Editor’s Note: This is the first installment for our “Meet the RF Experts” series in which contributors to the Understanding the RF Path e-book elaborate on subjects in their areas of expertise.

People familiar with wireless network concepts know that either frequencies or codes are re-used repeatedly throughout the network in order to sustain the enormous amount of voice and data traffic. Operators employ base station antenna pattern control to decrease interference between cells operating on the same frequencies or codes. As the cell sites become denser, the coverage area from each cell must diminish.

One way to decrease the coverage area from a cell site is to bring down the height of the antenna radiation center. However, this is often undesirable since it could place the antenna below many nearby obstacles such as buildings or foliage. Since most cellular networks now use sector antennas, a second way of reducing the coverage area is to tilt the vertical pattern of the antenna downwards thus constricting the coverage on the horizon—where interference to adjacent cell sites occurs.

 The easiest way to do this beam tilting is to mechanically tilt the entire sector antenna using adjustable brackets available

 

Mechanical Tilt

 

from most antenna vendors. However, this approach has a significant downside since it does not reduce coverage consistently on the horizon over the sector. It reduces coverage more in the bore sight direction and less at other angles away from bore sight, so the result is not a consistent decrease of the cell coverage. CommScope has come up with a quantifiable term for this phenomena—pattern blooming. The degree of acceptable mechanical downtilt is a function of the antenna’s height (aperture), and the tolerable amount of blooming should not exceed 10%.

 When travelling, I am often fascinated by the excessive amount of mechanical downtilt used at local cell sites. This technique often generates interference levels that can cause network inefficiencies and result in customer churn.

 The preferred method for tilting a sector antenna’s vertical pattern is by using electrical downtilt. This technique leaves the antenna mounted upright and accomplishes the beam tilting by manipulating the electrical phase delivered to each element using phase shifters. The consequent pattern is tilted consistently over the entire 360 degrees, reliably shrinking the coverage area. For electrically downtilted antennas, pattern blooming does not increase regardless of the amount of tilt.

 Another advantage of electrical downtilting is that it can be done remotelyby connecting a motor to the phase shifter

 

Electrical Tilt

 

mechanism. This feature will become even more important as next generation technologies such as LTE mature. One concept associated with LTE is the self-organizing network (SON), where the network regularly re-optimizes itself based on demand. Versatile coverage adjustment capabilities such as electrical downtilt are key to the deployment of the SON concept.

You can learn more about electrical downtilt and other features of today’s base station antennas in chapter three of CommScope’s Understanding the RF Path e-book, which I helped author. Use the link to view the online version or request access to a mobile application.

And feel free to ask questions about electrical downtilt in the Comments section below.

 

About the Author

Lou Meyer

Louis Meyer, P.E., is director of technical sales for CommScope Mobility Solutions. Lou has spent a lifetime advancing RF technology, taking it from the drawing board to practical use. Over the years—in various roles with Allen Telecom, Andrew Ltd. and CommScope—Lou has been responsible for supporting the sales teams for such solutions as remote antenna control systems, transmission lines, diplexers and other important components. Prior to joining Allen Telecom, Lou worked with Decibel Products as vice president of Antenna Design and vice president of International OEM Relations. Earlier, Lou worked with Harris Corporation in RF communications and Bendix Corporation in their Missile Systems division. Lou holds 10 patents and was previously chair and vice-chair of the TIA’s TR-8.11 Antenna Standards subcommittee. He earned his Bachelor of Science degree in electrical engineering from Marquette University in Milwaukee, Wisconsin, and is currently a registered professional engineer in the state of Texas.

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RF Solutions Wireless

Comments

7 comments for "Mechanical Downtilts Can Hurt Antenna Performance"
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Mari

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David Fallon

Great topic! How does the SON use Electrical Tilt in regards to surface obstacles? Should they be taken into account when integrating phase shifters to achieve the etilt? Also, can you recommend a phase shifter product (from the CommScope ecatalog, maybe) as an example?

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Lou Meyer

Hi David — The whole topic of SON is pretty exciting. The 3GPP Standards divide SON up into 3 separate categories: Self Configuration, Self Optimization and Self Healing. I believe your question on surface obstacles relates to the re-optimization caused by a change in propagation characteristics such as new building construction or even foliage changes over different seasons. A more in-depth article on SON can be found here: http://www.radio-electronics.com/info/cellulartelecomms/self-organising-networks-son/self-optimisation-optimization.php The phase shifters that CommScope employs are specifically designed to be an integral part of the antenna. They typically have one input and multiple outputs and the outputs feed the individual radiating elements in the vertical array. As the phase shifter is adjusted, it changes the phase taper to the elements in the array, thus causing the elevation beam to tilt electrically. Since these are not connectorized components, they are not sold as stand-alone products by CommScope.

Simon

Dear Lou,

Do you have any information on the relationship (Formula) between the various parameters used for tuning the antenna to give a specific electric tilt? eg there is a formula for relating power to h and D in Okumurata Hata model

Thanks

Simon

Lou Meyer

Hi Simon,
The Okumura-Hata model is a well-accepted set of formulas that calculate path loss vs distance and antenna heights; however, they do not take antenna patterns into account. Coverage prediction software tools use these path loss equations in conjunction with the antenna's azimuth and elevation patterns to calculate the signal strength in the coverage area - which is normally divided up into numerous pixels and a calculation is done for each pixel. The geometry between the base antenna and the user antenna defines a distance and an angle for each pixel. The digitized pattern data for each model (at different tilts) supplied on our website is used to provide the antenna's gain for each pixel calculation.
Hope this helps,
Lou

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