Radio Frequencies. Oscillator Feedback loop Oscillator As the output of the amplifier is fed to the...
Transcript of Radio Frequencies. Oscillator Feedback loop Oscillator As the output of the amplifier is fed to the...
Radio Frequencies
OscillatorFeedback loop
Oscillator
• As the output of the amplifier is fed to the input, feedback or oscillation occurs
Tuned Oscillator
Oscillator
• When properly tuned to a high enough frequency, the oscillator will produced radio frequencies
• Frequencies begin to take on the characteristics of radio frequencies (RF) at about 30,000 KHz
• RF can travel great distances, and can be modulated to carry information (audio & video)
RF
• Antennae are conductive rods that will easily radiate RF
• When the output of the oscillator is connected to an antenna, the electrons in the antenna begin to vibrate or oscillate at RF frequencies
• Waves of electromagnetic radiation will emanate from the antenna at those RF frequencies (+30KHz)
RF spectrum
• Very Low Frequencies – 3 – 30 KHz• Low Frequencies 30 – 300 KHz• Medium frequencies 300 KHz – 3 MHz• High frequencies 3 – 30 MHz• Very High frequencies 30 MHz – 300 MHz• Ultra High frequencies 300 MHz – 3 GHz• Super High frequencies 3 – 30 GHz• Extremely High frequencies 30 – 300 GHz
RF and beyond• As particles continue to accelerate, creating
waves of decreasing wavelengths, energy takes on other characteristics
• Infrared• Visible light – red, orange, yellow, green, blue,
indigo, and violet• Ultraviolet• X-rays• Gamma rays• Cosmic rays
Spectrum management
• AM radio (MF) – 535 – 1705 KHz• Channels are 10 KHz• Radio receivers tune to the center frequency
in the channel or carrier
565535 545 555
540 550 560
Spectrum management
88.0
88.1
• FM radio (VHF) – 88 – 108 MHz• Channels are 200 KHz• Radio receivers tune to the center frequency
in the channel or carrier
88.2
+/- 75 KHz deviation
Spectrum management
82.0
83.25 MHz video carrier
• TV (VHF and UHF)• Channels are 6 MHz• TV receivers tune to the center frequency in
the channel or video carrier• In this example: Channel 6
88.0(1.25 MHz above lower edge of channel) (Audio 4.5 MHz
+/- 25 KHz)
(30 KHz above lower edge of channel)
Spectrum management
• http://frrl.files.wordpress.com/2010/11/frequency-allo-chart.jpg
Calculating wavelength
Velocity Frequency
WL =
Velocity = speed of light = 300,000,000 meters per second
Calculating wavelength
300,000,000 Frequency
WL =
Calculating wavelength
300,000,000 102,500,000
WL =
WHIZ-FM – 102.5 MHz
Calculating wavelength
300,000,000 102,500,000
2.93 =
Radio wave is 2.93 meters from crest to crest
Calculating wavelength
300,000,000 102,500,000
2.93 =
Calculating wavelength
300,000,000 1,240,000
WL =
WHIZ-AM – 1240 KHz
Calculating wavelength
300,000,000 1,240,000
241.93 =
Radio wave is 241.93 meters from crest to crest
Calculating wavelength
• Higher frequencies = shorter wavelengths• In the highest bands of the usable spectrum,
wavelength are measured in nanometers or angstroms
• Wavelengths in the “microwave” bands are so short that atmospheric moisture affects transmission
Calculating wavelength
• These calculations are used to determine ideal antenna length
• In FM and TV, antennae use half-wave dipole construction
• The antenna is one-half the length of the wave, and the pole is cut in half
Half wave dipole
“Stacking” the elements forces waves toward horizon. This adds gain to the antenna.
Quarter-wave vertical
• In AM, the longer wavelengths dictate the use of quarter-wave vertical antennae
• The tower itself becomes the antenna• AM also uses a ground array to propagate the
ground waves
How waves behave
• HF and above (FM, TV, satellite, etc.) travel in direct waves, or line-of-sight
• Direct waves will not “bend” or pass through solid objects
• LF and MF frequencies travel in sky waves, which bounce off the ionosphere
• LF and MF frequencies travel in ground waves, which follow the curvature of the earth
Antenna location
• Best location for an FM or TV antenna is on the highest unobstructed hill, building, tower, peak
• Best location for an AM antenna is low marshy location
Antenna schematics
FM antenna AM antenna
Ionosphere
• A layer of the atmosphere where hydrogen atoms become “ionized”
• Ionization occurs when the hydrogen atoms become “charged” because they give up electrons
• Heating of the ionosphere by the sun causes the ionization
• The will determine the behavior of sky waves
Sky waves
• May be absorbed during the daylight hours• May pass through during the daylight hours• May be reflected at night when the
ionosphere cools
Sunspots
• Affect terrestrial communication, telecommunications, broadcasting, computers, other electronics
• Occur in 11 year cycles• Activity builds and subsides slowly
1990 2000 2010
Modulation