Comparison of Methods to Evaluate Semianechoic Chamber Performance

 

The performance of a 3-m semianechoic chamber is usually determined by performing a normalized site-attenuation (NSA) measurements series and comparing them to a reference standard.^2–4 To obtain NSA, site attenuation must first be measured. This is accomplished by connecting a signal generator to a transmit antenna. Next, a receive antenna is connected to a receiver and placed 3 m from the transmit antenna. Site attenuation in decibels (dB) is determined by subtracting the maximum voltage measured by the receiver from the voltage measured when the cables are connected together while the receive antenna height is varied from 1 to 4 m. Finally, NSA (dB) is obtained by subtracting the free-space antenna factors (dB/meter) from the site attenuation (dB).

 

Figure 1. Comparison of normalized site attenuation (NSA) measured in a semianechoic chamber to theoretical NSA calculated using free-space antenna factors, vertical polarization (ht = 1.5 m, hr = 1–4 m, d = 3 m, 30–200 MHz).

 

NSA measurements were recently performed in a state-of-the-art 3-m semianechoic chamber. The inside of the chamber was 8.5 m long, 5.2 m wide, and 5.3 m high. It was equipped with high-performance 24-in. hybrid absorber and a 2-m diameter turntable. Both the turntable and the receive antenna were located along the centerline of the chamber.

The chamber's worst-case NSA occurred in the 30–200 MHz frequency range when the biconical antennas were vertically polarized and the transmit-antenna height (h_t) was set to 1.5 m. The NSA measurements are compared in Figure 1 to a theoretical NSA calculated using free-space antenna factors. These antenna factors were measured using the standard site method⁵ with the antennas horizontally polarized (h_t = 2 m), the receive-antenna height (h_r) varied from 1 to 4 m, and the horizontal distance between the antennas (d) set to 10 m. The measurement was performed on a near-ideal OATS equipped with an uncovered 50 x 80–m ground plane. However, the result was reasonably accurate free-space antenna factors because the standard site method properly accounted for the ground-plane reflection. This method also yields equivalent results when the measurement is performed on a more modest ground plane, which will be discussed later. The NSA in Figure 1 was measured with the transmit antenna placed at the center, front, left, and right of the turntable as specified in ANSI C63.4–1992.⁴ The chamber is acceptable for 3-m emission testing because the measurements in Figure 1, as well as all of the other required NSA measurements from 30 to 1000 MHz, are within ±4 dB of the corresponding theoretical NSA.

 

Figure 2. Comparison of normalized site attenuation (NSA) measured on a near-ideal OATS to theoretical NSA calculated using free-space antenna factors, vertical polarization (ht = 1.5 m, hr = 1–4 m, d = 3 m, 30–200 MHz).

The curves from 140 to 200 MHz in Figure 1 are significant. Although the largest difference between the theoretical and measured curves occurs over this frequency range, the difference between the measured curves is relatively small. This behavior suggests a significant error associated with the theoretical NSA model. To investigate this possibility, a similar NSA measurement was performed on the near-ideal OATS. The result, shown in Figure 2, is compared to the theoretical NSA from Figure 1. Because the ground plane on the near-ideal OATS is extremely large and the measurement error is small, the difference between the measured and theoretical curves in Figure 2 is mainly the result of the effects neglected by the theoretical model. These effects include the 1/r² and 1/r³ radiation terms, the mutual coupling between the antennas, the mutual coupling between the antennas and the ground plane, and the nonuniform illumination of the receive antenna. If the chamber NSA measurements in Figure 1 were directly compared to the NSA measured on a near-ideal OATS in Figure 2, the differences would be smaller. Such a direct site-to-site NSA comparison is shown in Figure 3.

 

 

The question as to whether a theoretical model or a measurement on a near-ideal OATS provides the best NSA reference standard has been rigorously debated during the last 20 years. The major advantage of the measured NSA reference or direct site-to-site comparison is that it can provide the greatest accuracy of the two methods as seen in Figures 1 and 3 and Table I. However, the NSA reference must be measured on a near-ideal OATS with an extremely large ground plane, such as the 50 x 80-m ground plane used for the measurements in Figure 3. This ground plane must be located on a large, level, obstruction-free area and be well connected electrically to the surrounding earth. The near-ideal OATS also must not be covered by any structure that prohibits measurements during adverse weather conditions. Such a site is expensive and of limited usefulness for purposes other than NSA measurements.

The performance of the near-ideal OATS must also be rigorously verified. Furthermore, to achieve the greatest possible accuracy, the same antennas, antenna masts, cables, and instrumentation must be used for both the chamber and the OATS measurements. Moreover, the placement of the cables, positions of any ferrite cores, and the locations where the cables penetrate the ground plane must also be identical in the chamber and on the OATS. Cabling is critical because of the coupling between the vertically polarized antennas and the sections of the cables that are parallel to the antennas.

The greatest advantage to using a theoretical NSA reference is that the free-space antenna factors (employed by the theoretical NSA model) can be measured on a modest OATS with a much smaller ground plane. An example of such a site is a 7 x 14–m ground plane constructed of paper-barrier insulating foil placed in a parking lot.⁵ Another advantage is that the requisite antenna factors are determined from a horizontal site-attenuation measurement in which the cables are orthogonal to the antennas. This orthogonality virtually eliminates the sensitivity to cable position, the need for ferrite cores, and the importance of the location where the cables penetrate the ground plane. The only disadvantage of the theoretical NSA reference is that the total measurement error is larger, which is the result of the effects neglected by the theoretical model that were described earlier. The total error is typically less than ±2 dB for horizontal 3-m NSA measurements from 30 to 200 MHz, and all 3-m horizontal and vertical NSA measurements from 200 to 1000 MHz. However, vertical 3-m NSA measurements from 30 to 200 MHz may exhibit a maximum total error of ±3 dB as shown in Figure 2. Both the theoretical model and the direct site-to-site comparison are specified in current ANSI C63 standards.

 

	Maximum NSA Difference (dB) from 30 to 200 MHz
	Horizontal Polarization		Vertical Polarization
	ht =1.0m	ht = 2.0 m	ht =1.0 m	ht =1.5 m
Theoretical	+2.2, –1.6	+2.1, –1.5	+3.6, –3.5	+3.7, –3.3
Measured on OATS	+0.9, –1.9	+1.2, –1.3	+1.8, –3.5	+1.7, –3.0

Table I. Comparison of normalized site attenuation (NSA) measured in a semianechoic chamber to theoretical NSA calculated using free-space antenna factors and NSA measured on a near-ideal OATS (h_r = 1–4 m, d = 3m).

ANSI C63.4 and ANSI C63.5

ANSI C63.4-1992⁴ states that a test site is acceptable for emission measurements if it provides NSA within ±4 dB of the theoretical NSA calculated using antenna factors that are measured according to ANSI C63.5-1988.¹ According to Section 4.1 of ANSI C63.5-1988, these antenna factors should be free-space antenna factors. This section specifies that antennas must be calibrated in a geometry where near-field and mutual-coupling effects are negligible. Such a calibration results in free-space antenna factors. The intent of ANSI C63.5-1988 is again stated in Section 5.1, which explains why the standard site method should only be used to determine antenna factors that most nearly approximate the results obtainable in free space.

Soon after publication of ANSI C63.4-1992, it became apparent that it is quite difficult to design and build a semianechoic chamber that provides measured NSA within ±4 dB of the theoretical NSA calculated using free-space antenna factors. An interpretation of ANSI C63.5-1988 was subsequently developed that allowed antennas to be calibrated using the same polarization, antenna heights, and separation used for the chamber NSA measurements. The result became known as geometry-specific antenna factors. However, this interpretation is questionable because it relies upon wording that permits the use of special geometries in unusual situations. It also produces measurements that exhibit significant near-field and mutual-coupling effects, which contradict the requirements in Section 4.1.

Unfortunately, this interpretation of ANSI C63.5-1988 has been accepted by the Federal Communications Commission (FCC) and become the industry standard. Worse yet, it is actually a direct site-to-site NSA comparison in disguise.⁶ When chamber performance is compared to a theoretical model that uses geometry-specific antenna factors, the chamber NSA is actually being directly compared to the measured NSA of the OATS where the antennas were calibrated! More importantly, ANSI C63.5-1988 does not specify the near-ideal OATS that must be used to achieve an accurate site-to-site NSA comparison.

ANSI C63.5-1988 contains no specific requirements for the OATS to which a chamber is compared because its objective was not to allow direct site-to-site NSA comparisons. Consequently, many OATS can be measured from which one can be selected whose NSA is similar to that of a chamber. Furthermore, an OATS can be modified to produce NSA that matches that of a chamber. Hence, it must be concluded that the accepted interpretation of ANSI C63.5-1988 offers no effective control of semianechoic chamber performance because chamber NSA can be compared to the NSA of an unspecified OATS.

This unfortunate situation should be remedied in the future. The recently published ANSI C63.5-1998 requires the use of free-space antenna factors and does not allow direct site-to-site NSA comparisons.⁷ Furthermore, ANSI Accredited Standards Committee C63, Subcommittee 1, Working Group 1-15.6 on Antenna Calibration is developing correction factors for the theoretical NSA model that include the effects that were previously neglected. A goal of the working group is an enhanced theoretical model that provides significantly improved accuracy for biconical antennas from 30 to 200 MHz. A future version of ANSI C63.4 will likely include these correction factors and reference a revision of ANSI C63.5-1998.

 

 

The objective of ANSI C63.5-1988 is to compare the NSA of a semianechoic chamber to a theoretical NSA calculated using free-space antenna factors. However, the accepted interpretation of ANSI C63.5-1988 allows chamber NSA to be directly compared to the NSA measured on any unspecified OATS. Therefore, semianechoic chamber performance is currently unregulated. Even if the near-ideal OATS were properly specified, the greater accuracy of the direct site-to-site NSA comparison would be negated by its disadvantages.

Adequate control of chamber performance is achieved with ANSI C63.5-1998. A chamber that is designed and built to meet this standard should provide state-of-the-art performance that is better overall than the performance of a chamber that only meets the requirements of a direct site-to-site NSA comparison. Such a chamber should also meet both the intent and the accepted interpretation of ANSI C63.5-1988 and can be expected to meet any requirements imposed by an enhanced theoretical NSA model. Improved control of chamber performance will occur if the enhanced theoretical NSA model is incorporated into a future version of ANSI C63.4 as anticipated.

 

 

Robert F. German is the manager of German Training and Consulting LLC (Boulder, CO). He can be reached at gtc@ieee.org. Charles L. Devor Jr. is the vice president of Lehman Chambers (Chambersburg, PA). He can be reached at charlie_devor@lehman-inc.com.