Table 2-1. Propagation Characteristics of the RF Spectrum

TM 11-490-5

beyond the line-of-sight distance.

Skywaves and

reflections at abrupt changes in the troposphere). Along

the surface of the earth, the maximum distance that a

tropospheric scatter waves undergo deviation in

direction along their propagation paths as a result of

ground wave is effective decreases with increasing

frequency of the wave.

Ground waves undergo

diffraction, reflection, and refraction as they encounter

variations in the media through which they travel. All of

deviation from normal, straight-line travel by contact

the above factors, in addition to absorption of the energy

with the troposphere, refraction, reflection from path

of the radio waves, are responsible for propagation path

discontinuities, diffraction, and propagation along the

losses encountered by radiated signals and the

curved surface of the earth because of the earth's

"freakish" interference which results from distant, low-

conductivity. It is for these reasons that ground wave

power transmitters.

propagation extends

Table 2-1. Propagation Characteristics of the RF Spectrum

Frequency

Band

Propagation characteristics

Typical uses

Below

3 kHz

ELF

Primarily ground waves; low attenuation, reliable, Very long distance point-to-point (greater

daytime absorption of skywaves greater than at

than 1000 nautical miles)

night

3-30

VLF

Same as ELF, except attenuation equally low,

Very long distance point-to-point. Tactical

kHz

day or night; reliable

broadcast communications

30-300

Same as ELF

Long and medium range (50 to 1000

kHz

nautical miles point-to-point com-

munications, marine, navaids

300-3000

Ground waves, but also some ionospheric

Broadcasting, marine communications,

kHz

skywaves; attenuation of skywaves low at night

navaids, harbor telephone, medium and

short range

and high in daytime. Subject to ground-

skywave interference for distances less than

500 nautical miles.

3-30

VHF

Primarily skywave transmission over great

Moderate and long distance communications

MHz

distances, depending on ionosphere. Varies,

of all types

greatly with time of day, season, frequency,

and portion of solar sunspot activity cycle.

Subject to ground-skywave interference at

short distances

30-300

VHF

Primarily line-of-sight transmission. Sporadic

Short distance, line-of-sight com-

MHz

ionosphere effects occur during high portions of

munications, television, FM broadcasting,

solar sunspot cycle

navaids, radar, troposcatter com-

munications, aero-navaids

300-3000

UHF

Substantially straight-line propagation analogous

Short-distance communications, radar,

MHz

to that of light waves. Unaffected by

television, aero-navaids, point-to-point

ionosphere

relays, troposcatter communications

3-30

SHF

Same as UHF

Short-distance communications, radar,

GHz

point-to-point relay systems, navaids,

satellite relays, tropospheric scatter

communications

Bands

ELF =Extremely low frequency

VLF =Very-low frequency

LF =Low frequency

MF = Medium frequency

HF= High frequency

VHF= Very-high frequency

UHF= Ultra-high frequency

SHF= Super-high frequency

These regions are believed to be generated by

(2) The ionosphere is a region of the earth's

ultraviolet light and "soft" X-rays from the sun. Radio

upper atmosphere which lies between about 40 to 50 km

waves propagate through, or are reflected and refracted

above the earth out to about 12,000 km above the earth.

by, these regions, depending on the frequency of the

There are within this space three ionized layers or

waves and the angle of incidence of the wave front

"regions," referred to as the D, E, and F regions,

related to the layers. The regions are not uniform

occurring at approximate heights of 50-90 km, 90-130

around the earth, but, since they are

km, and above 130 km, respectively.

2-11