Problem solve Get help with specific problems with your technologies, process and projects.

# Line-of-sight may not be good enough

## Fresnel zone clearances table for some popular frequencies and obstacle distances.

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) 0.6 FRESNEL ZONE CLEARANCE MIDPATH 2000 MHz 0.15 10 0.519 m 100 1.64 m 1000 5.19 m 10,000 16.4 m 4000 MHz 0.075 10 0.612 m 100 1.16 m 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, R m, is:
R m = 17.3 Ö(d 1xd 2/FD)

Where d 1 is the distance in meters from one end of the link to the obstacle and
d 2 is the distance from the obstacle to the other end of the link such that d 1 + d 2 = 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.

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.
This was last published in December 2004

## Content

Find more PRO+ content and other member only offers, here.

#### Start the conversation

Send me notifications when other members comment.

## SearchSDN

• ### NFV disaggregates firewall appliance to increase scalability

A traditional firewall appliance can incorporate many services, but NFV can virtualize firewalls to offer individual services ...

• ### Cisco, Hyundai to add software-defined platform to vehicles

Cisco and Hyundai will release jointly developed software-defined vehicles in 2019. These connected cars will boast a Cisco-built...

• ### Zayo SD-WAN available as a managed or on-premises product

Zayo Group has launched an SD-WAN product. The Zayo SD-WAN is available as an on-premises or managed product. Zayo is a bandwidth...

## SearchEnterpriseWAN

• ### How to calculate network bandwidth requirements

Figuring out how to calculate bandwidth requirements is vital to ensuring your network runs smoothly, and it's best to get the ...

• ### How types of noise in data communication systems affect the network

Learn about the different types of noise in data communication -- including thermal, intermodulation, cross-talk, impulse and ...

• ### The best VPNs for enterprise use

This slideshow highlights the best VPNs used in enterprise wide-area networks (WANs) and offers principles for designing and ...

## SearchUnifiedCommunications

• ### Emerging technologies to fuel collaboration industry growth

Business communications is getting more contextual and intelligent, thanks to emerging technologies, such as AI and APIs. UCC ...

• ### How to navigate the enterprise collaboration market

When evaluating enterprise collaboration services, first look inward at your company needs. Some key collaboration criteria ...

• ### Businesses slow to embrace products like Samsung Flip board

Samsung hopes to heat up the tepid digital whiteboard market, but the new Samsung Flip board will face tough competition from ...

## SearchMobileComputing

• ### A roundup of the leading EMM providers in today's market

Just as identifying the need for EMM is important, so is selecting the right vendor. Here's a rundown of leading EMM vendors and ...

• ### Compare the main mobile app development languages

Developing an app is all about visualizing what the users need and what they'll see in the UI. Get visual with this infographic ...

• ### A handy mobile app testing checklist for developers

Application testing is a necessity in the era of enterprise mobility. Define the right testing criteria and adopt the proper ...

## SearchDataCenter

• ### Three requirements for a hybrid cloud computing deployment

As the hybrid cloud computing approach gains steam, organizations will need to pay close attention to cross-cloud connectivity ...

• ### Five debunked myths about SSD issues

Solid-state drives are mature now, and the technology has eclipsed hard disk drives with superior performance, manageability and ...

• ### Future data center trends hinge on the edge, cloud and staffing

Edge computing, colocation, cloud and IT staffing issues lead the way as industry analysts make their predictions for the future ...

## SearchITChannel

• ### Riverbed Rise channel program reinvents partner requirements

The Riverbed Rise program will focus on partner performance, which Riverbed will measure by customer acquisitions, account ...

• ### Primary and secondary storage combos attract channel partners

Nontraditional storage vendors, such as ClearSky Data and Reduxio, are attracting resellers and managed service providers with ...

• ### Demand for cloud-enabling infrastructure soared in 2017, says IDC

IDC's latest cloud IT infrastructure market tracker shows the demand for public and private cloud-enabling servers, storage and ...

Close