AM stations rarely have regular listeners outside the 1 mV/m groundwave contour because noise levels preclude interference free service, and because few if any stations have night service to areas with weak day signals.  The solitary exception are class A stations which have large service areas and interference free night groundwave coverage.  Since most revenue for AM stations comes from within the 5 mV/m contour, it makes sense to optimize that coverage, since any coverage at below 1 mV/m is of marginal value.  The increase of the 5 mV/m signal is most easily achieved with greater power. In order to do this, increasing the domestic protected contour to a value that recognizes the increase in noise in the environment is required.  An increase to 1.0 mV/m for the Class B, C and D stations and the the 0.5 mV/m contour partially compensates for the overall noise increase in rural areas and increase in automotive noise interference..

 Protected Contours

In addition to an increase in protected contour, additional adjustments for the higher noise levels in highly populated areas are needed.  This may be readily accomplished using computer mapping and the information available from the US Census.  One proposal would be to delete any census tracts from an AM service where the population density exceeds 100 persons per square km and the signal level is less than 2 mV/m, and additionally to delete any census tracts where the population density exceeds 2000 persons per square kilometer and the signal level is less than 5 mV/m.

Another proposal is to delete from the protected area places within the 1.0 mV/m groundwave contour separated by more than 10 km of water from the main service area., unless it it within the same county, parrish, metropolitan area or Arbitron survey area as the principal community of the station.  The main service area is the area listed above, contiguous with the principal community of the station.

Protection Ratio

Analog

Co Channel protection ratio of 26.5 db has been built into the system since its inception, and has served well for Analog modulation.  The proposal is to maintain the current cochannel protection ration.  Mutual interference on first adjacent channels is built into the allocation system, as a result the service areas of first adjacent channel analog stations cannot overlap.  The transmitted bandwidth of the AM signal extends to the center of the first adjacent station.  No filtering can remove this overlap, so there needs to be a clearance between service areas.  The present  6 db protection ratio partially meets this need, and going forward seems appropriate.

Second and third adjacent channel protection  for analog modulation provides compensation for receiver filter and overload deficiencies and for undesired AM transmitter noise and emissions.  AM HD Radio Systems are the primary reason for second and third adjacent channel  protection.

Digital Modulation

AM HD Radio digital modulation in modes MA-1 and 2 (Hybrid) have co-channel protection ratio requirements very similar to Analog radio.  Digital only modes MA-2 and MA-3 are designed to operate with the present cochannel protection ratio for Digital to Digital system operation.  The interference potential for Digital only to Analog or Hybrid modes may be marginal or inadequate, but this interference will be transitory because Digital only operation is uneconomic until there is adequate receiver penetration to make complete conversion viable.  The upper and lower reference signals may need special protection consideration during night operation, since selective fading may cause unlocking of the system when an interfering signal is also carrying an AM HD Radio signal.  The present protection ratio is so locked into the system that any changes would have little effect due to the large number of stations allocated under the present standard.

The present FCC protection scheme  for second and third adjacent channels in the context of AM HD Radio is counterproductive.  It encourages the location of second and third adjacent channel stations at the edge, but within of a station’s service area. The weaker portions of the station service area is where the greatest interference will be caused by second or third adjacent channel signals.   Much less interference would be caused if the stations were clustered together where the strong desired signals will override the digital hash.  Interference will be minimized.  The  historic second adjacent channel overlap requirement of keeping the 25 mV/m contour clear of the 2 mV/m contour made more sense than the present defective 5 mV/m vs 5 mV/m standard because it kept the strong signals of the second adjacent channel station outside the primary  service area (within the  2 mV/m) of the desired station.  As a compromise between the international protection ratio and the interference potential of AM HD Radio, a second adjacent protection ratio of -14db is suggested for calculating the interference area caused by a second adjacent channel within the service area of an AM station.

Third adjacent channel interference in the context of AM HD Radio is the result of receiver overload, transmitter noise and receiver filter deficiencies in both Analog and Digital modes. There is no particular logic to avoid overlapping 25 mV/m contours.  No matter where the nuisance transmitter is within the service area of the “desired” signal it will cause interference.  If the two stations are similar in power the interference will be minimized by clustering the transmitters together instead of causing widespread interference in another area where the desired station’s signal is weaker.  Third adjacent stations will always cause some interference in the immediate area of each transmitter.  A methodology for mitigating that interference is required.  Overlap prohibition is a counterproductive allocation strategy for second and third adjacent channels.  One method is to permit a certain percentage of the service area and/or population of any station to receive interference from any second (and third) adjacent channel stations within its service area.  Existing areas of calculated interference would be grandfathered, and could be moved, but not increased.  Mutual power increases would maintain the same interference areas.

Alternate digital broadcast systems, such as DRM-30 are viable alternatives to HD Radio and have, even in their hybrid Analog-Digital modes an ability to cause much less interference to stations which share a service area or are in adjacent service areas.

Use of interference analysis

Presently, the FCC uses area and population of contour overlap in its analysis of allocations.  Grandfathered overlap may not be increased, nor caused to a station not already receiving overlap.

Overlap area does not accurately reflect service loss and gains..  The FCC should use both interference and service as the criterion of acceptability of an allocations situation.  Overlap is only an administrative convenience.  Computer analysis can properly determine the amount of service that will be gained and lost in a given allocations situation, and administrative convenience should not trump accuracy.

Overwater Paths

In addition to the limitation of service areas for water paths above, one proposal is to ignore nuisance signals received over a path which contains  more than 100 km of salt or brackish water.

Ground Conductivity

In recognition of the fact that for much of the country the FCC Figure M-3 overestimates ground conductivity, one proposal is to halve all M-3 conductivity values used in analysis of contour locations, while either permitting measured ground conductivity, or eschewing measured data.

Transitional Hours

When stations operate with high power during the hours proximate to sunset and sunrise (critical hours) there is a potential for interference between stations during periods of low sunspot activity. Protection of stations in other than  class A from  daytime skywave interference should be provided.  There is a separate document on this site to discuss this consideration, along with operation after local sunset and before local sunrise.

Negotiated Interference

The licensees of radio stations understand the relative importance of various portions of their service area.  The FCC should permit licensees to accept new interference to some maximum permitted level so as to maximize service.  One proposal is to permit negotiated interference up to 5% of a station’s service contour.