Radio Systems i.AM.Radio Transmitter Instruction Manual download pdf

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

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Summary of Contents

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AM Radio Field Strength Measurements with Confidence November 2004 Understanding the AM Broadcast signal The AM Broadcast service in the United State

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APPENDIX B Working with Antenna Factors Antenna Factors are the key to accommodating all of the external variables affecting the field strength meter

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APPENDIX C Measurement Unit Conversion Chart 0dBm = 1 milliwatt into 50 ohms (Note: 0dBuV is referenced to 1 uvolt into 50 ohms) mV uV

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proof of performance or field strength measurements are to be made, AM Stations should bypass any processing that makes the audio modulating waveform

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channel station. The null is bi-directional, however, sometimes limiting the measurement effectiveness when the station’s antenna pattern null is in a

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measurement instruments can accumulate accurate data much faster and more flexibly than early models. Equipment calibration The Z Technology Field

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For signal coverage measurements, the recommended antenna system is the AA-2R Ruggedized Active Monopole Antenna with AA-2M Magnetic Mount. This ante

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be used with a Z Technology BC-BCB block converter and appropriate antenna for AM Broadcast measurements. A sample step-by-step measurement checklis

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APPENDIX A Procedure for Mobile Field Strength Measurements 1. Assemble your measurement system: a. Programmable field strength meter with fresh b

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i. Select direct low frequency readout using meter Function 13. This is done by selecting F13 on the meters front panel and setting the instrument to

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b. Select readout units in dBuV to be sure signal at the FSM input is within the range +90dBuV to +10dBuV. Install 50 ohm pad at input to block conve

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Radio Systems i.AM.Radio Transmitter Instruction Manual

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