Antennas

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

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 fieldCreates changing magnetic field Produces EM wave.Produces EM wave.  This is an antenna.

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.  At an angle the effective field is reduced.

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 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 = f  Solve for the wavelength. Speed of light Frequency f = 9.39 x 10 7 /s = c/f = 3.19 m  Antennas should be slightly shorter than size to work well. next