Line-of-sight may not be good enough
Roger Freeman
12.14.2004
Rating: --- (out of 5)
If I can see the distant end antenna, there is line-of-sight and a good microwave path. This is a trap that many prospective microwave users fall into. There are rules of clearance that should be obeyed or suffer the consequences. If a user does not follow the rules, the received signal at the distant end can get corrupted with multipath energy. This "corrupted" energy derives from reflections and refraction off obstacles in and around the path. The downside for the user is that more money must be spent to raise tower heights to achieve the additional clearance. If we allow 0.6 of the first Fresnel zone as clearance, we are pretty secure from corrupting multipath. As one might expect, this clearance is a function of the emitted signal's wavelength and obstacle distance from the emitter. However, here are a few 0.6 Fresnel zone clearances in the table below -- for some popular frequencies and obstacle distances (m).
0.6 fresnel zone clearances with distance to obstacle
|
FREQUENCY
|
EQUIVALENT
WAVELENGTH (m)
|
DISTANCE (m)
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0.6 FRESNEL
ZONE CLEARANCE MIDPATH
|
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2000 MHz
|
0.15
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10
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0.519 m
|
|
|
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100
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1.64 m
|
|
|
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1000
|
5.19 m
|
|
|
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10,000
|
16.4 m
|
|
4000 MHz
|
0.075
|
10
|
0.612 m
|
|
|
|
100
|
1.16 m
|
|
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|
1000
|
3.67 m
|
|
|
|
10,000
|
11.6 m
|
|
6000 MHz
|
0.05
|
10
|
0.3 m
|
|
|
|
100
|
0.948 m
|
|
|
|
1000
|
3.0 m
|
|
|
|
10,000
|
9.48 m
|
|
12,000 MHz
|
0.025
|
10
|
0.21 m
|
|
|
|
100
|
0.67 m
|
|
|
|
1000
|
2.1 m
|
|
|
|
10,000
|
6.7 m
|
The equation used to calculate the radius of the first
Fresnel zone, Rm, is:
Rm = 17.3Ö(d1xd2/FD)
Where d1 is the distance in meters from one end
of the link to the obstacle and
d2 is the distance from the obstacle to the other
end of the link such that d1 + d2 = D, the length
of the
link.
F = the frequency of the emitted signal in GHz. The values in the right hand column of the table are 0.6 the value of Rm in each case. Increasing clearance over an obstacle implies raising tower heights which costs money. Sometimes a lesser value than 0.6 of the first Fresnel zone may be used depending on modulation type and waveform configuration. Never-the-less, we recommend keeping with the 0.6 criteria and adding a safety factor of 3 m on top of that. This will help greatly to avoid degraded error performance due this self interference phenomena.
What results when we bend these rules and decrease obstacle clearance values? More RF energy is then reflected off the obstacle and additional refracted energy gets passed to the distant receiver. This energy is delayed and tends to spill into the time slot of the following bit(s) or symbol(s) confusing the receiver. Error performance degrades as a result.
About the author:
Roger Freeman has worked in telecommunications since 1946 when he joined the Navy and became an aviation radioman. Later, Roger served as a radio officer in the merchant marine for nearly 10 years. He then held several positions with ITT assigned to their Spanish Standard Electrica subsidiary. He also served the International Telecommunication Union as Regional Planning Expert for Northern Latin America based in Quito, Ecuador. Roger is bilingual. His last employee position was principal engineer with the Raytheon Company, Marlboro, MA where he took early retirement in 1991 to establish Roger Freeman Associates, Independent Consultants in Telecommunications. He has been giving seminars in telecommunication disciplines at the University of Wisconsin, Madison for nearly 20 years. Roger has been writing books on various telecommunication subjects for John Wiley & Sons since 1973. There are seven titles which he keeps current including the two-volume work, Reference Manual for Telecommunication Engineers, now in 3rd edition. He holds two degrees from NYU. His Web site is www.rogerfreeman.com and his e-mail address is rogerf67@cox.net.
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