Detuning Wireless Communications Tower
Near AM Broadcast Station Antennas
A Primer for Non-Engineers
B. Benjamin Evans, P.E.
Approximately 40% of the AM broadcast stations operating today in the United States use a directional antenna, that is, an antenna consisting of two or more vertical radiating towers placed close together (either in a line or in some geometric pattern), and whose RF current magnitudes and phases are controlled in such a way as to achieve desired levels of radiated signal intensity, or field strength, in certain directions. This “directional radiation pattern”, as it is known in the industry, is characterized by high levels of field strength in one or more directions, and lower levels of field strength in other directions. As you would expect, the received signal in the direction of high fields is greater than that in the direction of low fields. Many directional radiation patterns have “nulls”, or places in the pattern where the field strength is extremely low or virtually non-existent.
In contrast, a non-directional, or single-tower, AM antenna produces essentially the same level of radiated field strength in all directions (think of a circle whose center is at the AM tower).
Underlying reasons for using a directional antenna are: to increase the signal in a major population area, to avoid wasting RF power over unpopulated areas such as large bodies of water or mountains, and, probably most important, to avoid the occurrence of interference to other AM stations on the same or adjacent frequencies.
Whatever the reason for using directional antennas, AM stations are licensed by the FCC to maintain their radiation pattern at or below specific levels of field strength in specified directions from the antenna. The problem that directional AM stations face is that other metallic structures of 100 feet or more that are close to the AM antenna array, such as power line supports, water towers, or other antenna towers, can reflect, or re-radiate, the AM signal back to the AM radiation pattern and change its shape. The effect can either increase or decrease the AM signal, depending on the distance and direction to the AM antenna. In either case, the effect is undesirable; if the signal decreases in a certain direction, the station could lose listeners, and a signal increase somewhere in the pattern could cause the station to operate outside the field strength levels specified in its FCC license.
The problem is particularly acute when the re-radiating object is in the direction from the station where the radiated field strength is highest, and the AM directional pattern has very deep nulls. The reflected or re-radiated signal, when mixed with the original signal, could then substantially increase the radiation levels in the nulls of the pattern, resulting in operation outside of the license.
Non-directional AM stations are not as susceptible to re-radiation effects, since their radiation patterns have no nulls; the field strength is essentially constant in all directions. However, if a re-radiating object is within a half-mile of a high power AM non-directional station (10 kilowatts or more), the circular radiation pattern of that AM station could become measurably distorted.
The common method for mitigating this re-radiation effect is to detune the re-radiating tower or structure, in other words, to make it electrically invisible to the AM station’s signal. In this paper, we deal with how to detune towers and monopoles used in the wireless communications services (cellular and PCS).
Explosion of Wireless Tower Construction
Since the advent of cellular and PCS mobile phone service, there has been an explosion of wireless base station tower construction in every corner of the country. Some of these towers were built less than two miles from directional AM stations, and caused the stations to operate outside of FCC limits. The heights of these towers often approach one-quarter of a wavelength on the AM band (150 feet or more), and therefore make the towers formidable re-radiators.
In the mid-1990’s, the FCC acknowledged the problem, and amended their rules to spell out the responsibility of the wireless service providers who propose to build a tower in the vicinity of an AM station to assure that the tower does not adversely affect the operation of the AM station. SEE COPY OF RULES. The responsibility does not stop at wireless services; it applies to any FCC licensee that builds a transmitting tower. Furthermore, all expenses incurred in detuning a tower and/or taking measurements to confirm that the tower does not affect the AM station must be borne by the proposed tower owner.
It is up to the wireless service provider to search for and identify all directional AM stations within two miles and non-directional AM stations within 0.6 mile of a planned base station tower. The FCC has a program on its web site that performs such a search. Any AM stations within the distances specified must be notified prior to construction. Next, a preliminary determination of whether any stations within these distances would be adversely affected is conducted. This determination is best performed by a broadcast consulting engineer experienced in re-radiation disturbances to AM station operation. If the determination finds that an AM station’s radiation pattern would be significantly affected, the re-radiating structure should be detuned.
What does a tower that is “detuned” mean? It means to eliminate or greatly reduce the RF current induced into the tower by the incoming AM radiated field. It is this induced RF current that causes the tower to re-radiate. The most effective method for detuning a short tower or monopole (of a quarter-wavelength or less on the AM frequency) is to install a detuning skirt (also called a folded unipole). The detuning skirt reduces the induced RF current in the structure to the point that the tower does not re-radiate significantly.
A detuning skirt consists of at least three wires about a 3/8 of an inch thick, space equally about the structure and strung alongside the length of it, between 18 and 30 inches out from the face of the structure. At the top of the skirt, the wires are bonded to the tower, and at the bottom, the wires are connected together by a wire “hoop” around the structure. Insulators are placed between the hoop and the mechanical attachment of the bottom of the skirt to the tower. Neither the skirt wires nor the hoop make electrical contact with the structure, except at the top of the skirt. Stand-off insulators are used to prevent long lengths of skirt wire from oscillating in the wind and striking the structure. A pigtail wire connects the hoop to an electrical tuning circuit. This circuit is usually housed in an aluminum weatherproof box which is mounted near the bottom of the tower where it can be easily accessed. The other side of the circuit is connected to the structure’s electrical ground. See the schematic sketch of a typical detuning skirt on a tower.
The tuning circuit usually consists of a variable vacuum capacitor, or a fixed value capacitor in series with an inductor. The tuning circuit is the apparatus that, when adjusted, brings the RF current in the structure to a minimum or near zero, thereby achieving the detuned condition.
If you’ve been exposed to RF transmission line theory, you can think of a detuning skirt as a quarter-wave coaxial transmission line with one end shorted (the top of the skirt attached to the tower) and the other end open (the hoop). The tower is the center conductor and the wires form the outer conductor. This “quarter-wave stub”, as it is called, has a very high impedance at the open end, which effectively chokes off the current into the tower.
Before installing detuning equipment on a structure, it is vitally important that all transmission lines on the tower be grounded to the tower at intervals of no more than 50 feet. Also, if the structure is a guyed tower, the guy wires should be broken up with insulators every 50-70 feet (which should be done when the tower is being built). The reason for doing this is that transmission lines and guy wires can also re-radiate in the presence of strong AM fields, and the detuning skirt will do nothing to minimize those induced currents. Insulating the guy wires and grounding the transmission lines will electrically break them up, thereby preventing significant RF currents from forming.
If any antennas are added to the structure after it is detuned, the tuning of the skirt should be checked for minimum RF current, and adjusted if necessary.
If a re-radiating structure must be detuned for more than one AM station, it is practical in some cases to install additional wire skirts, each with its own tuning circuit.
Towers taller than a quarter-wavelength can be detuned in this manner by using two or more skirts, one above another.
Another method of detuning a tower is to build the tower on an insulator, thereby breaking off any RF current. However, this method has significant disadvantages. All power and transmission lines must be electrically isolated from the tower at the AM frequency to prevent them from shorting the tower to ground, which can be very troublesome and expensive to accomplish. Also, when the tower is insulated in this manner, the only continuous path for lightning strikes to dissipate is through the transmission lines, which exposes the transmitting equipment to significant risk of damage (installing a spark gap across the base insulator will reduce the likelihood of this happening). Furthermore, a tower taller than a quarter-wavelength at the AM station frequency may require insulators between tower sections in addition to the insulator at the base.