AVIATION RADIO FUNDAMENTALS
By Major Rob Robinette
Electromagnetic Spectrum (diagram)
Radio waves are similar to visible light, light waves are just higher in
frequency
Radio waves are electromagnetic energy
Electric lines of force and
magnetic lines of force, 90 degree angle to one another
Electric
lines of force define horizontal or vertical polarization
The C-141 "stinger" HF antenna is horizontally polarized. The VHF
and UHF antennas are vertically polarized
Radio waves travel at the speed of light
186,000 miles per second
300 million meters per second
Frequency measured in Hertz –
cycles per second
Kilohertz
(kHz) thousands of cycles per second
Megahertz
(MHz) millions of cycles per second
Gigahertz
(GHz) billions of cycles per second
Wavelength – distance from wave peak to
peak (example: 17 meter shortwave)
Sound 10
– 20,000 Hertz
Very Low
Frequency (VLF) - Submarine communication
Low
Frequency (LF) – NDB 190 to 1750 kHz (1.75 MHz)
Medium
Frequency (MF) – AM radio broadcast (1210 kHz, 1.21 MHz)
High
Frequency (HF) – 3 MHz to 30 MHz (short-wave)
Very High
Frequency (VHF) – 30 MHz to 300 MHz (FM broadcast 93.7 MHz)
Ultra High
Frequency (UHF) – 300 MHz to 3 GHz (cell phone 870 MHz, GPS 1.5 GHz)
Super High
Frequency (SHF, a.k.a. microwaves) - > 1 GHz (satellite 4.4 GHz and up)
ADF Radio
Automatic Direction Finder
Low and Medium Frequency
AM and CW (continuous wave or carrier)
190 to 1750 kHz (1.75 MHz)
- Non-Directional Beacons (NDB) normally 190 - 535 kHz CW
Can tune in AM broadcast stations
VHF Radio
AM 108 - 138 MHz
Navigation 108 - 118 MHz, voice 118 - 136 MHz
DME 962 - 1213 MHz (1.213GHz); glide slope 329.15 - 335 MHz; outer, middle, inner
markers 75 MHz
Line of sight
Guard 121.5 MHz
Two antennas #1 on bottom, #2 on top (usually better for ground ops)
UHF Radios
AM 225 - 400 MHz
Line of sight
Bounces off objects better than VHF
Two antennas #1 on bottom, #2 on top (usually better for ground ops)
Guard 243 MHz (VHF guard times 2)
HF Radios 2 - 30 MHz
400 watts minimum
Range 0 – 13,000 miles (anywhere on Earth)
Antenna and coupler (tuners) in T-tail
Coupler is pressurized to 7 psi +-1 to prevent
arcing
Has dual tuners, dual
receive but can transmit on one at a time
Max tune time 10
seconds (change freq and try again or use another freq)
Transmitter Duty Cycle: five minutes transmit, five minutes off
Single Side Band (SSB), upper side lobe
Can transmit AM but no reason to
Use AM to listen to broadcast short-wave stations
(BBC, VOA)
Most HF noise (static) is caused by lightning
Ground wave (0-90 miles, lower freq=more distance)
Dead zone between ground wave and reflected wave
(diagram)
Sometimes weak
scatter signals can be heard
Propagation (bending & bouncing of radio waves, a.k.a. skip)
Skywave radio wave bounces off E and F layers of
ionosphere
Can have multiple
bounces
Ionosphere – free electrons & ions form a
charged layer
D Layer – absorbs
radio waves
E Layer – 70 miles
high, highest ionization at noon, unreliable
F1 Layer – 100
miles high, some reflection
F2 Layer – 200
miles up, main cause of reflection
Check WWV time signals for propagation
2500 5000
10,000 15,000 and 20,000 kHz (AM)
HF radio waves reflecting off the ionosphere, from The ARRL Antenna Book.
Refraction and reflection of HF radio waves, from The ARRL Antenna Book.
Long Path – the long way around
the earth (causes echo)
Maximum Usable Frequency (MUF)
Goes up
and down with sun
Ionization highest at noon,
lowest at midnight
Freqs
close to MUF are best
Higher freqs have too much energy, break through ion layer
Lower freqs get absorbed by D layer
Sunspots
More sunspots = more ionization and better HF propagation
Can have too much of a good thing
Solar flare can stop HF com and
damage satelites
Runs in 11 year cycles, Summer 2000 will be max, 2006 will be minimum
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