CONCEPTS IN TRANSMISSION TRANSPORT
usually display a 55% efficiency or somewhat better. With such an efficiency, gain (
in decibels) is then
= 20 log
D + 20 log
F + 7.5
F is the frequency in gigahertz and D is the diameter of the parabolic reflector
in feet. In metric units we have
= 20 log
D + 20 log
F + 17.8
D is in meters and F is in gigahertz.
Running a Path/Site Survey
. This exercise can turn out to be the most
important step in the design of an LOS microwave link (or hop). We have found through
experience that mountains move (i.e., map error), buildings grow, grain elevators appear
where none were before, east of Madrid a whole high-rise community goes up, and
Another point from experience: If someone says "line-of-sight" conditions exist on a
certain path, don't believe it ! Line of sight must be precisely defined. We reiterate that
for each obstacle in the LOS microwave path, earth curvature with proper
be added to obstacle height, 0.6 of the first Fresnel zone must be added on top of that,
and then 50 ft for trees and 10 ft more for growth must be added if in a vegetated area
(to avoid foliage-loss penalties).
Much of the survey is to verify findings and conclusions of the path profile. Of course,
each site must be visited to determine the location of the radio equipment shelter, the
location of the tower, whether site improvement is required, the nearest prime power
lines, and site access, among other items to be investigated.
Site/path survey personnel must personally inspect the sites in question, walking/driving
the path or flying the path in a helicopter, or a combination thereof. The use of GPS
receivers are helpful to verify geographical positions along the path, including altitudes.
Fades, Fading, and Fade Margins
In Section 22.214.171.124.2 we showed how path loss can be calculated. This was a fixed loss that
can be simulated in the laboratory with an attenuator. On very short radio paths below
about 10 GHz, the signal level impinging on the distant-end receiving antenna, assuming
full LOS conditions, can be calculated to less than 1 dB. If the transmitter continues to
give the same output, the RSL will remain uniformly the same over long periods of time,
for years. As the path is extended, the measured RSL will vary around a median. The
signal level may remain at that median for minutes or hours, and then suddenly drop
and then return to the median again. In other periods and/or on other links, this level
variation can be continuous for periods of time. Drops in level can be as much as 30 dB
or more. This phenomenon is called fading. The system and link design must take fading
into account when sizing or dimensioning the system/link.
As the RSL drops in level, so does the
. As the
decreases, there is a
deterioration in error performance; the BER degrades. Fades vary in depth, duration, and
frequency (i.e., number of fade events per unit of time). We cannot eliminate the fades,
Often it is advisable to add 10 ft (or 3 m) of safety factor on top of the 0.6 first Fresnel zone clearance to
avoid any diffraction-loss penalties.
GPS stands for global positioning system.