In the past few years nearly every radio station has been looking for an additional source of revenue. AM broadcast stations are no exception and are generally the most in need of a boost to the bottom line. One area that is frequently overlooked, yet provides a great source of non traditional revenue, is the vertical real estate of broadcast towers.
With the widespread acceptance of cellular phone service, many of the service providers are in need of a tower to hang their antennas on. Sometimes it’s easier to locate the cellular antennas on an existing tower, rather than go thru the process of building a new cell tower. This is where the AM broadcast tower has its appeal and cellular service providers are willing to pay the broadcaster for that vertical rental space.
AM broadcast towers that are located in areas that were once rural are now being surrounded by high density residential development. This residential growth is appealing to cellular service providers because, more residents can provide more potential cell phone users.
In the early days of AM broadcasting, attaching anything to an AM broadcast tower was considered bad engineering, an invitation for trouble. The iso-couplers could fail and short the tower to ground causing the station to go off the air. Any addition of small antennas to the tower affected the towers base impedance in bad ways. And of course if the tower was part of a directional antenna system, attaching anything to the tower created never ending grief.
Fortunately technology has progressed since those days and now there are solutions that allow a cellular antenna site to co-exist on an AM broadcast tower. Modern cellular technology has the RF transmitting gear located on the tower; this eliminates the long coaxial cable runs up the tower. The only cables going up the tower are data cables containing the phone calls.
For the AM broadcaster, one solution for cell service co-location involves the use of iso-couplers on all cellular site cabling. A good source for these particular iso-couplers is LBA & Associates and Kintronic Labs. They have complete solutions that will provide isolation between cellular equipment cables and a hot AM tower.
Another solution is to “skirt” the tower and use the skirt for the AM station. The tower is then grounded at its base and the cellular equipment is mounted to this grounded tower without the need of iso-couplers. There are several manufacturers that provide tower skirt kits. The kits include the skirt wires, mounting hardware and accompanying ATU.
A third solution is to allow a cellular service monopole tower to be built on your AM antenna property and “detune” the cellular tower. The process of detuning the cellular tower is common practice these days and is the least intrusive approach. Detuning the cellular tower essentially makes it electrically invisible at the AM broadcast stations frequency and eliminates the impact on your AM broadcast signal pattern. I know of one directional AM facility, which has 5 cell towers located within 100 feet of its 6 tower directional array…and the 6 tower array hosts two AM stations diplexed together. All the cell towers are detuned at both AM frequencies and the AM array is not impacted by them.
While I’ve addressed the co-location of cellular phone equipment, the techniques described can be applied for nearly any wireless service that wants to co-locate their equipment on an AM broadcast tower.
The next time someone wants to mount their antennas on your tower, think of the sound of money jingling in your pocket before you dismiss their offer.