Appendix II. PROPAGATION OF RADIO WAVES

TM 11-5820-919-12

APPENDIX II

PROPAGATION OF RADIO WAVES

ionosphere makes it possible to communicate over great

A2-1. GENERAL.

distances using hf radio transmissions.

A2-2.

This appendix provides information on

A2-9. MAXIMUM USABLE FREQUENCY. At any given

propagation of radio wave, in the high frequency hf band

time for a predetermined path length (skip distance there

used by' the radio set. The data and illustrations are

will exist a .Maximum Usable, Frequency,' UF. This

based on average propagation characteristics of various

MILF is the highest frequency, radio wane that all he

hf band segments and indicate relative effectiveness

reflected back to earth in a specified geographical region

during the day and night The are to he used only as a

for that path length. The MUF is a predictable nut,

general guide.

Also discussed are two situation

number and is a available in the form of propagation

examples for optimizing communications.

prediction charts. A sample of a propagation prediction

chart is shown In figure A2-3. The NMUF is highest at

A2-3. RADIO WAVES.

noon or earl, afternoon and drops to its lowest in the

early morning hours. The MUF is lower on short paths

A2-4. Radio waves in the 2 thru 30 MHz region of the

and higher on long paths.

electromagnetic spectrum are propagated in two ways:

the ground wave and the sky wave.

A2-10. FREQUENCY OF OPTIMUM TRANSMISSION

(FOT). If the .MUF is known. the best frequency

A2-5. GROUND WAVE (Figure A2-1). There are two

selection would be one that is sufficiently close to the

types of ground waves which exist in various magnitudes

MUF to be reflected but not so close that it is affected by

depending on the frequency', type of antenna and terrain

minor changes in ionospheric density. This is around 85

conditions.

percent of the MUF. (Example: if MUF is 10 MHz,

optimum frequency will be 8.5 x 10 MHz or 8.5 MHz.)

A2-6. Surface Wave. This electromagnetic wave

This frequency is called the Frequency of Optimum

travels directly along the earth's surface and is

Transmission (FOT) and is commonly referred to as the

dependent on the ground conductivity. Because ground

Optimum Traffic Frequency.

conductivity is generally quite poor, unless propagated

over water, this wave is rapidly attenuated.

This

A2-11. MULTI-SKIP PHENOMENON. There will be

attenuation is particularly pronounced at frequencies

instances when the skywave is reflected between the

greater than 3 MHz making it unreliable for low-power

ionosphere and the earth surface several times. This

communication networks beyond about 10 miles.

multi-skip phenomenon occurs quite frequently making

Although attenuation is greater as frequency increases,

global communications possible in the hf band. In

better efficiency of the whip antenna and a drop in

addition to multi-skip there also exists a multipath

atmospheric noise favor the use of higher frequencies for

propagation characteristic. If this occurs, two waves

links dependent on surface wave.

that took different paths will reach the receiver

simultaneously. Depending on their relative phases, the

A2-7. Direct Wave. This is the "line-of-sight" wave

signal strength will be enhanced or reduced. If the two

which exists at all frequencies. The range of this wave is

waves are of equal amplitude and phase shifted 180

limited by the horizon. The direct wave is utilized most

degrees, total cancellation will result and the received

frequently for short range communication.

signal will fade out. Operating near the FOT will

minimize multipath degradation.

A2-8 SKYWAVE (Figure A2-2). Skywaves (ionospheric

waves) are basically direct waves that have been

A2-12. HF BAND SEGMENTS. The following is a

reflected (skipped) back to earth by the ionosphere. The

breakdown of the hf band and a general description of

ionosphere exists at altitudes between 30 and 300 miles

the behavior of each segment. There will be exceptions

and consists of several layers of ionized gases. These

but the descriptions will hold true in most cases. Figures

gases are ionized by several factors, the primary of

A2-4 and A?-5 show relative

which is ultraviolet radiation; the greater the level of

ionization the more reflective the ionosphere will be. The

ultraviolet radiation level is directly proportional to the

amount of sunspot activity and is predictable with a high

degree of accuracy.

This reflective power of the

A2-1