Antenna Far Field Distances

Antenna Far Field Distances

Antenna specifications can seem confusing or, worse yet, can be misinterpreted. Of all the salient antenna specifications, antenna gain seems to present the biggest problem. The antenna gain provided in data sheets is generally understood to be valid in the so called “far-field” region. In this region, the field energy primarily consists of radiated RF with an angular field distribution that tends to be independent of distance from the antenna and a field level that is inversely proportional to the separation between the antenna and EUT. As the EUT is moved closer to the antenna, a point is reached where the induction field begins to affect energy level, and standard equations are no longer valid. This is referred to as the “near-field” region. The key element here is the point where the transition from near-field to far-field takes place. While the precise point is generally unknown, two commonly used criteria suffice for most applications. Depending on the type of antenna used:

Dipole or log-periodic antennas

$$ Far - Field = \frac{λ}{2•π} (Eq. 1) $$

Horn antennas

$$ Far - Field = \frac{2•D^2}{λ} (Eq. 2) $$

Note: that the distance D is the largest dimension of the antenna’s radiating aperture and is transverse to the direction of propagation.

AR's Horn Antenna Far Field Distances

The above reference graph details AR RF/Microwave Instrumentation’s horn antennas far-field distances plotted as a function of frequency using the above equations.

Conclusion

It can be seen from the above equations that the transition to far-field decreases for a dipole or log-periodic antenna (eq. 1) as frequency increases and increases as a function of frequency for a horn antenna (eq. 2). Since the determination of just where the far-field begins is somewhat vague at best, and the far-field equations for antenna gain are approximations, it is always best to use actual test data or calibration data supplied by the antenna manufacturer. Furthermore, test data taken at the required test distance is most desirable. Accordingly, ar RF/Microwave Instrumentation supplies most of our antennas with field strength graphs based on actual measurements at the most sought after test distances of 1 meter and 3 meters.