PART 1: WHAT IS TOWER SHADOWING?

INTRODUCTION

A few months ago, I was contacted by a municipality who had just upgraded their single-site 800 MHz enterprise public safety trunking radio system to a two-site simulcast system.  Instead of getting more range on the new system, the range was reduced considerably and the cities sharing this system were in desperate need of help.

This system was designed by one of the major manufacturers in the business.  The city did choose the towers that were available and must be used, and the manufacturer used that fact as the basis for the system needing more towers to be effective.

In trying to solve the problem at hand, I did make a detailed analysis of what it would take to fix the problem.  I wrote a 24-page report on the system and how to fix the problem, and the manufacturer wrote a 68-page rebuttal that did not address the problem, but suggested that my analysis was wrong and the city just needed more sites.

My initial diagnosis of the problem was that tower shadowing was blocking the radio signals, and the engineers from the radio manufacturer and from the antenna manufacturer immediately accused me of being wrong.  The radio manufacturer had convinced the city that I was wrong and more sites were needed.  This required me to conduct a thorough study of the system to prove to the city and the radio manufacturer that they did not install the antennas for optimum coverage.  I was hindered in supporting my point of view because there is little literature that is published that explains tower shadowing.  Hopefully, this article will aid others with similar problems.

Every day since the system went on the air, hundreds of police, fire, EMS personnel, and other city employees were putting their personal safety in jeopardy due to the poor radio coverage in their jurisdictions.  I have worked in public-safety communications for over four decades, and I know that this system was a danger to those people who put their dedication to others before their own needs.  In the end, my only concern is for the users of the system and not the SITYS (SEE I TOLD YOU SO).

This article is written to explain to engineers, technicians, and radio system managers what tower shadowing is, and how to use it properly.

ANTENNA PLACEMENTS

In every radio system, the antenna is the most important component that sets the range of the system.  The height of the antenna sets the theoretical maximum range, but the actual range is usually reduced by obstructions, such as trees and foliage, buildings, hills and ground elevation obstruction.  In addition, the mounting location on the tower or rooftop has a lot to do with the range.  Sometimes, engineers use the tower shadow to protect against interference to and from other systems.  This section will look at the effects of antenna placement of the mounting structure in relation to the antenna’s performance.

The ultimate place to mount an antenna is the top of a tower with no other antennas near the antenna.  In real life, most towers have multiple antennas at the top, and since the side of the tower has plenty of room, there are as many antennas in use on the sides of towers as there are on the top of the towers.

The lone antenna at the top of a tower will have a 360-degree radiation pattern called OMNI-DIRECTIONAL.  If you place any object near the antenna, whether it is another antenna or the side of a tower, the pattern will then have an offset pattern.  The pattern can be predicted, and is usually shown in the catalogs from the antenna manufacturers as how the pattern is distorted from the tower.

The same goes for buildings rooftops.  In fact, buildings have an advantage in that each antenna has less distortion from the other antennas, as there is usually enough room at a building site as to not have the antennas crowding each other and causing severely distorted patterns.  There is also less attenuation on the coaxial transmission line on a rooftop location for most sites.

In some systems, it is desirable to have the antenna have a directivity so that all of the power to and from the system is in one or two directions, and the directions where the signal is not needed or wanted is reduced substantially.  Some of the antennas that are designed to have a concentration of power in one direction and have substantial attenuation in other directions include:

• Yagi
• Corner Reflector
• Cardiod
• Elliptical
• Offset
• Parabolic Reflector

If you have another system on the same channel (called co-channel) and you want to reduce the interference with this other system, you would normally use the tower to shade the signal in the direction of the co-channel system and this normally does fix many co-channel problems.

You can also use the tower to shield one system from the other on the same tower by placing the antennas on opposite sides of the tower, and letting the tower shadow keep each signal down by 20 dB (x 100 power) or more.  You can also use the vertical separation by placing the antennas vertically apart which provides a great deal of isolation from each other electrically, usually in the order of 40 dB (X 10,000 power) or greater.  In these last two examples, the tower shadow or vertical separation is used to enhance the operation of the system.

In the system for this municipality, the tower located in the north part of the territory had the transmitting antenna mounted on the north side of the tower.  This cast a large shadow to the south, where the majority of the users are located.  The south tower had the receiving antenna on the south side of the tower, and all of the users are located north of this tower.  In addition, the transmitter antenna for the south tower was mounted on the west leg, and the cities to the east had minimal coverage from this site.  In summary, more than 60% of the cities in this system were affected by the tower shadows.  One of the cities was located in the shadows from both sites, and their radio coverage was so bad, the police department had to use cellular telephones for keeping up with their field units.

A second city called just a few weeks ago, and again, the initial diagnosis after making a field visit to the municipality was that the tower shadow was the problem here.  Just as the radio manufacturer denied the problem in the first case, the other manufacture had the exact same reaction here.  Again, I made a detailed study of the signal strength at exactly 1 mile from the tower at 30-degree intervals, and the results were quite revealing as to the problem.  The area were the town was having problems was exactly the area that was found to be shadowed by the tower.  The first city uses the 800 MHz band, while this second city uses the 460 MHz band.  This just proves that the problem with shadows is not band specific.

BASIC PHYSICS

The one thing about radio waves is that they always obey the laws of physics.  This fact allows engineers to be able to very accurately predict how the radio waves will perform.  The performance can then be measured, which is called EMPIRICAL measurements, and the results are always the same.  In fact, if the field measurements do not match the predicted values, you normally send out a technician or engineer to find out why the measured values do not equal the predicted values.  Once the problem is found, the predicted values will match the measured values.

In the first system that I was looking at, the predicted value for the field strength at 1 mile was supposed to be -46.5 dBm, and the measured value was -47.0 dBm.  This difference is within the specification of the accuracy of the spectrum analyzer that was being used.  In the shadow areas, we measured -66 dBm at 1 mile when the antenna was on the north side of the tower.  When we went back to the exact same spot after we moved the antenna to the southwest side of the tower, the signal rose 21 dB to a -45 dBm.  This is a 120 times (12,000%) increase in signal strength power.  Again, we were within the measurement accuracy of the spectrum analyzer between the predicted value and the measured signal strength at that site.  Once the shadow was controlled, there was an immediate improvement of coverage for the cities involved.

In the second system, the predicted signal strength was –42.0 dBm, and the measured signal was also –42.0 dBm at the points where there were no obstructions of the clear Line-Of-Site path to the 1-mile mark from the tower.  In the tower shadow obstructed areas, the signal was down at least 20 dB, which translates to 1/100 of the power getting out in the direction of the shadow area.

Some of the laws of physics that we will discuss here that are pertinent to predicting the signal strength of the radio waves include:

• Inverse Square Law
• Free Space Attenuation
• Phasing
• Antenna Gain
• Downtilt

The Inverse Square law says that the field strength will drop by the square of the distance.  In simple terms, if you measured a value at a given point, and you double the distance, you will drop the field intensity by 1/4.  If you tripled the distance, you would drop the field intensity by 1/9.

Radio engineers have a formula that allows them to calculate the field intensity of a signal using the formula for Free Space Attenuation.  This formula is:

ATTN (dB) = 36.6 + 20 X LOG (Frequency in MHz) + 20 X LOG (Distance in Miles)

This formula holds true as long as you have Line-of-Sight propagation.

Radio signals can bounce off of objects and if they arrive in phase from the source to the receiver, the signals will add and give gain.  If the signal arrives out of phase, it can cause the field intensity to drop significantly, depending upon the amplitude of the out of phase signal and the phase relationship to the direct signal.

In some systems, the antenna can be so high in elevation that the main signal can literally pass over the intended coverage area.  You will see this in parts of the country where a mountain top site can be thousands of feet above the area you are trying to cover.  When this is the case, you can use beam tilt to bring the signal down to have better coverage in the desired coverage area.  If your antenna height is less than one thousand feet above the desired coverage area, you do not want to use beam tilt.

When an engineer designs a system, all of the factors go into the selection of the antenna site, antenna mounting, antenna type, coax feed line attenuation, plus the transmitter power and other pertinent factors to meet the coverage requirements of the system being designed.

IN PART 2, WE WILL LOOK AT REAL-WORLD SITUATIONS AND OTHER ISSUES THAT GREATLY AFFECT SYSTEM PERFORMANCE