T/G Layout 1

Radio frequency signals are electromagnetic waves that generally include frequencies

above 10 KHz. The waves are usually generated by periodic currents of electric charges

in wires, electron beams, or antenna surfaces. Radio and TV transmissions (collectively

referred to as radio signals) work on a line-of-sight basis. Signal transmission and recep-

tion is dependent upon an unobstructed straight line that connects transmitters and

receivers. Radio frequencies represent the different channels or stations that are broad-

cast—higher channels correspond to higher radio frequencies.

The waves most often received from satellites are in the range of 30 MHz up to 30

GHz. Those frequencies include the electromagnetic spectrum from Very High

Frequency (VHF) to Super High Frequency (SHF).

U.S. polar-orbiting satellites broadcast Automatic Picture Transmission (APT) at 137

MHz, and GOES geostationary satellite Weather Facsimile (WEFAX) at 1691 MHz (see

pages 92, 96).

The tracking and ranging capabilities of radio systems were known as early as 1889,

when Heinrich Hertz showed that solid objects reflect radio waves. Bouncing signals

off the ionosphere (upper atmosphere) increased the signal area, but the erratic atmos-

phere relayed signals of varying clarity. The radio signal area was also expanded by

using series of transmitting towers, 31 to 50 miles apart. Oceans, deserts, and lack of

towers all limited the relay.

In the early 1950s, U.S. army engineers unsuccessfully tried bouncing radio signals off

the moon (resulting signals were diffused and unfocused). In 1960 NASA launched a

satellite named Echo I to reflect radio signals. Since that first NASA launch, satellites

have graduated from being passive reflectors to actively relaying signals and carrying

sensors that obtain and relay additional information.

figure 44.

TH E RA D I O FR E Q U E N C Y SP E C T R U M

VLF LF MF HF VHF UHF SHF EHF IR Visible Ultraviolet

Ligh t

100kHz 3MHz 30MHz 300 MHz 1000 MHz 30 GHz 300 GHz 10^14 Hz 10^16 Hz

10 kHz 1 MHz (1 GHz)

APT 137-

138 MHz

WEFAX

1691 MHz

162 MHz

NOAA

Weather

Radio

HRPT

1698 MHz

AM

radio

short-

wave

FM

radio

two-way

radio

satellites &

microwave

hertz (Hz)

The international unit of frequency equal to

one cycle per second. Radio frequencies are

usually expressed in

kilohertz (kHz) = 1,000 cycle per second, or

megahertz (MHz) = 1,000,000 cycles

per second, or

gigahertz (GHz) = 1,000 MHz