6.6. Comparison: Hardened vs. Non-hardened Telescope
The inherent operational sensitivity of the KAT-7 radio telescope requires EMI mitigation measures surpassing that of most commercial applications. The objective of hardening a telescope was to create a shielded lower pedestal that minimises RFI entering or exiting this volume. This required installing continuous metallic interface barriers to serve as the floor and ceiling of the pedestal. All braided cables entering the pedestal would do so through individual 360º connectors with the braiding correctly terminated. Compressible RF gasketting would be used on the access door to ensure a galvanic seal once it is closed. EMC cabinets would house the dish control and antenna feed electronics. Collectively all these shielding measures would combine creating a pedestal hardened against RFI.
The shielding measures described, with exception of the EMC cabinets, were post-build additions. This made their ideal implementation difficult. The interface barriers had to be multi- sectioned (Figure 6.21 (a)) to enable installation. The various sections were joined at seams, each of which required gasketting for reasonable galvanic connection. However, each join was a possible weak point in the shielding interface. Also, as seen from Figure 6.21 (b), tabs provided the connection between the interface and the cable braiding for the cables entering through the
barrier. Such an interface had to be used because removing and reconnecting the cables was not possible. The asymmetrical connection between the tab and the cable braiding is reasonable but less effective than a 360º connection. The cable braiding, in some instances, did not extend to connect to the EMC cabinets, and was not properly terminated in the upper pedestal.
Despite the hardening measures not being implemented ideally, it was the best first-effort. Improving the shielding in a developing project such as this is an interactive process, which requires feedback to justify expenditure of both time and money. This section presents the findings of the investigation made on the initial hardening attempt. This was accomplished by investigating the differences in the exterior current levels measured between a hardened and non- hardened telescope. The added shielding influenced the amount of CM current entering the pedestal and this reflected in the current flowing on the outside of the structure.
(a) (b)
Figure 6.21: The hardening policies used in one of the KAT-7 telescopes include (a) a multi-sectioned metallic interface plate and (b) gasketting added to interface for braided cables.
6.6.1. Hardened vs. Non-hardened for Direct Injection
Comparison between a hardened and non-hardened telescope is firstly made for the direct current injection method. The increase in measured CM current as a result of the shielding measures is clearly noticeable when comparing aligned and misaligned results for a particular connection. The results shown in Figure 6.22 are for the west-facing connection for scenario one.
(a) (b)
Figure 6.22: Current measured on the west-facing connection for scenario one aligned and misaligned compared for (a) a non-hardened telescope and (b) a hardened telescope.
It is assumed that for the aligned case the LDCs on the upper pedestal, along with the copper shoes and earthing connections on the lower pedestal, form a direct current path to ground. CM current flowing down the pedestal to ground does so either on the outside or the inside of the pedestal. If the assumption of a more direct path for the aligned scenario is correct, less current will flow on the inside of the pedestal. As a consequence the current measured on the earthing connections for the non-hardened telescope should be higher when the dish is aligned, than when it is misaligned. The misalignment interrupts this direct current path. This leads to more current flowing on the inside of the pedestal attempting to find an alternative path to ground. The results shown in Figure 6.22 (a) do in fact verify this point and confirm a change in CM current.
The hardened telescope, on the contrary, shows only a small difference when comparing the aligned and misaligned results (Figure 6.22 (b)). It is only a small difference because the bulk of the current is already flowing on the outside of the pedestal as a consequence of the hardening policies that were added. The hardening policies, despite being incomplete, are therefore providing a certain level of shielding. The aligned scenarios do not indicate this point very clearly. The misaligned scenarios which creates a less defined path, increases the amount of current wanting to enter the inside of the pedestal. As a result, a more substantial difference between a hardened and non-hardened telescope is seen for these scenarios. This result is consistent for the remaining connections which are shown in Appendix C.
0 20 40 60 80 100 10 20 30 40 50 60 70 80 90 Cu rr en t ( d Bu A) Frequency (MHz)
Current Distribution Direct Injection: Scenario 1 Hardened West Aligned Hard West Misaligned Hard
0 20 40 60 80 100 0 10 20 30 40 50 60 70 80 90 C ur rent ( d Bu A ) Frequency (MHz)
Current Distribution Direct Injection: Scenario 1 Non-hardened West Aligned Non-hard West Misaligned Non-hard
6.6.2. Hardened vs. Non-hardened for Induced CM Current
A comparison is made for the induced currents from the radiated measurements presented in section 6.5.2. Results for the west-facing connection are shown with comparison between a hardened and a non-hardened case. As before, for the illumination measurements, only scenario one and two aligned were measured. The comparison for scenario one aligned is shown in Figure 6.23. 20 30 40 50 60 70 80 90 100 -5 0 5 10 15 20 25 C u rr e n t (d B u A ) Frequency (MHz)
Current Distribution Radiated Induction: Scenario 1 Aligned West Rad Non-hardend West Rad Hardend
Figure 6.23: Current compared for a hardened and non-hardened telescope for scenario one aligned.
From the results shown in Figure 6.23 neither of the two cases shows a definitively higher current level. The average measured level for the non-hardened telescope was 11.4 dB and for the hardened telescope 11.2 dB. This is as expected with an aligned scenario, because most of the current seems to be already flowing on the outside of the pedestal due the clearly defined current path. Similar results can be seen for scenario two aligned which is shown in Figure 6.24. The hardened case is only higher for certain parts of the investigated band. Here average levels of 11.2 dB and 15.3 dB were measured for the non-hardened and hardened telescopes respectively.
20 30 40 50 60 70 80 90 100 -10 -5 0 5 10 15 20 25 30 C u rre n t (d B u A ) Frequency (MHz)
Current Distribution Radiated Induction: Scenario 2 Aligned West Rad Non-hardend West Rad Hardend
Figure 6.24: Current compared for a hardened and non-hardened telescope for scenario two aligned.
6.6.3. Discussion
This part of the chapter shows that the shielding measures, despite not being completely refined, do provide some level of additional shielding. A convincing indication of this is seen in the comparison of results for a hardened and non-hardened telescope in a misaligned position. Results were compared for the direct current injection and illumination methods. Findings indicate that if a more defined current path can be provided on the outside of the telescope, non- ideal shielding measures could be considered more acceptable. The fact that the cables do not enter through 360º connectors, or that the interfaces have seams providing possible points of entry, will not cause such a dramatic increase in CM current levels inside the pedestal.
Presently, the current path between the upper and lower pedestal is established by the two LDCs mounted on the upper pedestal and the four copper shoes mounted on the lower pedestal as discussed in section 6.2. The four earthing connections around the circumference of the foundation are always in alignment with the copper shoes. Therefore, by increasing the number of positions the LDC on the upper pedestal connects to the rotation ring, an aligned connection between the upper and lower pedestal will be established more often as the dish rotates. This will create defined paths to ground for the CM current to follow more frequently. This is confirmed by the finding in [5] Chapter 6 section 6.4.5. Additional verification will be done using measurements on the physical scale model as part of future investigations.