A transmitting antenna generates two types of fields--a near field
and a far field.
The pattern or shape of the antenna, particularily a directional antenna, will not form until
several wavelengths away from the antenna (the far field)
In the 1930's, Dr. George H. Brown of RCA developed several formulas that allowed the
measurement of antennas in the near field.
These same formulas can be used to determine the amount of re-radiation from a tower from a nearby radio signal.
The formulas allow a close approximation to determine if a tower is causing a pattern disturbance to a nearby AM broadcast station. This is particularily usful in the event pre-construction field strength measurments were not completed.
The procedure calls for a minimum of ten (10) measurements to be taken with a calibrated AM field strength meter (FIM21 or FIM41) on a direct line 90-degrees from the AM radio station. Two measurements are taken--one directly to the radio station and the other approximately 90-degrees from the radio station. Measurements are typically taken from fifty (50) feet to one-hundred and fifty (150) feet from the tower being measured.
Contact AM Coordination Services information of Proximity Measurements. amcoordination@aol.com
Proximity MeasurementsThe pattern or shape of the antenna, particularily a directional antenna, will not form until
several wavelengths away from the antenna (the far field)
In the 1930's, Dr. George H. Brown of RCA developed several formulas that allowed the
measurement of antennas in the near field.
These same formulas can be used to determine the amount of re-radiation from a tower from a nearby radio signal.
The formulas allow a close approximation to determine if a tower is causing a pattern disturbance to a nearby AM broadcast station. This is particularily usful in the event pre-construction field strength measurments were not completed.
The procedure calls for a minimum of ten (10) measurements to be taken with a calibrated AM field strength meter (FIM21 or FIM41) on a direct line 90-degrees from the AM radio station. Two measurements are taken--one directly to the radio station and the other approximately 90-degrees from the radio station. Measurements are typically taken from fifty (50) feet to one-hundred and fifty (150) feet from the tower being measured.
Contact AM Coordination Services information of Proximity Measurements. amcoordination@aol.com
For a location, P, on a plane earth as shown above for a vertical radiator of height G and distances in electrical degrees the ratio between near field flux density K is:
K = {[cosR1 -(cosG)(cosR0)]2 + [sinR1 - (cosG)(sinR0)]2}
[1 -cosG]
The field strength meter antenna is a loop that responds to magnetic flux density and is the meter is calibrated in terms of field strength. Thus:
E(far field) E(near field) = K
1 G. H. Brown, “Directional Antennas” Proceedings of the IRE,