By Garrison C. Cavell and Erwin G. Krasnow
Performing due diligence is critical when you’re planning to buy a radio or TV station. Last week, Cavell, Mertz & Associates President Garrison Cavell and Garvey Schubert Barer attorney Erwin Krasnow described how to assess a station’s true coverage. Today, they delve into what those contour maps are really telling you.
You should know that, for FM and TV stations, the coverage map should not just show simple coverage contours – while these are useful for FCC administrative purposes, and are relatively easy to understand, they don’t really tell the coverage story and often are misleading. Instead, use a coverage analysis that gives you more of a “real world” picture of the station’s reach – we typically recommend a Longley-Rice “terrain limited” map. These maps display coverage using overlays of color keyed to different signal levels, so you can see discreet areas of coverage, weakness, and interference areas. People are often surprised at the extent to which coverage may extend, or be significantly reduced, from what the coverage contours suggest.
For FM and TV stations, coverage is a pretty much a “line-of-sight” matter – if the receiving antenna cannot “see” the transmitting antenna, the received signal level will be low or non-existent. Longley-Rice maps carefully consider the line of sight paths from the transmitting antenna across all of the intervening terrain in the coverage area while the conventional FCC coverage contour maps only consider the terrain starting at 2 miles and ending at 10 miles from the transmitter site – that leaves a lot of room for things to be missed. We recommend that you ask the map preparer to use a receiving antenna height that is typical of what your listeners would use. The default for FCC contour maps and most Longley-Rice maps is a 30-foot high receiving antenna – most people listen at antenna heights of 6 feet above the ground or less.
Occasionally you will hear a fluttering in a station’s signal – this is called multipath and is typically caused by reflected signals off objects and terrain. Often there is very little you can do to overcome multipath, other than changing antenna location. In the worst situations, a change in location merely relocates the multipath area. On occasion, a change in antenna design (changing antenna type or bay count) will help. Multipath effects also can occur at stations with badly tuned antennas or station RF systems with problems. If the multipath is unique to one station and does not occur at other stations that are similarly situated, there is often a technical reason involved. If you are still interested in purchasing the targeted station, a consultant should be called in to investigate possible causes and remedies.
Also, the benchmark signal strengths people reference in coverage maps by rote often bear no resemblance to what are the truly meaningful coverage levels. Ask your consultant for a map showing coverage that recognizes today’s reception environment. What is the sufficiency of received signal strength for your station? For FM stations, suburban coverage is expected at signal levels of 1 mV/m (60 dBµ) or higher. City coverage levels are considered to be 3.16 mV/m (70 dBµ) by the FCC, but higher signal strengths are often needed in heavily urbanized areas.
Satisfactory stereo reception is often possible, in the absence of interference, at signal levels as low as roughly 0.25 mV/m (48 dBµ). Mono reception, when no interference is involved, can often be achieved down to 50 µV/m (34 dBµ), but this is NOT a typical listening level – only station owners and the mothers of DJs report reliable coverage at this modest signal strength.
It is important to know that the FCC “protected” FM contour is 1 mV/m for most classes of stations, except for Class B stations where it is 0.5 mV/m (54 dBµ) and Class B1 stations, where it is 0.7 mV/m (57 dBµ). While real world service may extend beyond these protected contour limits, keep in mind that it is not protected from interference by the FCC.
For AM stations, local coverage is a matter of radio waves that travel along the ground (“ground wave propagation”), like the waves in the ocean. Line of sight is not a major factor and only the most severe terrain impacts local AM coverage. For AM stations, signal density maps can be developed (just like the Longley-Rice maps in appearance, but using a different prediction method), but these maps are not commonly used. Instead, most individuals rely upon conventional contour maps since in the case of AM, they provide a reasonable representation of statistical signal levels.
In evaluating AM stations, there is a general consensus that the 2 mV/m contour is the most useful measure of effective coverage. This signal level seems to work in populations of 2,500 or higher, but stronger signal levels are needed in urbanized areas and downtown regions. The FCC requires a minimum of 5 mV/m for “city of license” coverage, but as a practical matter these days, this is more of a suburban or small city signal level. We often suggest 25 mV/m or higher (often 40 mV/m) if congested area “downtown” coverage is essential, and as much as 14 mV/m may be needed in densely populated suburban areas. Yes, reception is possible in rural areas at 0.5 mV/m level or less, but the growing ambient noise levels from energy saving and computerized devices typically make this impossible.
At night, incoming interference from distant stations occurs because their signals bounce off the ionospheric layer of the atmosphere. This can have the effect of limiting usable reception, not unlike entering a crowded room and trying to enjoy a conversation. When this happens, you need to stand closer or talk louder – such is the case for nighttime AM reception. Your consultant can predict the expected “nighttime interference free” signal level for you and plot that on a map so you can visualize the realistic nighttime service you can expect later in the evening. If nighttime signal reach is important (ball games and other nighttime events come to mind), you need to take a critical look at the reach of the station at night and consider if any means are available for improvement.
The Next Steps There are several pre-purchase items that should be reviewed before approaching the closing table. In this article, we touched the tools you need to start thinking about whether your proposed acquisition will give you the kind of performance you need to achieve your goals. Next we’ll help you look under the hood and evaluate engineering paperwork traps.
Garrison C. Cavell, the President of the Cavell, Mertz & Associates, Inc. consulting firm, has worked in management and technology positions in communications and broadcast systems for over 40 years. Gary is a former President of the Broadcast Technology Society of the IEEE and is presently the Editor-in-Chief of the 11th Edition of the NAB Engineering Handbook. He and his staff provide professional services to a wide array of facilities and organizations across the country and are regularly engaged to speak at industry functions on engineering and executive management topics. He can be reached at [email protected] and (703) 392-9090.
Erwin G. Krasnow, the co-chair of the Communications Group of Garvey Schubert Barer, is a former General Counsel of the National Association of Broadcasters, Washington counsel to the Media Financial Management Association, coauthor of Profitably Buying and Selling Broadcast Stations and Washington counsel to the Media Financial Management Association. He can be reached at [email protected] and (202) 298-2161.
