Prospects of combined antenna systems in onboard electronic protection systems of aircraft

Introduction

The basic characteristic of radio engineering facilities with combined transceiver systems is isolation, i.e. space coupling ratio W between the receiving and transmitting antennas. Ensuring a required level of W between the receiving and transmitting antennas allows to avoid the problem of discerning signals in the receiving device against the background of transmitter radiation. For real sensitivity of the station receiver, a correct isolation level is required, therefore it is necessary to go for the maximum attainable W, while trying to preserve the optimal directional and matching characteristics of antennas. However, in so doing it is important to find a compromise between the required characteristics of transmitting antennas and their isolations with the receiving antenna.

Isolation between antennas also characterises their near-field and/or far-field coupling. It is exactly the near-field coupling that makes a greater contribution to the space coupling ratio. Contribution of the far-field coupling between the antennas is minor but difficult to eliminate. To reduce it, it is necessary to optimise the antenna directivity in the main and broadside direction of radiation or reception.

One of the primary requirements imposed on airborne countermeasure stations is their compact size. In this respect, an engineering task arises to provide for such arrangement of station’s receiving and transmitting antennas that ensures the necessary level of isolation in a restricted space. As a rule, under onboard equipment operating conditions it is unfeasible to separate the receiving and transmitting antennas to a sizeable distance, therefore various absorbing structures [1], materials [2], specific geometry features of construction [3], and other means are applied.

The tendency for reducing mass and dimensional parameters of jamming stations is associated with element base improvement, hence the requirements to overall dimensions when arranging antennas in a unit become ever more stringent. Working for the long run, it would not be right to consider just one of the options for improving isolation in the transceiving path, so attention should be paid to possible combination methods [4].

It is possible to check the effectiveness of isolation methods by means of 3D electrodynamic simulation in the CST software, with subsequent fabrication of a mockup and experimental study of the obtained characteristics. Application of the confidence regions method in electrodynamic simulation enables to calculate with high accuracy the influence of transmitting antennas on the receiving antenna operation across the entire operating frequency band of the station. By combining various options of shielding and arrangements of antennas in 3D environment, it can be possible to improve space coupling ratio W.

Although the number of papers dealing with the known methods for improving isolation is fairly large, they do not offer any elaborated solution to the problem of reducing space coupling ratio W between the receiving and transmitting antennas of combined systems.

This paper discusses a scientific and engineering solution to the problem of optimising the arrangement and geometry of antennas, with account for interactions in the near and far fields that affect the magnitude of edge currents. The novelty of the presented material consists in solving the problem of reducing the ratio of space coupling between the receiving and transmitting antennas of combined systems. There is also a description of application of combined methods for increasing isolation in a compact aircraft-based electronic countermeasure station.

There is a possibility to increase W through application of an integrated approach, comprising implementation of specific techniques of improving isolation and calculation of parameters by the method of 3D mathematical simulation. The obtained results were checked and verified on a mockup sample.

Original antenna unit

In the original antenna unit with dimensions 200×220×230 mm, to reduce coupling ratio W, only the method of applying screening partitions between the transmitting and receiving antennas was used, being one of the most viable methods. However, with such a form of the shielding surface, a given arrangement of antennas, and using only the above-mentioned method for reducing W (Fig. 1), the attainable isolation between the transmitting and receiving antennas within a specified (limited) volume of space ensures the correct operation of an electronic countermeasure station only when using time selection of signal receiving and transmitting modes.

Fig. 2 shows dependencies of coupling ratio W between antennas in the original unit by the example of transmitting antenna А1 and receiving antenna А3 [5]. Fig. 2 shows dependencies of the coupling ratio between transmitting antenna А1 and receiving antenna А3 of the computer model and the original design. In this example, comparative evaluation of isolation effectiveness of options А1–А3 and А2–А3 is not discussed, since transmitting antennas А1 and А2 are inherently combined by design. Divergence of the study results is possible due to the fact that the simulation is implemented using idealised material properties, without account for probable diffractions in an anechoic environment, contrary to a real experiment under laboratory conditions. A graph of the experimental values (see red line in Fig. 2) was obtained using the domestically made measuring instrument R2М-18 with an activated inter-frame averaging function. It should be noted that the measurement results without the said function were dynamically unstable during the experiment under available laboratory conditions.

Upgraded unit

An upgraded unit was essentially modified with the use of combined measures for reducing coupling ratio W.

The modification included (Fig. 3):
• electrodynamic simulation at all upgrading stages for the purpose of identifying the best measures for obtaining the most optimal characteristics;
• the maximum feasible diversity of the receiving and transmitting antennas in a given space and under the selected structure of the external box and radome;
• arrangement of each antenna in individual compartments, isolated from one another by bevelled shielding partitions;
• applying absorbing material of KhV grade [6], 4 mm thick, on the shielding partitions;
• changing the receiving and transmitting antennas to orthogonal polarisation by turning antennas А1 and А2 through 90°.

Let us elaborate on propagation of surface waves over the unit structures shown in Fig. 4. A pseudocolour image demonstrates distribution of the propagating electric surface waves (ESW) over the antenna unit’s inner walls between the receiving and transmitting antennas, with the position of maximum shown as well. Marked with red colour are ESW “stagnant” areas. In those areas the energy is not radiated into space, due to which “standing waves” are produced, which increase the level of reflections at the antenna inputs. Marked with blue colour are the areas of ESW amplitude minimum, i.e. those where there is no ESW at that time. It can be seen that at both frequencies the pattern of field distribution in pseudocolour representation is identical in both compartments with the transmitting and receiving antennas. It confirms high near-field coupling due to imperfection of the original design.

Each compartment where receiving antenna А3 and transmitting antennas А1 and А2 are located is screened from the others (see Fig. 4, b), with absorbing material KhV secured on the compartment walls. For this reason, no identical brightness points and identical patterns of field ESW propagation, as in Fig. 4, a, can be observed.

In this way, the near-field coupling between the receiving and transmitting antennas is reduced, which is illustrated by the graph in Fig. 5.

The isolation is characterised mostly by the far-field coupling, i.e. spatial and polarisation diversity of antennas and their height, as well as the thickness of screening partitions between them. As compared with the values in Fig. 2, isolation improvement can be seen. The fabricated mockup is shown in Fig. 6.

Conclusion

The results of the technical solution to the problem of increasing isolation in a combined antenna unit of a compact aircraft-based electronic countermeasure station are presented.

In development of a new antenna unit, a combined parameter improvement method was used. As a result, the isolation values increased from 28…45 dB to 54…76 dB. Comparing study results of the presented antenna units, it can be concluded that the use of combined options is the most promising method for improving isolation.

When using the combined option considered, the following methods for improving the modulus of the coupling ratio between antennas can be applied:
• electrodynamic simulation;
• spatial diversity of antennas;
• special forms of screening partitions;
• use of absorbing materials.

The obtained results can be used in development of aircraft onboard systems with closely arranged antennas.