## 1 Introduction  
High-power microwave (HPM) radiation presents a great threat to an aircraft's electronic system and has been widely used for military applications [1; 2; 3; 4]. As microwaves can heat dielectric materials effectively [5; 6; 7; 8; 9], in a high-speed air current the absorbing coating on the aircraft's surface will suffer a harmful heating process. There are also non-heating effects that have not been clearly understood by researchers and may cause some damage to the absorbing material [5; 10; 11; 12]. Therefore, the damage effect of high-power microwaves on absorbing coatings has attracted our attention. To study this damage effect, high power microwave experiments play an important role. Such experiments require high microwave power density at X band to be injected into a wind tunnel so that the heating effect can be measured in a high-speed wind field [3]. The whole experimental system is illustrated in Figure 1. The injected microwave can be harmful to the equipment inside the wind tunnel and to the research workers outside the wind tunnel because it can propagate along the wind tunnel and leak from some connecting junctions. Meanwhile, the field distribution on the testing object is different from the realistic one, due to the scattering effect. In this work, considering that the absorbing materials attached on the wall cannot effectively attenuate the part of the scattered microwave with the direction along the wind tunnel, the absorbing array of dielectric bars in the middle of the wind tunnel is studied so that this part of the microwave can be attenuated more effectively. In another work, we tried to eliminate the influence of the tunnel on the testing object under microwave radiation by designing the focusing antenna, the geometry of the testing object and the absorbing arrays attached on the wall; this will be introduced in another paper. The absorption of the injected microwave can be a problem. On one hand, the extremely high-speed airflow restricts the application of common absorbing techniques as the material of the absorbing apparatus should be able to endure strong pressures. The total cross-section area of the absorber is also required to be much less than that of the wind tunnel so that the experimental wind speed is not influenced. Simple structures such as bars or sticks have a very small cross-plane area. For this reason, rectangular dielectric lossy bars (RDLBs) are investigated as the absorbers in the microwave radiation effect experiment. For simplicity, the propagating modes on the RDLB are assumed to be excited by an incident plane wave whose propagation direction is parallel to the longitudinal direction of the RDLB, as shown in Figure 1, for the following reasons: (i) the geometrical size of the testing object and the distance between the testing object and the RDLBs are both much larger than the wavelength of ~3 cm, so the far field condition can be easily satisfied; (ii) it has been mentioned above that the absorbing array in the middle of the wind tunnel is used to attenuate the incident wave whose propagation direction is parallel or nearly parallel to the bar; as for the scattered waves not parallel to the bar, they will be absorbed by the absorbing structures attached on the side wall; (iii) although the amplitude of the incident wave varies in the cross section, the amplitude of the incident wave near a bar can be approximately uniform if sufficient gaps are provided between adjacent bars; thus, in the following discussions, the incident plane wave is assumed. In addition, the required microwave power can be produced by a travelling wave tube that can deliver an output power of ~1 MW contained in a pulse of thousands of microwave periods. Therefore, the signal of the electromagnetic wave is assumed to be a time-harmonic sine signal and the transient effects are therefore not considered in this work. It is also assumed that the incident plane wave has the same polarization orientation as that of the microwave field radiated by the linearly polarized focusing antenna.