[BC] AM daytimers

[Dan Strassberg]

Paul: Your definition of a daytimer doesn't quite agree with the FCC's--but
you're close. The FCC defines a station as daytimer (that is, as a Class D
AM) if its nighttime pattern RMS is less than 140.85 mV/m @ 1 km. That's the
equivalent of 250W into an antenna of minimum efficiency for Class B and D
AMs (281.7 mV/m/kW @ 1 km). So stations that run 250W or the equivalent at
night aren't Class Ds (because they are over the maximum Class D night
power) and some stations that run less than 250W at night but have antennas
more efficient than the Class D minimum are also not Class Ds (because they
are over the maximum Class D night RMS). Class D AMs are the only class of
AMs whose nighttime service is considered secondary and therefore is not
protected from interference from other stations. Class D AMs are also not
required to provide any nighttime service to their communities of license.
Indeed the FCC does not require Class D AMs with nighttime authority to use
that authority. That is, they can remain silent at night if they choose to.

A few Class C AMs (AKA graveyarders) that run 250W may still be around. The
minimum efficiency for Class C AMs is 241.5 mV/m/kW @ 1km. A 250W Class C
could thus have an RMS of only 120.8 mV/m @ 1 km, yet the FCC would still
not consider it a daytimer. The station would be required to stay on the air
at night for some number of hours per week. I think Class Cs are required to
operate at least 2/3 of the daylight hours and 1/3 of the nighttime hours
unless their licenses specify limited-time or shared-time operation. Years
ago, Class Cs (which at that time were known as Class IVs) were not required
to provide a NIF (nighttime interference-free) signal to any specified
portion of their COLs but they still had to meet the minimum operating-time
requirement. Probably the majority of those stations deliver NIF signals to
substantially less than 80% of the COL. (Class B AMs are required to deliver
an NIF signal to at least 80% of their COL, although waivers of this
requirement are common.) In the case of Class Cs, I think all stations that
existed at the time of the rule change (meaning all but the newest US Class
Cs) had whatever NIF coverage they provided grandfathered.

--
Dan Strassberg, dan.strassberg at att.net
eFax 707-215-6367