Rule section 47 CFR 73.525 addresses the potential for interference from noncommercial educational stations operating in the FM band on Channels 201 through 220 (88.1 MHz through 91.9 MHz). Because the Channel 6 television band (at 82 to 88 MHz) is immediately below the FM band, interference will sometimes occur where a strong FM signal is received along with the Channel 6 television signal. The interference may appear as "herringbone" lines on the television set's picture, or the TV audio may be garbled or, the FM station may be audible over the television set. This does not mean that the FM station is operating improperly or that the FM station was established in the wrong place, but is rather a reflection of the capability of the individual TV receiver to ignore the unwanted signal.

It must be emphasized that interference does not always occur to Channel 6 reception, even when the television receiver is located well within the predicted FM interfering contour. Whether or not interference occurs is dependent on many factors, including the relative position of the television receiver with respect to the TV and FM stations, the channel on which the FM station is operating, the effective radiated powers (ERP) and antenna heights above average terrain (HAAT) of the FM and TV stations. Some generalizations can be made, though. Interference is less likely to occur when the FM noncommercial educational station operates on a higher frequency. Interference is also less likely to occur when the FM station operates with a lower power and antenna height above average terrain (HAAT). Using a rotatable rooftop TV antenna to pick up the TV signal increases the received TV field strength while (in most cases) reducing the received FM interfering signal. Rabbit-ears antennas can often be adjusted to improve television reception. Tuned filters (tuned to block the FM signal) are often helpful in resolving interference.

TV receiving installations which use signal booster amplifiers are not protected from interference caused by other sources, including noncommercial educational FM stations. This is because television signal amplifiers are typically wideband in nature, so that they can pick up the whole VHF range of television signals, from Channels 2 through 6, which lie below the FM band, to Channels 7 through 13, which lie well above the FM band. The wide band of reception causes the booster amplifier to amplify not only the TV signals, but the already strong FM signals as well. This unwanted amplification of the interfering signal itself can increase the perceived interference level. Thus, if interference occurs, the first step should be to remove or bypass the boosters to see if the interference is reduced or eliminated. Some amplifiers contain a switch to "notch out" the FM band: this may help to reduce the FM signal level.

The Audio Division has constructed a Internet program which can be used to approximately define the potential interference area around an FM noncommercial educational station. As defined in the rule, FM noncommercial educational stations are only required to address interference caused within the TV Channel 6 station's Grade B service contour (47 dBu, F(50,50) propagation curves). On the Tiger Census Map available through this program, this is shown as a blue circle around the TV station's transmitter site. An inner blue circle represents the Channel 6 station's 68 dBu Grade A service contour. Because application of 47 CFR 73.525 treats interference within the Grade A contour and Grade B contour somewhat differently, both circles are displayed.

The program will determine the approximate location and shape of the predicted potential interference area. Users may obtain a table of bearings and distances to the interfering contour, a list of the latitude and longitude points making up the TV Channel 6 service and FM interfering contours, Tiger Census maps referenced to the TV and FM transmitter sites (which can then be rescaled), or a copy of the Tiger Census #tms-marker file used to generate the map.

Users should be aware of the limitations of this stand-alone program:

• This program does not use actual terrain data, therefore the actual locations of contours will differ somewhat from the locations computed herein.

• The interfering contour is assumed to be symmetric around the line between the FM and TV stations.

• Because the "contours" are actually made up of a large number of individual points, discontinuities in the contours will sometimes be noticed. However, the shape of the remaining points is sufficient to establish how the gaps between points should be connected.

• Directional antenna patterns for either the FM or TV station cannot be entered into theprogram.

• If the predicted interference area encompasses the TV Channel 6's transmitter site, the program will halt. However, this will occur in only a very few cases where the FM and TV transmitter sites are close together.

• Population data is not computed. However, the Census Bureau's block centroids can be used to to find population information. If a block centroid point (latitude and longitude) falls within the interfering contour, include the population; if it falls outside, exclude the population.

• Map generation may sometimes be quite slow. This is because the map query is directed first to another FCC program to set up the structure, and then the map query is redirected to the Census Bureau's Tiger Map servers for generation. Additional information is available to explain the plotting program in more detail.

Despite these limitations, we believe that you will find this program useful as a starting point in computing the predicted potential interference to Channel 6 reception.

Coordinate, effective radiated power (ERP), and antenna height above average terrain (HAAT) for FM stations can be accessed through the FM Query, and data for TV stations may be retrieved through the TV Query.

Questions about this program may be referred to Dale Bickel dbickel@fcc.gov, 202-418-2706.