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ANTENNA NEAR FIELD
As noted in the sections on RF propagation and the radar equation, electromagnetic radiation expands spherically (Figure 1) and the power
density at a long range (R) from the transmitting antenna is:
When the range is large, the spherical surface of uniform
power density appears flat to a receiving antenna which is very small compared to the surface of the sphere. This is why the far field wave
front is considered planar and the rays approximately parallel. Also, it is apparent that at some shorter range, the spherical surface no longer
appears flat, even to a very small receiving antenna.
The distances where the planer, parallel ray approximation breaks down is known
as the near field. The crossover distance between near and far fields (Rff
) is taken to be where the phase error is 1/16 of a wavelength,
or about 22.5°.
where λ is the wavelength and D is the largest dimension of the transmit antenna.
If the same size antenna is used for multiple frequencies, Rff
will increase with increasing
frequency. However, if various size antennas are used for different frequencies and each antenna is designed with D as a function of λ (λ/2
to 100λ), then Rff
will vary from c/2f to 20000c/f. In this case Rff will decrease with increasing frequency. For example: a 10λ
antenna at 3 GHZ has a D of 100 cm and corresponding Rff
of 20 m, while a 10λ antenna at 30 GHz has a D of 10 cm and corresponding
of 2 m.
While the above analogy provides an image of the difference between the near and far fields, the relationship
must be defined as a characteristic of the transmitting antenna.
Actual antennas, of course, are not ideal point source radiators
but have physical dimensions. If the transmitting antenna placed at the origin of Figure 1 occupies distance D along the Z-axis and is boresighted
along the Y-axis (ф = 90), then the geometry of point P on the sphere is represented in two dimensions by Figure 2. For convenience, the antenna
is represented by a series of point sources in an array.
When point P is close to the antenna, as in Figure 2, then the difference in
distance of the two rays r and R taken respectively from the center of the antenna and the outer edge of the antenna varies as point P changes.
Derivation of equation  is given as follows:
From Figure 2, the following
 r2 = z2 + y2
 z = r cos θ
y = r sin θ and
Substituting  and  into 
which puts point P into spherical coordinates.
Equation  can be expanded by the binomial theorem which for the first three terms,
In the parallel ray approximation for far field calculations
(Figure 3) the third term of  is neglected.
The distance where the far field begins (Rff
) (or where the near field
ends) is the value of r when the error in R due to neglecting the third term of equation , equals 1/16 of a wavelength.
is usually calculated on boresight, so θ = 90° and the second term of equation  equals zero (Cos 90° = 0), therefore from Figure 3, where
D is the antenna dimension, Rff
is found by equating the third term of  to 1/16 wavelength.
Sin θ = Sin 90 = 1 and z' = D/2 so:
Equation  is the standard calculation of far
field given in all references.
Besides  some general rules of thumb for far field conditions are:
r >> D or r >> λ
If the sphere and point P are a very great distance from the antenna, then the rays are very nearly parallel and this difference is small
as in Figure 3.
Figure 3 - Far Field Parallel Ray Approximation for Calculations.
The power density within the near field varies as a function of the type of aperture illumination and is less than would be calculated
by equation . Thus, in the antenna near field there is stored energy. (The complex radiation field equations have imaginary terms indicating
reactive power.) Figure 4 shows normalized power density for three different illuminations.Curve A
reference only and shows how power density would vary if it were calculated using
equation .Curve B
shows power density variations on axis for an antenna aperture with a cosine amplitude distribution.
This is typical of a horn antenna in
the H-plane.Curve C
shows power density variations on axis for a uniformly illuminated antenna aperture
or for a line source. This is typical of a horn antenna in the E-plane.Curve D
shows power density variations
on axis for an antenna aperture with a tapered illumination.
Generally the edge illumination is approximately -10 dB from the center
illumination and is typical of a parabolic dish antenna.Point E
- For radiation safety purposes, a general
rule of thumb for tapered illumination is that the maximum safe level of 10 mW/cm2 (-200 V/m) is reached in the near field if the level at R
reaches 0.242 mW/cm2
as can be ff verified by computing the power density at point E in Figure 4. (10 mW/cm2
E extrapolates to 0.242 mW/cm2
[16 dB lower] at R=Rff
, or Y axis value =1). Figure 1 in Section 3-6 depicts more precise
values for radiation hazard exposure.Point F
- Far Field Point. At distances closer to the source than this
point (near field), the power density from any given antenna is less than that predicted using Curve A. At farther distances, (far field) power
densities from all types of antennas are the same.
Figure 4 - Antenna Near-Field On-Axis Power Density (Normalized) For Various Aperture Illuminations
For Various Aperture IlluminationsFOR FAR FIELD MEASUREMENTS:
When free space measurements are performed at a known distance from a source, it is often necessary to know if the measurements are
being performed in the far field. As can be seen from Curve A on Figure 4, if the distance is halved (going from 1.0 to 0.5 on the Y axis),
the power density will increase by 6 dB (going from 0 to 6 dB on the X axis). Each reduction in range by ½ results in further 6 dB increases.
As previously mentioned, Curve A is drawn for reference only in the near field region, since at distances less than Rff
density increases less than 6 dB when the range is halved. In the far field, all curves converge and Equation  applies.
When a measurement
is made in free space, a good check to ensure that is was performed in the far field is to repeat the measurement at twice the distance. The
power should decrease by exactly 6 dB. A common error is to use 3 dB (the half power point) for comparison. Conversely, the power measurement
can be repeated at half the distance, in which case you would look for a 6 dB increase, however the conclusion is not as sure, because the first
measurement could have been made in the far field, and the second could have been made in the near field.
Table of Contents
for Electronics Warfare and Radar Engineering Handbook
Abbreviations | Decibel | Duty
Cycle | Doppler Shift | Radar Horizon / Line
of Sight | Propagation Time / Resolution | Modulation
| Transforms / Wavelets | Antenna Introduction
/ Basics | Polarization | Radiation Patterns |
Frequency / Phase Effects of Antennas |
Antenna Near Field | Radiation Hazards |
Power Density | One-Way Radar Equation / RF Propagation
| Two-Way Radar Equation (Monostatic) |
Alternate Two-Way Radar Equation |
Two-Way Radar Equation (Bistatic) |
Jamming to Signal (J/S) Ratio - Constant Power [Saturated] Jamming
| Support Jamming | Radar Cross Section (RCS) |
Emission Control (EMCON) | RF Atmospheric
Absorption / Ducting | Receiver Sensitivity / Noise |
Receiver Types and Characteristics |
General Radar Display Types |
IFF - Identification - Friend or Foe | Receiver
Tests | Signal Sorting Methods and Direction Finding |
Voltage Standing Wave Ratio (VSWR) / Reflection Coefficient / Return
Loss / Mismatch Loss | Microwave Coaxial Connectors |
Power Dividers/Combiner and Directional Couplers |
Attenuators / Filters / DC Blocks |
Terminations / Dummy Loads | Circulators
and Diplexers | Mixers and Frequency Discriminators |
Detectors | Microwave Measurements |
Microwave Waveguides and Coaxial Cable |
Electro-Optics | Laser Safety |
Mach Number and Airspeed vs. Altitude Mach Number |
EMP/ Aircraft Dimensions | Data Busses | RS-232 Interface
| RS-422 Balanced Voltage Interface | RS-485 Interface |
IEEE-488 Interface Bus (HP-IB/GP-IB) | MIL-STD-1553 &
1773 Data Bus |
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