Do Water Towers Make Bad Antenna Sites? By Alfred T. Yerger II, RF Engineering Specialist, Bird Technologies Group I'm often presented with the question, "Do water tanks make bad antenna sites?" The answer I give is what my wife calls the consultant catch-all answer it depends. There are a number of factors that determine whether a water tower system will be effective or not, including the type of water tower or tank, the frequency band of the system, the presence of other systems, the need for omnidirectional coverage, and the intermodulation situation. At first glance, the classic, iconic water tower would appear to be an ideal antenna site. First, water towers are usually placed in high locations in order to produce sufficient water pressure. Second, there are usually few zoning issues because the structure is already in place, and adding a few antennas won't make the thing look any uglier than it already is. Third, particularly for public safety systems, the land is often already owned by the local government or water authority, making the site available for little or no monthly rent. Despite these obvious advantages, there are also some downsides to the typical water tank that, depending on the nature of the proposed system, could or should be major show stoppers. Antenna Location Depending upon the type of tank, there may be a limited number of locations for mounting antennas. For a cylindrical tank or a water sphere design, you are probably limited to the top of the tank, although I have seen some cellular antennas mounted around the vertical support of a water sphere. The top is good for omnidirectional coverage, but there is very little real estate available, and all of the antennas are going to be horizontally separated. This creates very close antenna spacing, which results in poor antenna-to-antenna isolation.
Do Water Towers Make Bad Antenna Sites? 2 of 5 The more traditional design, as shown in Figure 1, has a walkway around the perimeter, providing a railing to which antennas can be mounted. This is fine for panel antennas or other directional antennas. Obviously the tank represents a significant obstruction to omnidirectional coverage, but if you are only interested in sector or directional coverage, this is likely to be acceptable. Figure 1: Typical water tower antenna site When the railing is not available or unable to support the additional stress of the antennas, sometimes mounts can be welded to the side of the tank. This can present some challenges, as the tanks are often lined on the inside with materials such as plastic that cannot withstand the heat of welding. To make matters worse, sometimes the people at the water authority that negotiate the use of the tower either don't know or don't realize this issue, resulting in a major problem to resolve later. Intermodulation All of the issues previously mentioned can be managed as long as you understand the concerns and limitations. However, intermodulation (IM) could pose additional, unavoidable problems for a water tower site. For certain users and frequency bands, IM may not significantly affect system quality. Obviously, transmit-only users like paging or low-power broadcast won't have an IM problem, although they may cause IM issues for other systems. Land mobile and public safety frequencies above 700 MHz will also have little or no problem, due to the large guard
Do Water Towers Make Bad Antenna Sites? 3 of 5 band between transmit and receive frequencies, which eliminates most low-order IM products. On the 800 MHz band, for example, the transmit-to-receive frequency spacing is 45 MHz, and the spacing between the lowest infrastructure transmit frequency (851 MHz) and the highest infrastructure receive frequency (824 MHz) is 27 MHz. Few systems have frequencies spread across that entire frequency range, so most real-world systems will be immune to intrasystem IM beyond the 9th order. The same applies to cellular and PCS, which is why you see many water towers sporting a good number of cellular antennas. The bands that are most affected by IM are VHF and UHF land mobile. These frequencies have essentially no guard band between the infrastructure's transmit and receive frequencies, making them highly prone to low-order IM. We strongly advise our customers to avoid installing frequencies at any site that has the potential to create 3rd order IM. However, under the wrong conditions, 5th, 7 th, and 9th order IM products can also be a problem. Unfortunately, water towers tend to support these troublesome conditions. The first IM condition is close antenna spacing. When the antennas are clustered together at the top of the tank, there is little spatial isolation, and antenna-to-antenna isolation is one of the biggest factors in IM mitigation. The second IM condition is based on the structure of the tank or tower itself. Water towers were designed and built to contain water, and no thought was given to how the structure will affect radio frequency energy. Thus, most towers are constructed of steel. Steel is a ferrous metal, which means it is affected by magnetic fields. When electromagnetic energy flows through a ferrous material, the magnetic field is distorted, opening the door for mixing and intermodulation. But aren't radio towers made of steel? Yes, but that steel is galvanized with a thick coat of zinc. This is usually accomplished by dipping the completed tower or tower component into molten zinc. Due to something called the skin effect, RF energy tends to only flow on the surface of a metal conductor (the higher the frequency, the thinner the depth of the current). As long as the zinc coating is thicker than the skin depth, the RF current will only flow on the zinc coating. Since zinc is a non-ferrous material, the coating doesn't distort the magnetic field, thereby greatly reducing the tendency to produce IM. Since water towers are not normally galvanized, they often act as excellent IM generators. Energy from a transmit antenna induces an RF current in the nearby steel structure. If multiple frequencies are present, they will mix together in the steel to produce the various sum and difference frequencies, which we call intermodulation. The steel then conducts the IM products throughout the structure. We have observed IM levels at receive antennas on water towers (as well as other non-galvanized structures) at levels much higher than one would estimate, based only on antenna separation and free-space loss.
Do Water Towers Make Bad Antenna Sites? 4 of 5 Transmit Antennas Receive Antenna Figure 2: Water tank with UHF panel antennas Figure 2 shows a water tower with three UHF panel antennas for transmit and a UHF panel antenna for receive. The receive antenna is located by itself (on the right), away from the transmit antennas (on the left). Due to the curve of the railing and the orientation of the antennas, the receive antenna is actually slightly behind the transmit antennas. This should provide high transmit-to-receive antenna isolation (actually measured at nearly 60 db of isolation). Despite the excellent antenna isolation, we were still able to measure 3rd order IM at -105 dbm at the receiver input. Since the two frequencies involved were on different antennas, it is extremely unlikely that the IM was generated within the transmit network. The IM observed was significantly higher than we would expect for externally produced passive intermodulation. The conclusion is that the RF from the transmit antennas is being coupled into the structure of the tank or the railing where the IM is being produced, and then conducted along the structure to the receive antenna. We have experimented with this type of IM in the past on building tops using a non-galvanized steel beam as an antenna mount. Measurement of the IM along the beam showed that the level remained relatively constant despite increasing distance from the transmit antennas. We believe that this same mechanism is in play in water towers, as well as any other ungalvanized steel structures.
Do Water Towers Make Bad Antenna Sites? 5 of 5 Conclusion Our conclusion is that water tanks and water towers can make excellent antenna sites if you are aware of the possible pitfalls and take the appropriate precautions from the beginning. With regard to intermodulation, we would recommend avoiding all IM products up to the 7th order and maximizing antenna-to-antenna spacing as much as possible. In addition, test the system for IM at your first opportunity, as products could be caused by mixing with other systems on the tower. You don't want to wait until the day of the coverage testing to find out that you have a problem. About The Author Alfred T. Yerger II is an RF engineering specialist for Bird Technologies Group, specializing in field engineering for the land mobile industry including antenna site design, noise and interference, and communications system coverage issues. Mr. Yerger has been working in the radio communications and broadcast industries since 1974, including 18 years with Motorola Communications. He joined Bird Technologies Group in January 2005, after running his own business, Antenna Site Technology Inc., for 6 years. He is the senior engineer in Bird s Site Optimization Services (SOS) department. In this position, Mr. Yerger is responsible for supporting Bird s spectrum monitoring and noise measurement services, interference mitigation, and training for the land mobile communications industry.