About AM Directionality
Most modern AM stations, other than the smallest low-power outlets,
use an array of transmitting antennas to provide a directional pattern
to their signal. Sometimes this is used to concentrate the station's
energy into the area which it is supposed to serve (and away from
unpopulated areas), as with WBZ; more often, it
is required by the FCC in order to protect a longer-standing station
from undesirable interference. (US stations are also required to
completely protect the Canadian and Mexican borders on certain
frequencies, and likewise for those countries on other
frequencies.)
There are four basic situations which commonly occur; here they are
along with the abbreviations commonly used to indicate them:
- ND
- Non-directional; the station emits the same amount of
energy in all directions, using a single tower.
- DA-1
- Directional, same pattern day and night.
- DA-N
- Non-directional day, directional at night.
- DA-2
- Directional, different patterns day and night.
There are also more exotic situations, such as three different
patterns, or directional day, non-directional night; there are no such
stations in the Boston area, however.
For daytime-only stations, the situation is similar. Non-directional
daytime stations are designated ND-D; directional ones are
DA-D. Post-sunset service authority is indicated by +N.
The FCC no longer licenses new daytime-only stations, so
these operations are becoming more and more rare as time passes.
The Boston Radio Archives
wollman@lcs.mit.edu
archives@bostonradio.org