Ambient Field Strength values are measured as follows.
the antenna at the proper distance from the source of
the emissions, or the product being tested. This will
be one, three or ten meters depending on the exact specification
the antenna at the product being tested. This includes
bore-site direction, and polarization (polarity).
the output of the receiving antenna to the input of the
receiving system (spectrum analyzer used to measure field),
using a low VSWR, low loss coaxial cable. The input impedance
for the receiving system, and the receiving antenna should
all be matched to 50W
. Most antennas are already matched to this impedance,
if they are EMI/EMC antennas, as should most receiving
systems. If this is not the case, the readings will require
correction for the impedance mismatch.
the frequency of interest on the receiving system. Ensure
that the receiving antenna is capable of providing accurate
measurements at that frequency. This includes ensuring
the antenna can receive a signal at the frequency of interest
by its designed frequency range, provided by the manufacturer,
and that the antenna is properly calibrated at that frequency.
the RF Voltage, Va, referenced to the receiving
system. The units should be in decibels referenced to
1 micro-volt, dBm
V. If the units are in absolute micro-volts (dependent
on the receiving system used), they must be converted.
The conversion factors are as follows, however, more information
about the conversion can be found in the Conversion Technical
V) = 20Log10[Va(mV)]
the reading is converted to dB micro-volts (dBm
V), it can be added to the antenna factor to determine
absolute field strength. The antenna factor is provided
with an antenna when it is calibrated or newly purchased.
If the antenna factor for an antenna is not known, typical
values can be used to approximate field strength, in development,
however, this is not acceptable for compliance.
Field Strength (dBm
V/m) = RF Voltage (dBm
V) + Antenna Factor (dB/m)
V/m) = Va (dBm
V) + AF (dB/m)
However, the losses due to the cable between the receiving antenna and the
receiving system should also be included. The cable losses
can be determined easily, by connecting the cable in question
between a transmitter (such as a tracking generator) and
the receiver (the spectrum analyzer) at the frequency
of interest, and examining how much lower the signal is
than the output source is set to provide. (for example,
if the tracking generator is transmitting at a given strength,
then this strength minus the strength measured at the
spectrum analyzer is the cable loss). The above formula,
including cable losses, now becomes...
V/m) = Va (dBm
V) + Ac (dB) + AF (dB/m)