Handbook of Propagation Effects for Vehicular and ... - Courses

Attenuation Due to Roadside Trees: Mobile Case 3-23
3.5.2 UHF (870 MHz)
In Chapter 2 it was shown that a 35% increase in the dB attenuation was experienced at
870 MHz when comparing attenuation from trees having no foliage and those having
foliage (winter versus summer) for the static measurement case. This case corresponded
to a configuration in which the vehicle was stationary and the propagation path
intersected the canopy. Seasonal measurements employing a helicopter as the transmitter
platform were also performed by the authors for the dynamic case in which the vehicle
traveled along a tree-lined highway in Central Maryland (Route 295) along which the
propagation path was shadowed over approximately 75% of the road distance [Goldhirsh
and Vogel, 1987; 1989]. Cumulative fade distributions obtained from measurements in
October 1985 (trees with leaves) and in March 1986 (trees without leaves) are shown in
Figure 3-22. The results demonstrate the following:
At f = 870 MHz, 1% ≤ P ≤ 80% :
A ( full foliage)
= 1.
24A(
no foliage)
(3-19)
Equation (3-19) states that over the percentage range from 1% to 80% of the seasonal
cumulative distributions, there is an average increase of 24% (at equal probability values)
in the dB fade of trees with leaves relative to trees with no leaves. The dB error
associated with the above formulation over this percentage range for the curves in Figure
3-22 is less than 0.2 dB. The percentage fade increase (seasonal) for the dynamic case
(24%) is less than that for the static case (35%) because the former case represents a
condition in which the optical path is always shadowed, whereas the dynamic case has
associated with it measurements between trees.
The question arises “Why is there a small difference between the “foliage” and “nofoliage”
distributions at UHF (e.g. 24% change), whereas at K-Band, there is a large
change between the distributions for these two scenarios (e.g., two to three times)?” This
question may be answered as follows: The major contributor due to tree attenuation at the
UHF frequency (wavelength of approximately 35 cm) for trees without leaves are the
branches, since the separation between contiguous branches along the path are generally
smaller than 35 cm. That is, since the branch separation is generally smaller than a
wavelength at UHF, no substantial difference exists between the “leaf” and “no-leaf”
cases as both scenarios result in attenuating environments. On the other hand, because
the K-Band measurements have an associated wavelength of 1.5 cm, the separation
between branches for the “no leaf” case is generally larger than this dimension resulting
in a smaller relative fading condition. On the other hand, the “full blossom” case is
highly attenuating at K-Band because of the continuous blockage caused by the high
density of leaves.