This paper addresses a type of deployable mesh antenna consisting of the double-ring deployable truss edge frame and the cable net reflector. The structural design concept of the deployable antennas is presented. The deployable truss is designed and the geometric relationship of each strut length is formulated. Two types of radial truss elements are described and compared. The joint pattern and the active cables of the final design concept are determined. The pattern of the cable net is the three-orientation grid. Two connection schemes between the reflector and the deployable edge frame are investigated. The design parameters and the shape adjustment mechanism of this cable net are determined. The measurement test technologies of the antennas on the ground including test facilities, deployment test, and measurement and adjustment test are proposed. The antenna patterns are analyzed based on the real surfaces of the reflector obtained by the reflective surface accuracy measurement. The tests and analytic results indicated that the accuracy of the reflective surface is high and is suitable for low-frequency communication.
The deployable mesh antenna consisting of the deployable edge frame and the spatially determined cable net has long been considered by space mission planners for the development of large-size spatial antennas. Many research and development efforts, covering innovative design concepts and analysis methods, were conducted to enhance the state of the-art of deployable antenna technology. The number of conference and journal papers published to report the results of these research efforts is very large [
The recent research works focused on the cable net reflector including shape finding, prestress optimization and shape adjustment, and so forth. A new approach based on a force density strategy was presented to calculate a geodesic tension reflector that ensures both appropriate node position and uniform tension by Morterolle et al. [
This paper reports a recently conducted effort that developed an innovative design concept of mesh deployable antennas. The structural stiffness of the edge frames is improved, and the design concept is intended for large size mesh antennas. The deployable mechanics of the edge frames and the connection schemes of the cable net reflector to the edge frames need to be investigated carefully.
The remainder of this paper consists of four sections. Section
For the mesh antennas, the most important component is the reflective surface. The reflective surface is stretched over the cable net and attached to the edge frames. The cable net consists of four kinds of cables: front, vertical, tie, and back cables. Front cables link the reflective surface nodes that are distributed uniformly on the parabolic surface. Therefore, the shape of reflective surface is determined by the location of the front cables. The vertical cables are for correcting the surface error which arises in the manufacturing process by adjusting the cable lengths.
It is difficult to satisfy the reflector precision requirement due to the flexibility of the cable net, especially for the space antenna with larger size. To reduce the surface error between the real reflector and the theoretical reflector, many qualifications are required for the structural design of the edge frames, such as the feasibility of deployment, the high stiffness, and the high packaging efficiency. A new design concept of deployable antenna is proposed and investigated, in which a double-ring deployable truss structure was employed as the edge frames.
Two types of double-ring deployable truss are proposed, and the geometric equations of the truss structure are formulated in this section. The double-ring deployable truss is a symmetric structure and consists of
Top view of the deployable truss.
Two types of double-ring deployable truss concepts.
The first truss concept
The second truss concept
There are two types of radial truss elements in the double-ring deployable trusses. The radial truss element in the first concept is a parallelogram mechanism, which is shown in Figure
Radial truss elements of the first truss concept.
Deployed configuration
Folded configuration
Activator joint F
The radial truss element of the second concept is also a parallelogram mechanism, which is shown in Figure
Radial truss elements of the second truss concept.
Deployed configuration
Folded configuration
Activator struts DE.
For the same section of the diagonal member, the stowed volume of the first concept is larger than that using the second one. The manufacture and assembly of the first concept is also more complex and difficult. Based on the comparison between these two types of double-ring deployable truss, the second concept was selected for the final demonstration model. The basic spatial deployable element of the demonstration model is a multiple-loop kinematic chain, shown in Figure
Spatial deployable element.
For the
Variations of the length ratio with the side number.
Then, the side number was determined as 18, giving a length ratio
As shown in Figure
Two types of truss element joints.
4-struts joint
7-struts joint
There are many kinds of cable network pattern for mesh antenna, such as radialized cable net, quasigeodesic cable net, and three-orientation grid cable net. After the comparison of the prestress distribution and the mechanical properties, the three-orientation grid cable net is determined for the deployable mesh antenna.
There are two schemes to connect the cable net reflector with the double-ring edge frame. The first one is shown in Figure
Attachment schemes of the cable net to the truss.
The first scheme
The second scheme
For the second scheme, there are some vertical cables placed between the outer ring and the inner ring truss. It is difficult to avoid the physical interference between the cable net and the deployable truss during the deployment process. The height of the folded configuration of the second scheme is larger than that of the first scheme. The load path of the second scheme is less direct than the first approach. Though the size of the reflective surface of the second scheme is larger than that of the first scheme, the first scheme is used to design the cable net reflector.
To decrease the height of the mesh antenna, the focus length of the front cable net reflector and the back cable net surface are designed to be different values. The major design parameters of the reflective surface of the demonstration model include the following. The aperture of the reflective surface was The ratio between the focus length and aperture of the front reflector was The ratio between focus length and aperture of the back surface was The mesh number of the radial cable is 4. The mass of the entire model is 15.1 Kg. The height of the folded configuration is 1.23 m and the diameter of that is 0.52 m.
The coordinate of each joint and the geometric topology of each cable are determined by the equation of the surfaces and the three-orientation grid pattern. The geometrical model of the cable net reflector is shown in Figure
Geometrical model of the cable net.
Kevlar cord, 0.7 mm in diameter, was used for the cable net. An inverse iteration algorithm based on the finite element analysis was used to determine the pretension force distribution of the cable nets. A lightweight adjustment mechanism was designed to adjust the prestress of the vertical adjustable cables, as shown in Figure
Adjustment mechanism of the vertical adjustable cables.
The purpose of this section is to sum up the measurement test technologies of the antennas on the ground. There are two important tests for the deployable mesh antenna: deployment test and surface accuracy measurement test. It is extremely difficult to evaluate the deployment characteristics of a large complete deployable structure on the ground. The gravity force affects the deployment process of the deployable mesh antenna heavily. The test facilities available to simulate space environments are designed for the deployment test. One type of these facilities is the suspend system, including the support frames, the plane orbit, the suspend cables, and the fix end mast, as shown in Figure
Suspension system for the deployment test.
Another test facility reducing the effect of the gravity force is shown in Figure
Universal hinge joint system.
Deployment test of the antenna model.
The cable net reflector is a flexible structure and therefore its surface accuracy was best determined using a noncontact measurement approach, to avoid any unintentional local deformation caused by contact forces. To meet this requirement, a photogrammetric measurement system based on the PhotoModeler software packages was selected. There are 61 targets fixed in the joints of the front cable net surface in the measurement test. Three cameras were needed in the measurement test, as shown in Figure
Measurement of the surface accuracy.
RMS in each measurement step.
The section aims at the antenna pattern analysis of the deployable mesh antenna. The Ticra software Grasp is utilized to analysis the antenna patterns based on the real coordinates obtained from the measurement test. The primary feed horn operates in two frequency bands of 0.5 and 1.0 GHz. The antenna patterns based on the configuration after measurement step 10 are showed in Figure
Antenna patterns of the surface from measurement step 10.
0.5 GHz
1.0 GHz
Antenna patterns of the final reflective surface.
0.5 GHz
1.0 GHz
When the frequency band is higher (3.0 GHz e.g.), the aberrance of the antenna pattern will be very bad and unacceptable, as shown in Figure
Antenna pattern of the final surface (3.0 GHz).
A type of deployable mesh antenna consists of the double-ring deployable trusses, and the cable net reflector was proposed and investigated. The structural design concept of the deployable mesh antennas was presented. The mechanical measurement tests of the antennas on the ground including test facilities, deployable test, and measurement and adjustment test were introduced. The mass of the entire model is 15.1 Kg. The height of the folded configuration is 1.23 m and the diameter of that is 0.52 m. The test results showed that the antenna driven by two motors can be deployed successfully, and the surface accuracy is high after adjustment and measurement. The antenna pattern was analyzed based on the real surfaces of the reflector obtained by the reflective surface accuracy measurement. The analytic results indicated that this type of mesh antenna is suitable for low-frequency communication. The reason of the bad antenna pattern for the mesh antenna was also discussed.
To apply this type of mesh antenna in high-frequency communication, the mesh number of the radial cable needs to be large and the size of triangle grids in the reflective surface is designed to be small. But this increases the number of the vertical adjustable cables, the adjustment after installation would be difficult, and the weight and the driven force for deployment would be increased as well. Therefore, the optimize design of the structure-electromagnetic performance for this type of deployable mesh antenna is interesting in the future.
This work was supported by grants from National Natural Science Foundation of China (Grant no. 51008271) and the Fundamental Research Funds for the Central Universities (2012FZA4026).