Antennas Free Charge A free charge has a

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Antennas

Antennas

Free Charge ] A free charge has a constant electric field and no magnetic

Free Charge ] A free charge has a constant electric field and no magnetic field. ] A charge at constant velocity has both fields. • Fields are constant so no wave forms. ] No waves are produced. • No radiation B field surrounds electric field, although there is no “current” flowing here

Accelerating Charge ] An AC power source creates an oscillating current. ] The charges

Accelerating Charge ] An AC power source creates an oscillating current. ] The charges in the conductor accelerate. ] This produces a changing electric field. • Creates changing magnetic field • Produces EM wave. ] This is an antenna.

Receiving Waves ] A conductor in an electric field will have a current. •

Receiving Waves ] A conductor in an electric field will have a current. • Charges accelerate ] An oscillating field generates an oscillating current. ] Antennas must be of a length appropiate to detect the oscillation.

Angled Antenna ] The conductor works best when parallel with the electric field. ]

Angled Antenna ] The conductor works best when parallel with the electric field. ] At an angle the effective field is reduced.

Radio Waves ] Radio waves are a type of electromagnetic wave. ] Broadcast antennas

Radio Waves ] Radio waves are a type of electromagnetic wave. ] Broadcast antennas put out electric and magnetic fields.

Wavelength ] A radio station broadcasts FM signals at 93. 9 MHz. ] Find

Wavelength ] A radio station broadcasts FM signals at 93. 9 MHz. ] Find the wavelength of the electromagnetic wave. ] First assume that the speed is the speed in a vacuum c. ] The speed of a wave is related to the wavelength and frequency. • v = lf ] Solve for the wavelength. • Speed of light • Frequency f = 9. 39 x 107 /s • l = c/f • l = 3. 19 m ] Antennas should be slightly shorter than size to work well. next