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howto:hambasics:sections:polarization

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How To Make A Radio Wave

Back on the 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:

  1. Get a length of conducting wire and lay it in a straight line.
  2. Cut it in half right in the middle and bend both ends at right angle.
  3. Connect the two middle ends to each side of an alternating current generator.

radiationpatternh.jpg

Voila! The antenna we've made is called a dipole and 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.1) As electrons move up and down the length of the wires, they create varying electric and magnetic fields that couple together according to Maxwell's Equations and propagate outward in a doughnut shape.2)

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.

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:

<latex> $$ \lambda = \frac{300}{f} \qquad \text{or} \qquad f = \frac{300}{\lambda}$$ </latex>

This explains the Band name in the table on the 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!

Cellphone Antenna

LTE band 1 operates roughly at 2100 MHz.3) How long would a dipole have to be?

  • 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.
howto/hambasics/sections/polarization.1574740281.txt.gz · Last modified: 2019/11/25 19:51 by ve7hzf