Instruction manual

measurement instruments can accumulate accurate data much faster and more flexibly
than early models.
Equipment calibration
The Z Technology Field Strength Meter is a 50 ohm input measurement instrument
calibrated and traceable to the National Institute of Standards and Technology over the
full range of frequencies, power levels, and operating temperatures specified in its
instruction manual. When measuring AM Broadcast signals, the field strength meter is set
to an IF bandwidth of 15 kHz (Narrow setting) and accepts the signal from a calibrated
unity gain BC-BCB block converter. This configuration covers a frequency range of
0.3 –3.0 MHz, with a power input level range of -97dBm to –17dBm (+10 dBuV to +90
dBuV).
In the development of recommendations, Z Technology has characterized measurement
system performance with AM Broadcast signals at several different input power levels
+30 to + 60dBuV; several input frequencies from 500 kHz to 2.0 MHz, and at various
modulation frequencies from 400Hz to 10 KHz. The AM signal source was an HP-8640B
signal generator calibrated with an HP-435A/8482A Power Meter and bolometer
traceable to NIST. Test results indicate that for AM modulation levels from zero to 60%,
the power level reported by the field strength meter was found to vary a maximum of 1.1
dB. A reading of -1.1 dB occurred with a 1KHz audio modulation of 60%. At all other
frequencies tested the worst power level reading was -0.8dB. At a 90% modulation level,
input power level was found to change by -3.2dB at 1 kHz. At all other frequencies tested
the worst power level change was -2.3dB.
Field testing with AM radio stations with several different program formats confirms
readings can be reliable and as expected for a modulated AM signal and are considered
within a reasonable range of calibrated measurement. To remove any uncertainty, it is
still recommended measurements not be taken during periods of excessive modulation.
AM Broadcast field strength measurement suggestions:
Operator safety and comfort are always worthy of consideration. Personnel should always
take care when working near power lines, in the presence of electrical storms or in high
RF fields.
Turn off any non-symmetrical audio processing. Audio leveling is OK, but heavily
compressed audio will increase average RF power and make the field strength reading
more program content dependant. Use the field strength meter’s AM demodulator and
speaker to confirm any undesired program content.
For spot location measurements, such as confirmation of monitoring points and
maintenance of the nulls in a directional transmitting array, the suggested antenna would
be the shielded loop antenna. The loop antenna is preferred because it seems to be more
resistant to noise and may be able to measure a deeper pattern null. It can also be
manually rotated to null any interfering co-channel signals that are off-axis to the
direction of the station’s antenna pattern null.