Page:Electronics Technician - Volume 7 - Antennas and Wave Propagation - NAVEDTRA 14092.pdf/15

 SKIP DISTANCE AND ZONE

Recall from your previous study that a transmitted radio wave separates into two parts, the sky wave and the ground wave. With those two components in mind, we will now briefly discuss skip distance and skip zone.

Skip Distance

Look at the relationship between the sky wave skip distance, skip zone, and ground wave coverage shown in figure 1-8. The skip distance is the distance from the transmitter to the point where the sky wave first returns to the earth. The skip distance depends on the wave's frequency and angle of incidence, and the degree of ionization.

Figure 1-8.—Relationship between skip zone, skip distance, and ground wave.

Skip Zone

The skip zone is a zone of silence between the point where the ground wave is too weak for reception and the point where the sky wave is first returned to earth. The outer limit of the skip zone varies considerably, depending on the operating frequency, the time of day, the season of the year, sunspot activity, and the direction of transmission.

At very-low, low, and medium frequencies, a skip zone is never present. However, in the high-frequency spectrum, a skip zone is m present. As the operating frequency is increased, the skip zone widens to a point where the outer limit of the skip zone might be several thousand miles away. At frequencies above a certain maximum, the outer limit of the skip zone disappears completely, and no F-layer propagation is possible.

Occasionally, the ﬁrst sky wave will return to earth within the range of the ground wave. In this case, severe fading can result from the phase difference between the two waves (the sky wave has a longer path to follow).

REFLECTION

Reflection occurs when radio waves are "bounced" from a flat surface. There are basically two types of reflection that occur in the atmosphere: earth reflection and ionospheric reflection. Figure 1-9 shows two

Figure 1-9.—Phase shift of reflected radio waves. 1-7