On Sun, 28 Dec 2003 22:20:17 +0000 (UTC), "Reg Edwards"
wrote:

Could someone please give me an idea of the field strength, in terms of
volts/meter, which broadcasters expect will provide a MINIMUM SATISFACTORY
service in various sorts of service areas.


With respect to MW broadcasting in the US, the FCC has established a
complex set of values. Here's a quick summary. These are found at 47
C.F.R. Sect. 73.182 if you want more detail.

The four classes of AM broadcast stations (Class A,B,C and D; these
relate to power and frequency) have in general three types of service
areas, i.e., primary, secondary and intermittent. (See Sec. 73.14 for
the definitions of primary, secondary, and intermittent service
areas.) Class A stations render service to all three areas. Class B
stations render service to a primary area but the secondary and
intermittent service areas may be materially limited or destroyed due
to interference from other stations, depending on the station
assignments involved. Class C and Class D stations usually have only
primary service areas. Interference from other stations may limit
intermittent service areas and generally prevents any secondary
service to those stations which operate at night. Complete
intermittent service may still be obtained in many cases depending on
the station assignments involved.

(d) The groundwave signal strength required to render primary
service is 2 mV/m for communities with populations of 2,500 or more
and 0.5 mV/m for communities with populations of less than 2,500. See
Sec. 73.184 for curves showing distance to various groundwave field
strength contours for different frequencies and ground conductivities,
and also see Sec. 73.183, ``Groundwave signals.''

(e) A Class C station may be authorized to operate with a
directional antenna during daytime hours providing the power is at
least 0.25 kW. In computing the degrees of protection which such
antenna will afford, the radiation produced by the directional antenna
system will be assumed to be no less, in any direction, than that
which would result from non-directional operation using a single
element of the directional array, with 0.25 kW.

(f) All classes of broadcast stations have primary service areas
subject to limitation by fading and noise, and interference from other
stations to the contours set out for each class of station.

(g) Secondary service is provided during nighttime hours in areas
where the skywave field strength, 50% or more of the time, is 0.5 mV/m
or greater (0.1 mV/m in Alaska). Satisfactory secondary service to
cities is not considered possible unless the field strength of the
skywave signal approaches or exceeds the value of the groundwave field
strength that is required for primary service. Secondary service is
subject to some interference and extensive fading whereas the primary
service area of a station is subject to no objectionable interference
or fading. Only Class A stations are assigned on the basis of
rendering secondary service.


Primary service area. Means the service area of a broadcast station in
which the groundwave is not subject to objectionable interference or
objectionable fading.


Secondary service area. Means the service area of a broadcast station
served by the skywave and not subject to objectionable interference
and in which the signal is subject to intermittent variations in
strength.

Intermittent service area. Means the area receiving service from the
groundwave of a broadcast station but beyond the primary service area
and subject to some interference and fading.



In addition, Section 73.24 of the FCC's Rules require that applicants
for MW broadcast licenses must demonstrate that their daytime 5mV/m
contour covers 80% of the population of the entire principal community
to be served, relaxed to 50% for stations in the 1605-1705 KHz band.


Jack K8ZOA