Chapter 12 
VARIATIONS IN RADAR COVERAGE 
ARIATIONS IN COVERAGE of radio and radar 
equipment are caused by atmospheric factors 
whith influence propagation of very short radio 
waves. 
The rapid and accurate evaluation of radar signals 
is dependent to a great extent upon our knowledge 
and understanding of the effects produced by the 
variable conditions of the lower atmosphere. 
Evaluation of radar signals influenced by weathei 
introduces problems of identification, actual range 
determination with second or third sweeps, and rada1 
coverage characteristics, each having a direct bearing 
on the tactical situation. 
Enemy ships far beyond the horizon have been 
located by radar and sunk by radar-controlled gun- 
fire. United States warships in the Pacific, in several 
instances, have picked up targets by radar at ranges 
four to five times those obtained under standard 
conditions. 
Army coastal radars have tracked convoys on 
some occasions to 20 or 30 miles beyond normal 
radar ranges. The same radars, a few hours later, 
may have failed entirely to pick up targets clearly 
visible to the eye. 
Allied forces are employing radar and VHF (very 
high frequency) equipment with steadily increasing 
effectiveness. But we are forced to revise and improve 
our early conceptions of the capabilities and limita- 
‘tions of these useful instruments of World War II. 
Serious.errors and false evaluation of radar presenta- 
tion may result if we do not take into consideration 
the effects of weather and atmosphere on radar 
ranges and VHF coverage. 
Complete reports of the variability of radar cover- 
age show that certain weather and atmospheric 
conditions prevailing along the transmission path 
may greatly modify the normal range characteristics 
of radar and VHF radio. The operator, at certain 
times, can “see” targets or hear messages far beyond 
the horizon, sometimes at unbelievable distances. At 
other times he is unable to contact, by radar or 
VHF, aircraft or surface craft well within the normal 
range limit. 
These effects of a nonstandard atmosphere might 
leave doubt in our minds as to the effectiveness of 
radar and the usefulness of VHF radio. But we should 
adopt the reverse view. We can, by understanding 
and allowing for these phenomena, make a useful 
instrument more effective—the weather will work for, 
instead of against, radar and microwave equipment. 
Unusual ranges are caused by bending or refraction 
of the radio waves by the atmosphere. A most import- 
ant special case of refraction is the concentration of 
127 
the wave energy in ducts within the atmosphere. 
This bending and duct formation is a direct result of 
the meteorological factors involved—factors of 
weather and atmosphere—peculiar, in many cases, 
to the locality and the season. Such factors are dis- 
cussed later. 
BENDING 
The VHF or radar operator usually assumes that 
short waves and microwaves, at frequencies above 
about 30 me, travel along the line of sight from the 
transmitter to the receiver and, in the case of radar, 
to and from the target. Experience has shown that 
this assumption, nearly true in many instances, may 
lead to serious errors or false evaluation if applied 
to radar operation and microwave communication. 
Radio waves are bent from a straight line path as 
a result of refraction by the lower atmosphere. This 
bending, or refraction, is generally recognized as a 
property of light. It is equally a property of radio 
waves. The underlying principles are exactly the 
same in both cases. 
The quantity that determines refraction is called 
the index of refraction. Refraction occurs whenever 
there is a change of index of refraction, as at the 
boundary of two substances. In the interior of a 
material of constant refractive index, the rays travel 
in a straight line. The change in angle at the bound- 
ary is the larger, the greater the difference in refrac- 
tive index from one material to the next. 
Radio waves are refracted or bent in the atmosphere 
because the index of refraction of the atmosphere 
changes with height. The properties of the atmos- 
phere which determine the refractive index and which 
change with height are temperature, pressure, and 
moisture content. These changes from one level to 
‘another are very small compared with that from 
water to air, and the resulting refraction itself is 
small. Nevertheless this refraction is of great import- 
ance in radar operations and radio communications 
above 30 me. 
If the atmosphere wefte composed of a number of 
successive layers each having a different index of 
refraction, a wave passing across the successive boun- 
daries of the layers would be abruptly deflected at 
each surface. The atmosphere does not consist of such 
distinct layers. Instead, the change in its physical 
properties and its index of refraction is gradual, 
continuous. There is, then, no sudden change in 
direction of the waves; the change in direction 
becomes gradual and zontinuous. In other words, a 
bending of the waves occurs as they pass through 
the atmosphere. Radio waves passing through the 
