Do Water Towers Make Bad Antenna Sites?

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.
As seen in the 4/12/11 edition of the RF Globalnet (www.rfglobalnet.com) newsletter.
Do Water Towers Make Bad Antenna Sites?
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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
As seen in the 4/12/11 edition of the RF Globalnet (www.rfglobalnet.com) newsletter.
Do Water Towers Make Bad Antenna Sites?
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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, 7th, 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.
As seen in the 4/12/11 edition of the RF Globalnet (www.rfglobalnet.com) newsletter.
Do Water Towers Make Bad Antenna Sites?
Transmit Antennas
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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.
As seen in the 4/12/11 edition of the RF Globalnet (www.rfglobalnet.com) newsletter.
Do Water Towers Make Bad Antenna Sites?
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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.
As seen in the 4/12/11 edition of the RF Globalnet (www.rfglobalnet.com) newsletter.