
  
 +This next animation shows the <fc #4682b4>direct wave</fc> going from the transmitter to the receiver, as well as a <fc #008000>wave reflected</fc> by the horizontal axis.
  
 +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.
  
-====== Next ======+{{ggb>/howto/hambasics/sections/multipath.ggb 700,300}}
  
 +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.
  
-  * Tropospheric bending on 2m +In this example, if the receiver moves straight up, the signals will interfere destructively every 5 wavelength-units or so.  This means that on the 2m band, the signal will fade every 10 meters or so.  This is why the signal strength of a mobile station sometimes goes up and down rapidly as the car moves, which we call [[wp>Picket-fencing |picket fencing]].
-  * Ducting +
-  * Sporatic E +
-  * Auroral propagation +
-  * Scatter +
-  * Meteor Scatter+
  
-See [[wp>Radio_propagation]]+For more details, see the [[wp>Fresnel_zone |Fresnel Zone]] Wikipedia article.
  
-Questions: 
-   * B-007-007-002 -> B-007-008-011 
  
 ====== Questions ====== ====== Questions ======
-  * B-007-004-007 +  * B-007-004-003 -> B-007-004-007 
-  * B-007-004-010+  * B-007-004-008 -> B-007-004-011 
 +  * B-007-008-002 -> B-007-008-005
  
-[[sections |{{/back.png }}]] [[propagation |{{  /next.png}}]]+[[polarization |{{/back.png }}]] [[propagation |{{  /next.png}}]]
    
  
howto/hambasics/sections/waveinteraction.1574301231.txt.gz · Last modified: 2019/11/20 17:53 by ve7hzf