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howto:hambasics:polarization [2019/11/26 07:00] – [Effect on Communication] ve7hzfhowto:hambasics:sections:polarization [2020/11/08 16:10] (current) va7fi
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 ====== How To Make A Radio Wave ====== ====== How To Make A Radio Wave ======
-Back on the [[intro#hz |Intro Page]], we introduced to the idea of frequency and saw that+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 [...] +<WRAP indent prewrap 80%> 
-> +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. + 
-> +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. 
->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. + 
 +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.  
 +</WRAP>
  
 It's now time to add a few more details to this story.  Here is a basic recipe for making a radio wave: It's now time to add a few more details to this story.  Here is a basic recipe for making a radio wave:
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   - Connect the two middle ends to each side of an alternating current generator.   - Connect the two middle ends to each side of an alternating current generator.
  
 +<WRAP centeralign>
 {{dipole.gif}}{{radiationpatternh.jpg}} {{dipole.gif}}{{radiationpatternh.jpg}}
 +</WRAP>
  
 Voila! The antenna we've made is called a //dipole// Assuming that its length matches the frequency of the current generator (more on this shortly), 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]])) Voila! The antenna we've made is called a //dipole// Assuming that its length matches the frequency of the current generator (more on this shortly), 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]]))
  
 +{{ youtube>JvPBlKLXVyo }}
  
 ====== Horizontal vs Vertical Polarization ====== ====== Horizontal vs Vertical Polarization ======
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 ===== Effect on Communication ===== ===== Effect on Communication =====
-The polarization of an antenna is determined by the orientation of the electric field relative to the Earth's surface.  In practice, polarization is more important for VHF and UHF communication because signals go directly from the transmitting station to the receiving one.+The polarization of an antenna is determined by the orientation of the electric field relative to the Earth's surface.  So a horizontal antenna will produce a radio wave that has an horizontal electric field.  The magnetic field part of the wave is always perpendicular to both the electric field and the direction of propagation, so in this case, it would be vertical.
  
-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.+In practice, polarization is more important for VHF and UHF communication because signals travel more or less directly from the transmitting antenna to the receiving one.  For skywave HF communications, however, 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 because you never know what polarization your signal will end up having once it arrives at its destination.
  
  
 ====== Wavelength and Antenna Length ====== ====== Wavelength and Antenna Length ======
  
-As we saw previously, the wavelength (λ) in metres of the wave is dictated by the frequency (fin MHz and the speed of light:+As we saw previously, the wavelength (λ) in metres of the wave is dictated by the frequency \$f\$ in MHz and the speed of light:
  
-<latex> $$ \lambda = \frac{300}{f} \qquad \text{or} \qquad f = \frac{300}{\lambda}$$ </latex>+<WRAP centeralign> 
 +\$$ \lambda = \frac{300}{f} \qquad \text{or} \qquad f = \frac{300}{\lambda}\$$ 
 +</WRAP>
  
-This explains the Band name in the table on the [[intro#full_frequency_list |intro page]].  For example, the frequency range of the 2m band is 144 Mhz -- 148 Mhz.  If we calculate the wavelength of 146 Mhz, we get: 300 ÷ 146 = 2.05 m+This explains the Band name in the table on the [[intro#full_frequency_list|intro page]].  For example, the frequency range of the 2m band is 144 Mhz -- 148 Mhz.  If we calculate the wavelength of 146 Mhz, we get: 300 ÷ 146 = 2.05 m
  
 Now it turns out that the size of the antenna is very closely related to the wavelength of the signal we wish to transmit or receive.  For a dipole, the total length is roughly half of the wavelength. So an antenna for the 2m band should be roughly 1m long, while an antenna for the 160m band would be roughly 80m long! Now it turns out that the size of the antenna is very closely related to the wavelength of the signal we wish to transmit or receive.  For a dipole, the total length is roughly half of the wavelength. So an antenna for the 2m band should be roughly 1m long, while an antenna for the 160m band would be roughly 80m long!
 +
 +We say "roughly" here because as we'll see later, the speed of electricity in a conductor is a bit less than the speed of light, so the antenna length end up a being roughly 5% shorter than calculated here.
  
 ===== Cellphone Antenna ===== ===== Cellphone Antenna =====
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 ====== Questions ====== ====== Questions ======
  
-[[sections |{{/back.png }}]] [[waveinteraction |{{  /next.png}}]]+[[empos |{{/back.png }}]] [[waveinteraction |{{  /next.png}}]]
  
howto/hambasics/sections/polarization.1574780414.txt.gz · Last modified: 2019/11/26 07:00 by ve7hzf