Radio Frequencies
Oscillator
Feedback 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
540
535
560
550
545
555
565
Spectrum management
• FM radio (VHF) – 88 – 108 MHz
• Channels are 200 KHz
• Radio receivers tune to the center frequency
in the channel or carrier
88.1
88.2
88.0
+/- 75 KHz deviation
Spectrum management
• 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
83.25 MHz video carrier
82.0
(1.25 MHz above lower
edge of channel)
(30 KHz above lower
edge of channel)
(Audio 4.5 MHz
+/- 25 KHz)
88.0
Spectrum management
• http://frrl.files.wordpress.com/2010/11/frequ
ency-allo-chart.jpg
Calculating wavelength
Velocity
WL =
Frequency
Velocity = speed of light = 300,000,000 meters per second
Calculating wavelength
300,000,000
WL =
Frequency
Calculating wavelength
300,000,000
WL =
102,500,000
WHIZ-FM – 102.5 MHz
Calculating wavelength
300,000,000
2.93 =
102,500,000
Radio wave is 2.93 meters from crest to crest
Calculating wavelength
300,000,000
2.93 =
102,500,000
Calculating wavelength
300,000,000
WL =
1,240,000
WHIZ-AM – 1240 KHz
Calculating wavelength
300,000,000
241.93 =
1,240,000
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