Instruction manual

AM Radio Field Strength Measurements with Confidence
November 2004
Understanding the AM Broadcast signal
The AM Broadcast service in the United States and many other countries is implemented
in 10 kHz channel steps with center frequencies 540 kHz through 1700 kHz, at carrier
power levels from 250 watts to 50 kilowatts.
AM field strength or field intensity measurements are made in support of a reference
proof or partial proof for stations with directional antenna arrays, as defined in Section
73.151 and 73.186 of the FCC rules. Knowledge of field strength confirms primary
coverage over the city of license, quantifies additional areas served by the station, and
assures directional protection of distant co-channel stations. To make consistent and
repeatable measurements, it is useful to understand the amplitude modulation process.
Amplitude Modulation products are contained in
upper and lower sidebands whose frequency
components represent audio frequencies and
energy levels represent loudness. Sideband
energy is generally contained within +/-10.2 kHz
of assigned channel frequency, for an audio
bandwidth of approximately 10 kHz. To the right
is an example of a talk radio format, with little
energy above 7 kHz.
A perfect AM transmitter, modulated with a single
sine wave tone would radiate a constant average
power regardless of modulation level, with
spectral power shared among the assigned
frequency carrier and two sideband frequencies
above and below the assigned frequency by the
frequency of the modulating tone. A field strength
meter, or calibrated power bolometer, would read
a constant value, with or without a modulating
tone. The example at left is from a signal
generator modulated with a 5 kHz tone.
Audio Processing
AM radio stations will often use audio processing equipment to maintain a relatively
even modulation level. When the audio processor is adjusted to make the audio seem
louder, the modulating audio will become non-symmetrical, with negative peaks
compressed to avoid driving the transmitter to a zero carrier cutoff and positive
modulating peaks stretched to increase the average level of audio available to the
receiver. Much development has been done to make audio both loud and good sounding,
but the net result is to increase average signal power during audio peaks. When audio