Friend and foe
A few months ago, I was contacted by a municipality that had just upgraded its single-site 800 MHz public-safety trunking radio system to a two-site simulcast system. Instead of getting more range on the new system, they got considerably less. The system was designed by one of the major manufacturers in the business, but the city chose the towers the system would use. The manufacturer used that as the basis for determining that the system needed more towers to be effective.
My initial diagnosis of the problem was that tower shadowing was blocking the radio signals, a finding that both the radio and antenna manufacturers immediately rejected. I was hindered in supporting my analysis of the situation because little has been published that explains tower shadowing. This article was written to help engineers, technicians and radio system managers better understand tower shadowing so that they can properly use it.
In every radio system, the antenna is the most important component, as it sets the range of the system. The height of the antenna sets the theoretical maximum range, but the actual range usually is reduced by obstructions, such as trees and foliage, buildings, hills and ground-elevation obstructions. In addition, the mounting location on the tower or rooftop greatly affects range. Sometimes, engineers use the tower shadow to protect against interference to and from other systems.
The ultimate place to mount an antenna is the top of a tower with no other antennas nearby. In real life, however, most towers have multiple antennas at the top, and because the tower has plenty of room on its sides, there are as many antennas in use there as on the tower’s top.
The lone antenna at the top of a tower will have a 360°, or omnidirectional, radiation pattern. If you place any object near the antenna, such as another antenna, the radiation pattern will be offset. This offset pattern can be predicted; indeed, antenna manufacturer catalogs usually depict how the pattern is distorted from the tower.
The same goes for building rooftops. In fact, buildings have an advantage in that each antenna is distorted less by the other antennas, as there is usually enough room at a building site so that the antennas are not crowding each other and causing severely distorted patterns. There also is less attenuation on a rooftop’s coaxial transmission line for most sites.
In some systems, it is desirable to concentrate signal power in one or two directions, and to substantially reduce power in the directions where the signal is not needed or wanted. Some of the antennas that are designed to concentrate power in one direction and to significantly attenuate it in other directions include:
- Corner reflector
- 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.
You also can use the tower to shield one system from another on the same tower by placing the antennas on opposite sides of the structure, and letting the tower shadow keep each signal down by 20 dB (x 100 power) or more. You also can use vertical separation, which provides a great deal of isolation for each antenna, usually on the order of 40 dB (X 10,000 power) or greater. In these examples, the tower shadow or vertical separation is used to enhance the operation of the system.
In the system for the aforementioned 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 its radio coverage was so bad that the police department had to use cellular telephones to keep up with their field units.
A second city called, and again the initial diagnosis after making a field visit was that the tower shadow was the problem. Just as the radio manufacturer denied the problem in the first case, the manufacture involved in this case had the same reaction. Again, I made a detailed study of the signal strength at exactly 1 mile from the tower at 30° intervals, and the results were quite revealing. The area where 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, which proves that the problem with shadows is not band specific.
The one thing about radio waves is that they always obey the laws of physics. This fact allows engineers to very accurately predict how they will perform. The performance then can be measured and the results are always the same — which is why they are called empirical measurements. In fact, if the field measurements do not match the predicted values, a technician or engineer normally is sent 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, 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 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. Again, the difference between the predicted value and the measured signal strength at that site was within the measurement of accuracy of the spectrum analyzer. 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 that are pertinent to predicting the signal strength of the radio waves include:
- Inverse square law
- Free space attenuation
- Antenna gain
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 doubled the distance, field intensity would diminish 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. (See Equation 1.) This formula holds true as long as you have line-of-sight propagation.
Radio signals can bounce off 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, the field intensity could 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 literally can pass over the intended coverage area. You will see this in parts of the country where a mountaintop 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 achieve better coverage in the desired coverage area. If your antenna height is less than 1,000 feet above the desired coverage area, you do not want to use beam tilt.
Next month: A look at real-world situations and other issues that greatly affect system performance.
Ira Wiesenfeld, P.E., is a consulting engineer who has been involved with commercial radio systems since 1966. He can be reached at [email protected].