AM tower installation
Feb 1, 2001 12:00 PM, By John Battison, P.E., technical editor, RF
If you have the good fortune to be responsible for the installation of a directional AM station, you will find that there are many important details that must be carefully planned. Depending on the circumstances, these may be combined into major groups.
- Antenna System
- Primary power distribution
- Installation (station) wiring
- Audio and ancilliary wiring
The most important and critical part of the installation is siting the towers. In critical arrays, azimuths as close as 0.5 degrees are sometimes involved. The required accuracy can normally only be provided by a professional civil engineer or surveyor. Any error in placing antenna bases is extremely costly to correct once the array is completed. Locating DA towers used to be akin to taking a sight in celestial or marine navigation, referencing the Pole Star and using involved math. No matter how the site is plotted, absolute accuracy is essential.
The use of handheld GPS units is becoming very popular for tower siting; however, be sure that the model you use is referenced to the 1927 NAD. Many models use the new NAD and can be misleading. When siting a tower for an existing license, the surveyor will check your original site coordinates, and if necessary provide a corrected set of coordinates to file with the License Application Form.
When the consulting engineer modeled the antenna pattern, a point was selected to which all tower bearings are referenced. This is the reference tower. In the case of a simple two-tower array, there is only one tower bearing. In a multitower array there will be more. Every azimuth or bearing is measured from the True North Meridian, and tower spacing is measured from the reference tower and sometimes from others as well. Here also, absolute accuracy in spacing is essential. Azimuths are never measured from any meridian other than the True Meridian. Not observing this is certain to result in trouble.
Most surveyors measure from the Magnetic North Meridian. There is almost always a difference of several degrees between the True and Magnetic Meridians; this is called variation. USGS maps show the variation at the date when the map was printed and usually the rate of change. It changes slightly each year. Surveyors also express bearings as direction in one of the quadrants. For example, a surveyor would express a bearing of 015� as N-15-E, and unless otherwise specified, it would be magnetic, and could be as much as 5 or more degrees different from True. If you are working with a magnetic compass, be sure to avoid errors from nearby metals, such as poles, fences or chain lengths being used to measure radials.
It is essential that this point be completely clarified with the surveyor prior to commencing any layout work. When examining plats and site layouts and boundaries (which are measured from the Magnetic North Meridian) look for the meridian symbol. It is a vertical line representing north. If it has a full arrowhead, it is True, if only a half arrowhead, it is Magnetic. The Commission now requires a statement from a qualified surveyor certifying that the array coordinates and antenna layout angles agree with the Construction Permit values.
It is the usual practice to use sheathed, flexible transmission line between the phasor and ATUs. These line lengths will have been carefully considered by the consulting engineer when designing the phasing and ATU units. It is important to follow the lengths in the application. Any unapproved changes will probably put the pattern out of tolerance.
The choice of above-ground supported or buried coax is left to the engineer. Keep in mind that replacing a buried line often ruins the ground system.
The sampling lines should all have the same length to facilitate monitor calibration. These should be placed in a conduit and buried. To avoid errors due to erratic line changes, any excess line should be buried so that all parts of the line experience similar environmental conditions. The choice of current transformers or loops for detecting phase and current in the antennas depends on personal preferences, local conditions and tower height. I prefer current transformers safely ensconced in the ATU housing. The FCC also seems to prefer transformers.
Other cable runs to the dog houses include electrical power for convenience use, security lighting and perhaps heat, and tower lighting if required. A multipair cable is often installed for communications, signaling, monitoring and ATU switching and condition reporting. These should also be in conduit for ease of replacement.
The ground system is a complex issue. For more information, see the April 1999 issue of BE Radio in part four of the antenna series.
Primary power system
Power utilities usually allow stations to either own the main transformer or just use it. If a stations owns the transformer, it can exclude other users from tapping into the transmitter’s primary power circuit. This can avoid a heavy non-constant load from affecting primary power.
I always feel that the greater the distance that the line can be buried, the better it is for avoiding line surges. In any case, power from the transformer should come underground. Adequate surge protection is essential where it enters the transmitter building. Electrical power installations must contain a main disconnect switch to isolate circuits for servicing without endangering anyone.
If a standby generator is to be installed as part of the new facility, be sure that all the loads, including air conditioning, heat and ancillary equipment, are included in the power total. Ensure that the generator is checked out on full load as well as lower loads and is also exercised regularly for at least one hour each time.
The article that originally appeared as part of the antenna series on ground systems from the April 1999 issue of BE Radio can be found on www.beradio.com. Follow the link to this article online. Click on This Month in Print and then RF Engineering.
E-mail John at: firstname.lastname@example.org.