RESULTS AND ANALYSIS

Practical and Simulated Results

Using Eqs. 3.4 and Eq. 3.5, the theoretical and practical Q values have been found out. Additionally, Eq. 3.5 is also substituted with simulated e-field readings to find out a simulated Q-factor. Figure З.Ю shows a graph of all three Q values.

The field uniformity is calculated for simulated results. Figure З.П shows the graph of practical and simulated field uniformity. The red line indicates the field uniformity limit specified by IEC for effective reverberation.

Analysis of Results

The difference in the practical and simulated readings can be attributed to various reasons. First, the simulations are performed at Ю probe positions, whereas practical measurements are carried out at eight probe positions, because increasing the number of probe positions in simulation does not increase simulation time, but while conducting practical measurements, the time required increases with increase in number of probe positions, as the probe has to be manually placed at every position sequentially. The number of stirrer positions, on the other hand, has a different relationship with the measurement and simulation times. While conducting simulations at lower frequencies, computational time is low; therefore a large number of stirrer positions can be considered for better accuracy. But at higher frequencies, the computational time increases manifold; hence the minimum required number of stirrer positions has to be considered, as specified by MIL standard 461G [21].

FIGURE 3.11 Graph of practical and simulated field uniformity.

On the other hand, the time required to conduct practical measurements in the chamber is not a function of frequency. The number of required stirrer positions is less at higher frequencies because at higher frequencies, the wavelengths are much smaller compared to the dimensions of the stirrer, and thus even a small change in the position of the stirrer causes the field levels to change drastically. Hence, there is little or no correlation between readings taken at adjacent stirrer positions. This makes the field more random in nature at higher frequencies.

CONCLUSION

Advantages

The RC has a number of advantages over the traditional anechoic chamber, which is more popular than its former counterpart.

The construction of RC is easier and cheaper because all the surfaces need to be made up of metal. In case of an anechoic chamber, the construction requires several pyramidal structures made up of a dielectric material, such as foam or polyethylene impregnated with graphite, which covers the entire chamber from inside, including the floor. A narrow walkway has to be constructed to allow the user to move inside the chamber which should be made up of a dielectric material and should occupy minimum total surface area to make sure absorption is above a certain limit. No such challenges exist in the designing of a RC. Therefore, comparatively fewer number of variables have to be taken into consideration while physically designing a RC.

Since the RC operates on the principle of stochastic electromagnetic theory, a tight shielding is not necessary. On the other hand, the anechoic chamber requires a good amount of shielding effectiveness, i.e., it cannot permit leakage of EM radiation, and thus special care is needed in designing the air vents doors and cable connectors.

• 3. There is no need to move or rotate the equipment under test while testing. Since the field in the reverberation is polarization-less, and is incident in all the directions, neither the EUT nor the transmitting antenna is required to be moved. In an anechoic chamber, either the EUT or the transmitting antenna is needed to be rotated, and the antenna has to be placed in both planes to see the effects of different polarizations. This becomes very critical, especially when dealing with large EUTs such as aircraft.
• 4. Large electromagnetic fields can be generated over the test volume using comparatively lower pow'er. This is due to the constructive interference of the SW within the RC. Anechoic chambers have been seen to require about 10 times the input power to generate the same level of electromagnetic fields for the same volume [7].

Disadvantages

• 1. Since the stirrer is movable, the boundary conditions are constantly changing, thus making it difficult to solve Maxwell’s equations. Therefore, it is difficult to mathematically model the chamber.
• 2. A minor change in the placement of the EUT or transmitting antenna can change the entire field, which causes non-repeatability of measurements.

Conclusion

Because the RC functions efficiently from 200 MHz onwards, the LUF can be said to be experimentally validated to be 200 MHz. This verifies that the LUF theoretically predicted in Section 3.3.4 is correct. The same can be said for Q-factor.

RCs have time and again proved to be the best candidate for radiated immunity tests.

The resemblance in practical and simulated results for Q-factor and field uniformity highlights accuracy of simulation definition and encourages engineers to incorporate 3D modelling and simulation of RCs not only as a prerequisite step in the manufacturing process of an RC, but also possibly to conduct radiated emissions and immunity tests virtually, thus cutting down costs manifold. As far as time is concerned, virtual tests can promise faster computation of results with minimal compromise in accuracy, provided a system with good configuration is used.

Future Scope

There are two main areas in the RC field which require the attention of researchers - RC testing and improved RC performance. The testing of the RCs using standard performance indicators is a very time-consuming process. This is because the field levels in all three polarizations at multiple points must be measured at every stir state throughout the frequency range. Even if the entire process is automated, it still takes hours to analyse the performance of a stirrer.

The effectiveness of a stirrer alone cannot be virtually analysed. It has to be accompanied with the chamber, and only then can its effectiveness be measured. Research is needed to find a new method of analysing the performance of a RC which consumes less time and efforts without reducing accuracy.

As far as the RC performance is concerned, one of the most common improvements is lowering of the LUF. As mentioned before, the LUF is inversely proportional to the number of contributing eigenmodes, which in turn depends upon the volume. Therefore to lower the LUF further, the volume of the chamber must be increased. This will require a larger stirrer which in turn affects feasibility of construction and increases load on the motor used to rotate the stirrer. The same problem also arises while testing large subjects such as aircrafts or satellites.

Another challenge is the placement of the stirrer in the chamber, since a huge portion of the chamber is permanently occupied by the stirrer. Moreover, many RCs employ more than one stirrer to meet required performance values. These disadvantages can be avoided by using an electronic stirring technique such as multiple antenna stirring [24] or source position stirring [25] which eliminate the need for a physical stirrer and increase the working volume, while making the RC more cost-effective.

Some of these alternatives may prove to be advantageous only if the RC is sufficiently large enough to accommodate the required extra equipment such as multiple antennas. Otherwise the same stirring techniques might end up occupying a considerable portion of the chamber. Similarly, the requirements of all the dependent parameters such as LUF, quality factor, maximum e-field strength, dimensions of equipment under test, etc. must be taken into account before the selection of stirring technique.

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