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
          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