TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY 149 
For SC and SA radars or for lower altitude installa- 
tions, they are optimistic.”’ 
Figure 24A shows the lobe structure for the 
standard atmosphere in which M increases 36 MU 
per 1,000 ft. It also shows the value of M — Mioo; 
that is, the M curve is drawn so as to pass through 
zero at the transmitter elevation of 100 ft. On 
diagrams B through E the lower portion of the 
standard lower lobe is indicated by a dash-dot line. 
The blind zones are cross-hatched, and their boun- 
daries represent the calculated limits of detection. 
An interesting feature of these diagrams is the 
appearance, in some cases, of blind zones of consider- 
able range and altitude along the surface. These 
cause “skip ranges” for ground targets that are 
significant in operational problem’. Ray diagrams 
were used in calculating the field strengths in 
Figure 24. 
The relative heights of the transmitter and the 
duct have an important bearing on the mechanism 
of transmiesion. The dutt may develop entirely 
below the transmitter site or entirely above, or the 
duct may include the transmitter. With these alter- 
natives a variety of propagation conditions is 
possible. 
One of the important concepts of radiation theory 
is contained in the principle of reciprocity. This 
principle states that when a transmitter is at a point 
in space A, and the receiver at a point B, the 
received intensity is the same when they are inter- 
changed, the transmitter being at B and the receiver 
at A. (It is assumed in making this statement that 
the transmitter and receiver may be regarded as 
point sources.) Similarly, for radar the signal inten- 
sity remains unaltered if the positions of radar and 
target are interchanged. It is known that there are 
serious limitations to the reciprocity principle where 
ionospheric reflections are involved, but for shorter 
waves and tropospheric propagation the principle 
may be applied without restriction. By means of 
the reciprocity principle any coverage diagram may 
be used to obtain the field strength when the heights 
of the target and the radar are interchanged. 
From a study of such evidence on coverage 
diagrams as is available, it appears that (a) the 
effects of superrefraction are most marked when the 
transmitter lies in the duct; (b) they exist to a lesser 
degree if the transmitter lies below the duct: in 
particular no excessively long ranges for targets are 
then found above the duct—sometimes the ranges 
are extended slightly, other times slightly decreased; 
(ec) for a transmitter above the duct no excessive 
changes in field strength occur below the duct—this 
can be deduced from (b) by using the reciprocity 
principle; (d) there is no appreciable superrefraction 
when the transmitter lies appreciably above the duct. 
For some time-after the discovery of superrefraction 
it was thought that the concentration of radiative 
energy in the duct might result in a decrease of the 
ALTITUDE 
FEET 
2 
M-M409 NAUTICAL MILES 
BLIND ZONE [=—7ZONE OF DETECTION 40 
200 MC TRANSMITTER ELEVATION 100 FEET 50 
Ficurs 24. Calculated coverage diagram. 
amount of radiation above the duct and hence in a 
reduction of coverage there. The cases illustrated in 
Figure 24, at least, are not in accord with this 
presumption. In spite of the great increase in ranges 
_ in the duct the amount of energy trapped is small 
compared to the total energy of the radiation field. 
Wave Picture of Guided Propagation 
It must be realized that while ray treatments give 
accurate results under certain conditions, there are 
features of the propagation problem which can be 
satisfactorily discussed only on the basis of the 
electromagnetic wave equations. As an aid to under- 
