howto:hambasics:sections:polarization
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+ | ====== How To Make A Radio Wave ====== | ||
+ | Back on the [[intro# | ||
+ | |||
+ | <WRAP indent prewrap 80%> | ||
+ | A Hertz (Hz) is a measure of how fast something vibrates [...] | ||
+ | |||
+ | Just seeing “Hz” doesn' | ||
+ | |||
+ | 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. | ||
+ | </ | ||
+ | |||
+ | It's now time to add a few more details to this story. | ||
+ | - 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. | ||
+ | |||
+ | <WRAP centeralign> | ||
+ | {{dipole.gif}}{{radiationpatternh.jpg}} | ||
+ | </ | ||
+ | |||
+ | Voila! The antenna we've made is called a // | ||
+ | |||
+ | {{ youtube> | ||
+ | |||
+ | ====== Horizontal vs Vertical Polarization ====== | ||
+ | |||
+ | {{ polarization.jpg}} | ||
+ | Here's the critical part though: | ||
+ | |||
+ | This " | ||
+ | |||
+ | |||
+ | ===== Effect on Communication ===== | ||
+ | The polarization of an antenna is determined by the orientation of the electric field relative to the Earth' | ||
+ | |||
+ | 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, | ||
+ | |||
+ | |||
+ | ====== Wavelength and Antenna Length ====== | ||
+ | |||
+ | 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: | ||
+ | |||
+ | <WRAP centeralign> | ||
+ | \$$ \lambda = \frac{300}{f} \qquad \text{or} \qquad f = \frac{300}{\lambda}\$$ | ||
+ | </ | ||
+ | |||
+ | This explains the Band name in the table on the [[intro# | ||
+ | |||
+ | 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. | ||
+ | |||
+ | We say " | ||
+ | |||
+ | ===== Cellphone Antenna ===== | ||
+ | LTE band 1 operates roughly at 2100 MHz.(([[wp> | ||
+ | |||
+ | * The wave length is: 300 ÷ 2100 = 0.14m = 14cm | ||
+ | * Half of the wave length would be 7cm, which is small enough to fit in the cellphone. | ||
+ | |||
+ | |||
+ | ====== Questions ====== | ||
+ | |||
+ | [[empos |{{/ | ||
howto/hambasics/sections/polarization.1574300066.txt.gz · Last modified: 2019/11/20 17:34 by ve7hzf