JED - June 2015 - 48

EW 101

Radio Propagation
Diffraction by a Rounded Obstacle

cont'd.

By Dave Adamy

Cross Section of
Cylinder Representing
The Ridge Line

Actual Ridge Line Contour

The Journal of Electronic Defense | June 2015

48

Figure 1: A ridge line with significant width can be represented by a cylinder.

L

accuracy, it is represented by a cylinder drawn so that it is
tangent to the prominent features of the terrain as shown
in Figure 1.
Figure 2 shows the model for calculation of the cylindrical diffraction loss. The circular cross section of the cylinder
is tangent to the two signal ray lines from
can be represented by a cylinder. the transmitter
and to the receiver. As shown in the figure, d1 and d 2 are
the horizontal distances from transmitter and receiver to
the point at which the two rays cross. These were entered
in kilometers last month, but we need them in meters for
this calculation. "D" is the horizontal distance (in meters)
D d1 2
between the2pointsdat which the cylinder is tangent to the
r=
signal ray a [d1 2 + d2 2]
lines.
CYLINDRICAL DIFFRACTION
We need to calculate "a," the angle between the transmitter
This discussion is based on calculations from an excellent
and receiver rays and "r," the radius of the cylinder. The angle
paper presented by Barry McLarnon at the 1997 TAPR/ARRL
a can be calculated from the formula:
Digital Communications conference.
a = atan[ H/d1] + atan [H/d2]
We will assume that the ridge line cannot be considered
a knife edge because it is too broad. To achieve the required
Where: H, d1 and d2 are in meters and a is in radians

ast month, we covered knife-edge diffraction as
a way to model the propagation over some object,
such as a ridge line, which is near the line-of-sight
path between a transmitter and a receiver, or which
blocks the line-of-sight path. Now, we will consider
diffraction from an object
ridge line)
Figure 1: reasonably be modeled as(for example a The actual
A ridge line with signi´Čücant width
that cannot
a knife edge.
contour of the ridge line is represented by a cylinder.
To use this technique, you first find the knife-edge diffraction
as described last month. Then, perform the operations described
in this month's column to determine the additional loss caused
by the width of the ridge line over which the signal is diffracted.

LCYL = 11.7 a

rf
96



JED - June 2015

Table of Contents for the Digital Edition of JED - June 2015

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
World Report
Leadership Interview
Missile Warning for Fighter Aircraft
Technology Survey: RWR/ESM Systems
EW 101
2015 AOC Election Guide
AOC News
Index of Advertisers
JED Quick Look
JED - June 2015 - cover1
JED - June 2015 - cover2
JED - June 2015 - 3
JED - June 2015 - 4
JED - June 2015 - 5
JED - June 2015 - The View From Here
JED - June 2015 - 7
JED - June 2015 - Conferences Calendar
JED - June 2015 - 9
JED - June 2015 - Courses Calendar
JED - June 2015 - 11
JED - June 2015 - From the President
JED - June 2015 - 13
JED - June 2015 - 14
JED - June 2015 - The Monitor
JED - June 2015 - 16
JED - June 2015 - 17
JED - June 2015 - 18
JED - June 2015 - 19
JED - June 2015 - World Report
JED - June 2015 - 21
JED - June 2015 - Leadership Interview
JED - June 2015 - 23
JED - June 2015 - 24
JED - June 2015 - 25
JED - June 2015 - Missile Warning for Fighter Aircraft
JED - June 2015 - 27
JED - June 2015 - 28
JED - June 2015 - 29
JED - June 2015 - 30
JED - June 2015 - 31
JED - June 2015 - 32
JED - June 2015 - 33
JED - June 2015 - 34
JED - June 2015 - 35
JED - June 2015 - 36
JED - June 2015 - 37
JED - June 2015 - 38
JED - June 2015 - Technology Survey: RWR/ESM Systems
JED - June 2015 - 40
JED - June 2015 - 41
JED - June 2015 - 42
JED - June 2015 - 43
JED - June 2015 - 44
JED - June 2015 - 45
JED - June 2015 - 46
JED - June 2015 - 47
JED - June 2015 - EW 101
JED - June 2015 - 49
JED - June 2015 - 50
JED - June 2015 - 51
JED - June 2015 - 52
JED - June 2015 - 53
JED - June 2015 - 54
JED - June 2015 - 2015 AOC Election Guide
JED - June 2015 - 56
JED - June 2015 - 57
JED - June 2015 - 58
JED - June 2015 - 59
JED - June 2015 - 60
JED - June 2015 - AOC News
JED - June 2015 - 62
JED - June 2015 - 63
JED - June 2015 - 64
JED - June 2015 - Index of Advertisers
JED - June 2015 - JED Quick Look
JED - June 2015 - cover3
JED - June 2015 - cover4
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