Page:Electronics Technician - Volume 7 - Antennas and Wave Propagation - NAVEDTRA 14092.pdf/21

 atmospheric levels in the ionosphere account for a large part of these energy losses. There are two other types of losses that also significantly affect propagation. These losses are known as ground reflection losses and freespace loss. The combined effect of absorption ground reﬂection loss, and freespace loss account for most of the losses of radio transmissions propagated in the ionosphere.

GROUND REFLECTION LOSS

When propagation is accomplished via multihop refraction, rf energy is lost each time the radio wave is reflected from the earth's surface. The amount of energy lost depends on the frequency of the wave, the angle of incidence, ground irregularities, and the electrical conductivity of the point of reflection.

FREESPACE LOSS

Normally, the major loss of energy is because of the spreading out of the wavefront as it travels from the transmitter. As distance increases, the area of the wavefront spreads out, much like the beam of a flashlight. This means the amount of energy contained within any unit of area on the wavefront decreases as distance increases. By the time the energy arrives at the receiving antenna, the wavefront is so spread out that the receiving antenna extends into only a small portion of the wavefront. This is illustrated in figure 1-15.

FREQUENCY SELECTION

You must have a thorough knowledge of radio-wave propagation to exercise good judgment when selecting transmitting and receiving antennas and operating frequencies. Selecting a usable operating frequency within your given allocations and availability is of prime importance to maintaining reliable communications.

For successful communication between any two specified locations at any given time of the day, there is a maximum frequency, a lowest frequency and an optimum frequency that can be used.

Figure 1-15.—Freespace loss principle.

MAXIMUM USABLE FREQUENCY

The higher the frequency of a radio wave, the lower the rate of refraction by the ionosphere. Therefore, for a given angle of incidence and time of day, there is a maximum frequency that can be used for communications between two given locations. This frequency is known as the MAXIMUM USABLE FREQUENCY (muf).

Waves at frequencies above the muf are normally refracted so slowly that they return to earth beyond the desired location or pass on through the ionosphere and are lost. Variations in the ionosphere that can raise or lower a predetermined muf may occur at anytime. his is especially true for the highly variable F2 layer.

LOWEST USABLE FREQUENCY

Just as there is a muf that can be used for communications between two points, there is also a minimum operating frequency that can be used known as the LOWEST USABLE FREQUENCY (luf). As the frequency of a radio wave is lowered, the rate of refraction increases. So a wave whose frequency is below the established luf is refracted back to earth at a shorter distance than desired, as shown in figure 1-16. 1-13