(What’s all this Fresnel Zone Stuff, Anyway?)

As wireless cell towers and antennas flourish, towns, cites and municipalities are challenged to adopt ordinances that let citizens receive the best telecommunications services available. Regulatory legislation hasn’t helped. The Telecommunications Act of 1996 was over 500 pages long; during debate, most legislators considered it to only be about deregulation. But buried inside the Act, in Section 704, lay a crafty clause about the siting of cell-phone towers that trips up planning and zoning groups.

For this and other reasons, zoning boards are challenged to approve high-quality wireless services. It doesn’t help when they don’t understand microwave propagation.

An appreciation of microwave propagation comes into play when carriers and network operators are faced with securing wireless transmission media (often a microwave link “backhaul”) that relay the wireless traffic to the landline network or between towers. As more educational institutions offer additional classrooms in distance education and remote video conferencing, their needs also increase for wireless systems and backhaul.

 

Collocation Tower with Cellular and Backhaul Antennas

 

As a case in point, a town in Illinois accepted an application from SBA Towers to replace an aging tower on a college campus with a taller monopole structure to allow better backhaul of college networks and to support more wireless tenants. The existing short tower was structurally limited but was situated in adequate campus space to meet setback and other pertinent city requirements. The application was presented to the town by wireless zoning attorney Richard Connor Riley, supported by expert testimony from LBA’s Chris Horne. Chris’ work included a microwave path study of the existing and proposed college backhaul link.

This is where it gets interesting.

The study was performed for a proposed new digital transmission system serving four video conference classrooms and also transporting all internet traffic between the college and a remote teaching facility. The included digital microwave link would be a backup to an existing fiber network between the two campuses.

Controversy arose centered on a proposed antenna dish that was needed to support the data requirements of four video conference classrooms. It would replace an existing microwave dish that served only one classroom and was an outmoded analog link with limited performance and capacity compared to the proposed digital replacement.

When LBA evaluated the microwave installation, it confirmed that greater height was needed for the new dish to support the required performance of the system. It had not been obvious to zoning authorities that such an increase was necessary; the devil was in an “invisible” gremlin!

A close evaluation of the microwave path between the campus and a far terminal revealed that a tall hotel building under construction several miles away was rising exactly in the path of the beam. At its final height, the building would penetrate the microwave Fresnel zone and block the microwave signal. Based upon LBA’s engineering evaluation, the town zoning board was persuaded to approve the SBA tower application, and clear the way for the higher microwave dish. The university learning network was saved!

So – just what is this mysterious microwave path Fresnel zone?

In order for radio waves emitted from a transmitting antenna to reach a receiver without attenuation of power, a certain amount of 3-dimensional space is required. The radio wave energy cannot reach the receiver via one straight line in space. Think of a hole the size of a needle in a concrete wall. If you shout across a pond to the wall, do you think all the sound penetrates only the hole?

The Fresnel zone or First Fresnel zone (0.6) is a three-dimensional elliptical space surrounding the line of sight path from the transmitter to the receiver. If a reflective object is placed anywhere on the edge of the ellipse, it will cause a reflected signal that, due to propagation delay, is delayed 180 degrees (“carrier phase shift”) with the line of sight signal at the receiving antenna. Digital radio equipment is typically robust in the presence of propagation fades and other noise. However, if there is an obstruction between the antennas (ground, buildings, natural objects, etc.), the radio waves will be reflected, resulting in multipath interference and unacceptable bit error rates–think old TV “snowy pictures.

 

Depiction of a Fresnel Zone in urban environment

 

A 0.6 Fresnel zone clearance is normally desired for microwave paths. While “line of sight” (LOS) is important, it obviously is not always adequate. Even though the path may have clear LOS, when obstructions penetrate the Fresnel zone, unwanted signal attenuation results.

This is precisely what happened in this case; a building encroached upon the 0.6 Fresnel zone of the proposed college microwave link. Such obstacles can wreak havoc on wireless backhaul including transmission errors and signal reception issues.

The take away here is that next time you plan a microwave backhaul link, be sure to consider the Fresnel zone, LOS obstructions, and other antenna placement factors. Consider every detail of the traversed terrain, including vegetation, existing, and planned construction. A detailed path study will provide valuable input results for an efficient tower placement scheme that meets both coverage and transmission quality metrics. A not unimportant byproduct is that it also provides the basis for a credible defense of a tower proposal in an increasingly contentious zoning environment.

LBA can help with all of the above. LBA also offers RF Engineering services including expert testimony and system design. We offer extensive zoning technical support here. Other capabilities include microwave system design, path study, and RF engineering services here.

About The Author

LBA Group

LBA Group, Inc. has 50 years of experience in providing RF asset solutions and risk management for industrial and telecommunications infrastructure assets. The group is comprised of LBA Technology, a leading manufacturer and integrator of radio frequency systems, lightning protection, and EMC equipment for broadcast, industrial and government users worldwide; the professional engineering consultancy Lawrence Behr Associates, and LBA University, providing on-site and online professional training. The companies are based in Greenville, N.C., USA.

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