## 3 Results and Discussion
### Tubular and Annular Plasma Elements' Intrinsic Radar Countermeasure Capability  
In the radar cross-section (RCS) testing experiments, to evaluate the potential of ultra-high-density cold plasma elements as transient antennas, it is necessary to measure their intrinsic wave-absorbing performance and the effect of plasma density on the RCS value. Therefore, the RCS results for two types of plasma elements were tested: a single tubular plasma element and an annular plasma element, as shown in Figure 8a,b. In testing the radar cross-section (RCS) performance of high-density cold plasma elements, horizontally polarized (HH) transmit-receive antennas were used. The tests covered three different bands--S, C, X--to ensure a comprehensive evaluation of the stealth performance of the plasma elements across key frequencies. For each band, the wave-absorbing capability of both tubular and annular high-density plasma elements was measured under seven plasma density states: inactive (0 cm\({}^{-3}\)), 5.46 \(\times\) 10\({}^{11}\) cm\({}^{-3}\), 9.11 \(\times\) 10\({}^{11}\) cm\({}^{-3}\), 11.24 \(\times\) 10\({}^{11}\) cm\({}^{-3}\), 11.68 \(\times\) 10\({}^{11}\) cm\({}^{-3}\), 12.11 \(\times\) 10\({}^{11}\) cm\({}^{-3}\), and 12.54 \(\times\) 10\({}^{11}\) cm\({}^{-3}\). The results are shown in Figure 9.  
Figure 8: Plasma element used for testing (**a**) Tubular plasma element, (**b**) Annular plasma element. Figure 9: RCS results of plasma element. (**a**) S-band HH polarization, tubular element; (**b**) C-band HH polarization, tubular element; (**c**) X-band HH polarization, tubular element; (**d**) S-band HH polarization, annular element; (**e**) C-band HH polarization, annular element; (**f**) X-band HH polarization, annular element. Through specific measurements and analysis, it can be observed that the tubular plasma elements exhibit significant interference and absorption effects on electromagnetic waves in the S and C bands. This finding is particularly important for the application of plasma in radar stealth technology and the subsequent use of plasma elements in radar countermeasure antennas. Specifically, when handling horizontally polarized (HH) electromagnetic signals, the elements themselves achieve a minimum RCS of \(-\)63 dBsm, indicating excellent radar countermeasure performance. As the plasma density increases, the RCS values show an overall downward trend, demonstrating that the activation of plasma and the increase in its density effectively reduce the reflection intensity of electromagnetic signals in this polarization direction. In the S band, the RCS can be reduced by up to 15.12 dBsm. At higher frequencies in the X band, the impact of plasma density changes on electromagnetic waves is relatively smaller due to the shorter wavelength of X-band electromagnetic waves. For annular plasma elements, an increase in plasma density in the S band results in a significant reduction in the RCS value of the elements. The results in the C band are even more pronounced; the RCS value in the inactive state is relatively low, and with an increase in plasma density, the RCS value decreases significantly. Particularly in the range of 5.46 \(\times\) 10\({}^{11}\) cm\({}^{-3}\) to 11.24 \(\times\) 10\({}^{11}\) cm\({}^{-3}\), the radar countermeasure capability of the elements improves markedly, reducing the RCS by approximately 10 dBsm overall. This indicates that annular plasma elements have better stealth potential in the C band, making them suitable for specialized plasma stealth antenna applications. In the X band, the wave-absorbing capability of the plasma elements is weaker compared to the S and C bands, but there is still an improvement in absorption performance with an increase in plasma density. This suggests that these elements have significant wave-absorbing capabilities and potential as plasma antennas. The results show that both tubular and annular plasma elements possess excellent radar countermeasure performance.