Everybody wants the top of the tower
Owners and managers of communications sites need to juggle zoning restrictions, technical issues such as intermod, loading capabilities of the tower and radiation exposure limits-while keeping an eye on profitability.
Church owns community repeaters, owns and operates radio communications sites and offers system design consulting services. He lives in Lebanon, NH.
Assume your new tower is finished and ready for occupancy. All zoning, FAA and local building requirements are complete. Usually towers are built with at least one user in mind, which can be broadcast, TV, cellular, SMR or a host of others. (Given the current public antagonism toward towers, especially in the Northeast, it is quite a feat to build a tower at all.) Obviously, one wants to maximize the rental income while providing an important benefit to the public.
Every prospective user wants the “top of the tower”-including the owner. If the prospect is a renter, things are different. The top of the tower has a distinct advantage in flat country, but in hilly or mountainous areas most of the elevation is gained by building on high terrain. The percentage increase in height gained by putting a 100-foot tower on a 4,000-foot hill is minimal. In this case, paying a premium rent to be at the top does not make sense because any location above the treetops will usually provide good radio coverage, especially if the ground drops off fairly steeply. The top of this tower will only provide two benefits: it allows more omnidirectional coverage, and it avoids damage from falling ice. Aside from the higher rent for the top of the tower, there are some other disadvantages in terms of greater transmission line loss, cost and, in the mountains of the Northeast above 2,000 feet, a dramatic increase in ice accretion, as shown in Photo 1 at the right. Many systems need directional coverage, so a side-mounted antenna lower down is often a better choice.
Tower owners and managers should be aware of the kinds of antennas tenants use. There are several commonly used frequency bands from 30MHz-50MHz to 1,800MHz-1,900MHz. In general, the lower the frequency, the longer the antenna, and in many cases, the higher the gain, the longer the antenna. Some protective filters in the 30MHz-50MHz band can be the size of a torpedo, and because the commodity you are dealing with is tower and building space, it may be appropriate to charge tenants in that band in proportion to the amount of space they occupy-or avoid them altogether. Antennas do not work well if they are side-by-side or too close to each other. Most communications engineers, equipment salesmen and technicians like to use “gain” antennas, which concentrate the signal toward the horizon, as compared to non-gain types, whose radiation is more nearly spherical-wasting energy toward outer space. In general, gain antennas are long, except for yagis, corner reflectors and panel types. A 10dB-gain antenna in the popular VHF band (140MHz-170MHz) or the UHF band (450MHz-470MHz) will be 18-21 feet tall. Shorter antennas with less gain are available and cost less, and should be used if a slight sacrifice in coverage can be accepted. A site owner/manager may not be in the business of designing systems, but his decisions will control site profitability.
There are other ways to deal with problems of tower and building space. Many tower sites use combiners, in which two or more radio systems in the same frequency band share a transmit and a receive antenna. The combining equipment is capable of handling several stations and can be provided by the site owner or by the users. The combiner consists of cavities: devices that are usually tuned at the factory for specific frequencies. A combiner can be provided with space for expansion, in which transmitter cavities and receiver multicouplers are added as the need arises. There should be an agreement in the site rental contract as to the ownership of this equipment, so that if a tenant leaves the site, he and the owner/manager know what belongs to whom. At the higher frequencies used by cellular and PCS, the combiner is usually built into the system because these users may have many frequencies and several antennas.
Another solution is to use a rental formula incorporating antenna length and height above ground. This can be tailored to adjust revenue to an acceptable figure. Some formulas include the volume of the radio cabinet and kilowatt/hour use. This allows the prospective tenant an opportunity for innovative selection of antenna and station equipment. A photographic “portrait” of the tower, with each antenna numbered, as shown in Photo 2 at the right, can be helpful, esp ecially after a few years of tenants coming and going. It is a good idea to post this photo in the equipment building and update it as needed.
If the tower is a large, substantial structure, some users, especially cellular and public safety systems, may need parabolic antennas (“dishes”) for point-to-point use. These devices are conspicuous and zoning boards hate them, but they are frequently necessary. They provide a large area for wind and ice loading, and their use should be approved by a competent engineer-preferably with the help of the tower manufacturer. In many cases, the function of a dish can be replaced by smaller structures such as corner reflectors, a panel antenna, a conditioned telephone line such as a T1 carrier, or a fiber-optic circuit. A local zoning board may not know much about such things-most do not even know what questions to ask-but they are getting more familiar with the technology all the time. Do not accept a dish on the tower unless the prospective tenant provides a written statement that there is no alternative and that he agrees to pay for an engineering stress analysis of the tower. A manager does not want one dish to stress the tower as much as several smaller antennas of possible future tenants.
Qualifications for a prospective tenant should always include the likelihood of interference between new equipment and existing systems, including interference to neighboring TV and broadcast receivers. On a well-run site, a compatibility analysis is required, at the expense of the new arrival. This should be done by an engineer fully qualified to do the computer study as well as to make recommendations for remedies in case of interference. There are simple computer programs to use for a preliminary look. If “Joe Technician” offers to do this, don’t discourage him, just see that the final product is from a fully qualified engineering service. The rule, on all sites that have rules, is that the new arrival is responsible for curing an interference problem whether it is to existing systems or to his own receiver(s). (See “Resolving Digital-analog ESMR-SMR Interference,” MRT March 1998.) Some sites are run as a real estate operation, whose owners have little, if any, interest in happy tenants. “Not for nothing” are some of these sites nicknamed “Intermod City,” as shown in Photo 3 at the left.
If your tower is favorably situated with respect to population centers, interstate highways and other main roads, potential tenants will include cellular, PCS, SMR, FM broadcast, TV, paging, local, state and federal agencies, electric and gas utilities, community repeaters and many others. An owner/manager will wish to obtain maximum revenue from the site commensurate with safe tower loading. Ask for a written contract with each tenant. Larger tenants will have “standard” contracts, which are carefully worded to protect the tenant, with little regard for the “landlord.” A cellular company will often want a phrase excluding other cellular or telephone companies. At one time, some companieswere claiming at zoning board hearings that there would be interference between cellular companies, and they were mutually incompatible. This was mainly competitive nonsense, and much to the disadvantage of the tower owner. Many cellular companies are now “collocated” because of pressure from zoning boards-their systems work fine. Any contract presented to a prospect will automatically cause his lawyer to rewrite it from start to finish. One should use the same basic contract for all tenants, rather than 10 or 15 variations. Usually the cost of electrical energy to run a 100W base station or repeater is less than $10 a month, which can be incorporated in the basic site rent.
However, higher powered paging stations with a heavy duty cycle, plus cellular and PCS, are much heavier energy users and should be metered separately. At a remotely located site, it is often not feasible for the power company to meter a number of small loads. This can be done by means of individual, solid-state meters connected to each station. The meters can be connected by an electrician without interruption to the station and will cost $500 to $600 installed, usually at the expense of the tenant. Be sure your billing agrees with local utility rates because site managers are not in the utility business. A privately owned watt- hour meter installed beyond the transfer switch of the standby generator will also record energy from that source.
Aside from the usual terms of lease agreements, there should be included a phrase covering Consumer Price Index (CPI) changes, which are traditionally small changes upward. To accept a flat rate for ten years, ignoring a 2% yearly CPI increase, means income from this source will have decreased 20% in buying power. When the time comes to renew a rental contract, there will be less “sticker shock” if the CPI has been applied all along. CPI applied each year is compound interest. Current CPI figures are available on a continuous tape provided by the U. S. Department of Commerce.
The FCC recently set new and more stringent requirements for human radio frequency (RF) exposure, and most radio sites must have a study performed to evaluate their radiation. (See page 10.) It is the responsibility of the licensees to provide evidence of compliance, and new FCC applicants should not expect to receive a license unless the site has had a survey. Several engineering companies are equipped to do these surveys. They will provide documentation for the site, as well as follow-up services in which the effects of new tenants’ transmitters are added to the original study.
In many cases a site with a few “push-to-talk” base stations and repeaters will not produce enough radiation to exceed exposure standards, either to the public on the ground or to tower workers. In some cases, however, areas on the ground may be subject to radiation above safe limits and warning signs or fencing may be required. The FCC guidelines assume that all transmitters are energized all the time, which is probably the only way to view the situation because duty cycles for communications stations are wildly variable. All tower owners are now required to file with the FCC an “Application For Antenna Structure Registration” (FCC Form 854). A copy may be obtained by calling 1-800-418-3676.
For more details on FCC guidelines, obtain a copy of OET Bulletin 65 from the FCC Web page: http://www.fcc.gov/oe’/info/documents/bulletins/#65. If the tower is on a busy “antenna farm,” the situation may be exceedingly difficult, considering the many owners, licensees and types of systems. Although the responsibility to the FCC is strictly the licensees’, it is obvious that the site manager is the person to make the radiation study happen. Although the FCC exempts certain installations from conducting an RF evaluation, one should know if and where the radiation limits are exceeded to protect workers and the public from overexposure.
While people on the ground near a tower will receive much less radiation than workers on the tower, the radiation threshold for those on the ground has been set five times “safer” than for site workers. However, at an “antenna farm” with closely spaced towers, climbers may be exposed to several sources of radiation other than from the tower on which they are working, where they may have some control over associated stations. This implies cooperation among all the systems using the site, which can only be achieved with the help of a site manager. For worst case situations, radiation protective suits are available.
In the case of complaints or inquiries from zoning boards, hikers or others who may gain access to the site, there should be a “Health and Safety Plan” in place, which includes an RF evaluation of the site, and shows that the owner/manager is proactive and earnest in his quest to assure safety to those who come to the site.
