Chapter 7 
PROPAGATION ASPECTS OF EQUIPMENT OPERATION 
GENERAL PROBLEM 
Introduction 
F@ A STANDARD atmosphere and with the basic 
assumptions set forth in Chapter 5, the 
relation between the factors affecting the power of a 
set and the gain factor A is given in equations (3) 
and (5) in Chapter 5. The problem of computing A 
depends on the set in the sense that some sets are 
designed to operate in free space, others with the aid 
of reflection from the sea, as in low-angle and surface 
coverage. 
The characteristics of a set as given in the manu- 
facturer’s description or in Tables 3, 4, and 5 at the 
end of this chapter may not represent the true values 
for a set in field use. Expected set performance, 
such as maximum range and coverage, can be calcu- 
lated on the basis of the set’s rated characteristics. 
Such performance can be termed “normal.” If a 
set is behaving abnormally, it may be that it is not 
functioning most efficiently. Unfortunately, the 
problem is complicated by the possible presence of 
atmospheric ducts and by the variability and diffi- 
culty of finding accurately the radar cross sections 
of aircraft and ships. 
Ducts are especially important for low antennas 
in surface search. In the case of communication 
sets, the most important item of information from a 
propagation standpoint is the maximum range. In 
the case of radar, not only is knowledge of maximum 
range wanted but also the ability to estimate the 
size and type of the target. 
The Performance Figure 
and Efficiency 
The maximum range of a set depends on the peak 
power output P, of the transmitter, the minimum 
detectable power Pyin (See discussion in Chapter 2) 
of the receiver, and the antenna gains G, and G2. 
These can be grouped to give a performance figure. 
For communication, this figure is (P,/Pmin)GiG2. 
For radar, the gains G, and G, are generally equal. 
The performance figure is then (P,/Pmin)G?. The 
ratio of the actual performance figure to the max- 
imum possible value, or the difference in decibels, 
gives the efficiency of the set. In field use, it is 
generally impossible to measure the working per- 
formance figure with any precision and methods for 
obtaining a rough measure must be employed. 
454, 
Effect of Reflection 
It has been pointed out in Chapters 5 and 6 that 
reflection may increase the maximum range of a 
radar up to twice the free-space value. This aids 
BEAM UP 8° 
HEIGHT IN METERS 
40 
DISTANCE — KILOMETERS 
30 
© Experimenta! Points 
Figure 1. Effect of beam tilt on coverage for a radar. 
the early detection of aircraft at low angles. How- 
ever, the minima which occur in the resulting inter- 
ference pattern prevent the continuous tracking of 
an airship coming in 
This effect can be counteracted in several ways. 
One way is to employ microwaves whose inter- 
ference lobes are narrow and close together. Vertical 
polarization is another means of filling in the nulls 
while gaining in maximum range at low angles. 
Another device is to tilt the antenna beam upward 
so that some radiation (but substantially less than 
half) falls upon the sea. The result is a gain in low- 
angle coverage while the high-angle coverage is that 
of free space, without minima. 
The effect of various percentages of specular 
reflection in comparison with the free-space pattern 
is shown in Figure 1 for various beam tilts of a 
radar with a comparatively narrow beamwidth (11 
degrees between half-power points). The experimen- 
