JED - July 2015 - 43

EW 101

Radio Propagation (cont'd)
Losses from Trees and Urban Areas
By Dave Adamy

LOSSES FROM TREES AND FORESTS
This discussion is based on calculations from an excellent
paper presented by Barry McLarnon at the 1997 TAPR/ARRL
Digital Communications conference which draws data from CCIR
report 1145 and the ITU-R report 567-4. Much of this discussion
comes directly from the referenced documents and from a 2009
paper in Progress in Electromagnetic Research B, Vol. 117.
While a single tree has little impact on radio propagation,
a dense forest can cause significant loss. There are differences
in attenuation depending on the types of trees and the time of
year, but the primary variable is the transmission frequency.
As shown in Figure 1, the attenuation from a forest depends
on the distance the signal path passes through the trees. Once
this path length is established, the approximate loss per meter
can be found from Figure 2. The scattering of attenuation values
for various conditions group around the line shown in this figure.

XMTR

Let's take an example: The path length through the forest is
100 meters. If the frequency is 500 MHz, the loss per meter is
0.2 dB. Thus, the additional loss above the clear air propagation
loss is 20 dB. (100 m x 0.2 dB/m).
Other Forest Propagation Factors
*	 	T he	loss	in	a	forest	is	less	for	horizontal	polarization	than	for	
vertical polarization below approximately 1 GHz, but there is
little difference above 1 GHz. This loss is caused by 5 to 15
dB greater depolarization loss in vertically polarized signals.
*	 	Cold	winter	weather	reduces	the	humidity	in	tree	trunks,	
and can reduce the loss by 2 to 3 dB.
*	 	I n	the	rain	(data	taken	at	1900	MHz),	the	loss	for	wet	trees	
in 50 to 150 meters increases by 29 to 32 dB.
*	 	One	report	showed	that	variations	between	no	leaf	and	full	
leaf conditions (data taken at 462 MHz) was only 8 dB.
*	 	T here	is	significant	diffraction	propagation	over	the	tree	
tops. Therefore, if the antennas are up near the tree tops,
there is a significant reduction in attenuation.

LOSS IN URBAN AREAS
Figure 3 shows an extremely simplified view of an urban
environment. The point is that a receiver will see many
different signals from multi-path off of structures. The

Distance

through f

orest

RCVR

LOSS = propagation loss + loss per km x distance through forest
Figure 1: Loss through a dense forest is a function of how far the propagation path travels through the forest.

The Journal of Electronic Defense | July 2015

T

he discussions in the last two columns dealt with
the effects of the shape of the terrain over which
radio waves travel. Now we will talk about the
attenuation caused by obstructions. Two cases are
considered. First, passing through dense forests
and then passing through urban areas.

43



JED - July 2015

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

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
World Report
The Modern MANPADS – an Evolving Threat
Technology Survey: RF Tuners and Tuner Modules for SIGINT Applications
EW 101
AOC News
Index of Advertisers
JED Quick Look
JED - July 2015 - cover1
JED - July 2015 - cover2
JED - July 2015 - 3
JED - July 2015 - 4
JED - July 2015 - 5
JED - July 2015 - The View From Here
JED - July 2015 - 7
JED - July 2015 - Conferences Calendar
JED - July 2015 - 9
JED - July 2015 - Courses Calendar
JED - July 2015 - 11
JED - July 2015 - From the President
JED - July 2015 - 13
JED - July 2015 - 14
JED - July 2015 - The Monitor
JED - July 2015 - 16
JED - July 2015 - 17
JED - July 2015 - 18
JED - July 2015 - 19
JED - July 2015 - 20
JED - July 2015 - 21
JED - July 2015 - World Report
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JED - July 2015 - 24
JED - July 2015 - 25
JED - July 2015 - The Modern MANPADS – an Evolving Threat
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JED - July 2015 - 28
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JED - July 2015 - 32
JED - July 2015 - Technology Survey: RF Tuners and Tuner Modules for SIGINT Applications
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JED - July 2015 - 36
JED - July 2015 - 37
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JED - July 2015 - 42
JED - July 2015 - EW 101
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JED - July 2015 - 45
JED - July 2015 - AOC News
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JED - July 2015 - Index of Advertisers
JED - July 2015 - JED Quick Look
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