Instruction manual

APPENDIX B
Working with Antenna Factors
Antenna Factors are the key to accommodating all of the external variables affecting the
field strength meter reading to finally display the signal field strength available to the
measurement system. The field strength meter reads power delivered at its input
connector in dBm, where 0 dBm equals 1 milliwatt into a 50 ohm load.
A table is easily generated to convert between dBm, a direct reading of power, and
voltage (including millivolts and microvolts) when the load impedance is known (50
ohms for RF measurement instruments). Such a table is included as APPENDIX C.
Working in logarithmic dBuV units, then, lets us easily account for antenna gain or
losses, cable loss, and any other losses in our measurement system. By simply adding all
of the losses to the dBuV reading, we can know the real field strength available to any
antenna we may place in that field. This field strength is usually expressed in
dBuV/meter.
A programmable field strength meter or a PC application can do these calculations and
measurement unit conversions for you, but you must provide the instrument or PC an
‘antenna factor’.
The antenna factor is derived from the sum of values measured with a calibration system
traceable to the National Institute of Standards and Technology. For measurement
antennas, the antenna factor is recorded on a calibrated antenna range by comparison to a
reference antenna on the same frequency. For measurement antennas in the AM
Broadcast band, the antenna remains physically the same over the band, and a table of
antenna factors will be developed by comparison to the reference antenna over the band.
It is important to sum all of the applicable correction factors for the system delivering
power to the 50-ohm input of the field strength meter. For example:
Field Intensity in dBuV/m= FSM reading in dBuV + cable loss in dB +pad loss +
Antenna Factor.
By providing the sum of all of the gains and losses between the field and the meter, the
field intensity may be calculated and displayed on the Field Strength Meter front panel
directly in dBuV/m. When a PC application is controlling/reading the meter, the PC can
do the calculating and display/record true field intensity readings for the operator.