Soxless Boss

Garrett Wollman
Sat May 6 19:26:49 EDT 2006

<<On Sat, 06 May 2006 16:00:19 -0400, Larry Weil <> said:

> But I always thought that directional AM's have no forward gain, that 
> the only purpose of being directional is to protect the signals of 
> other stations on the same or adjacent frequencies.  Am I wrong?

Yes, you are wrong.  All AM stations in the U.S. are licensed on the
basis of "nominal power".  For a non-directional station, nominal
power is defined to be the RF power presented to the antenna (with any
resistive load to decrease antenna-system efficiency considered part
of the antenna).  This can be computed as the square of the measured
base current times the pre-determined antenna impedance.  For a
directional station, however, antenna-system impedance, and thus
nominal power, is computed at the "common point" -- that is, the point
inside the phasor at which the input signal branches off to separate
L-C networks for each tower.

The practical upshot of this is that an AM station which reduces power
in one direction must consequently radiate more in another direction,
since the power is computed on the transmitter side of the phasing

There are other methods of computing power, but they are designed to
have the same result.  Then there's this language in 73.51(b):

# (1) For stations with nominal powers of 5 kW, or less, the authorized
# antenna input power to directional antennas shall exceed the nominal
# power by 8 percent.

# (2) For stations with nominal powers in excess of 5 kW, the authorized
# antenna input power to directional antennas shall exceed the nominal
# power by 5.3 percent.

(Essentially what this means is that the combination of phasing
network and transmission line is allowed to be 92.5% efficient for a
5-kW station but must be 95% efficient for a 50-kW station.)

The only reason "nominal power" exists is because of the pre-1985
restriction of stations to a small list of power levels.  If a station
had to replace its antenna with a more-efficient one before 1985, it
would have to either step back its power considerably, or install a
dissipative network to reduce antenna efficiency back to its old
value.  (The list, in kW, was 50, 25, 10, 5, 2.5, 1, 0.5, and 0.25.)
Otherwise, for non-directional stations, straight antenna-input power
could be used (as I believe is the case in Canada).  Other than the
"letterhead 50", there is no advantage to doing this in the current
regime, but existing stations are allowed to keep it until they change


More information about the Boston-Radio-Interest mailing list