机械电子工程中的科学计算方法（文献资料）卫星与天线相对位置 PIM CHARACTERISTICS OF THE LARGE DEPLOYABLE REFLECTOR ANTENNA MESH.pdf_大学文库

PIM CHARACTERISTICS OF THE LARGE DEPLOYABLE REFLECTOR ANTENNA MESH V. Lubrano(1), R. Mizzoni(1), F. Silvestrucci(1), D. Raboso(2) (1) ALENIA SPAZIO S.p.A., Via Saccomuro 24, 00131 Rome, Italy Email:v.lubrano@roma.alespazio.it Email:r.mizzoni@roma.alespazio.it Email:f.silvestrucci@roma.alespazio.it (2) ESA/ESTEC, P.O. Box 299, 2200 AG Noordwijk, The Netherlands Email:David.Raboso@esa.int ABSTRACT In the frame of Large Deployable Antenna (LDA) contract with ESA/ESTEC, ALENIA SPAZIO made a full Passive Inter-Modulation (PIM) product characterisation of the reflector mesh under development by S.P.A. EGS. The tests were made at three different frequency bands (Ku, C and L) and the results collected as function of several variables (PIM-order, power of carriers, mesh tension, micro-vibrations and temperature). Radiated as well as conducted (by means of contact-less PIM free flanges) test methods were adopted. The tests have demonstrated the adequacy of the mesh versus the PIM requirement provided that the mesh tension is at least 5 grams/cm and the quality of the coating is good. The PIM test campaign on the mesh samples is considered propedeutic to the tests at reflector level that will take place in the near future. INTRODUCTION Array-fed unfurlable reflectors are key constituents for L/S band satellite based communication systems. Digital Audio Broadcasting (DAB) and/or Universal Mobile Telecommunication Systems (S-UMTS) with point-to-point and/or broadcasting services from HEO or GEO constellations require contiguous cellular coverages consistent with a 12-m radiating aperture. Other missions are also possible at lower or higher frequencies (e.g. Radar, remote sensing, interferometers etc.), provided that the reflector surface accuracy is consistent with the operating frequency. Within an on going ESA/ESTEC program [1, 2], ALENIA SPAZIO is currently developing an EQM model of 12-m projected aperture unfurlable antenna together with Russian–Georgian EGS LTD (responsible of the reflector) and three European comp anies for the reflector arm. In Fig 1, the antenna geometry is illustrated. Depending principally upon the acceptable Passive Inter-Modulation interference (PIM), the antenna subsystem can be based on two separated Tx and Rx antennas or both functions can be combined within a single antenna unit. The critical components for a PIM free design are the unfurlable reflector mesh and the high power feed-array section beyond the Tx/Rx antenna filters to the radiating elements. A state of the art product asks for a reflector contribution significantly lower than the feed-array itself, with PIM power level as low as –140-dBm when illuminated by two carriers of 200-W via 16.5-dBi gain horn placed at the focal point. The two carriers frequencies are set to provide the highest PIM tone in the receive band consistent with the mission, so that typically a fifth to seven order is applicable at L/S band while at higher frequencies the third order may also apply. PIM tests on several reflector mesh samples, representative of the flight technology of the unfurlable antenna, were performed at ALENIA plant with proprietary set-up at C and Ku band and ESA/ESTEC bench facility at L band. Radiated as well as conducted (by means of contact-less PIM free flanges specifically designed and manufactured by ALENIA SPAZIO, to achieve a higher sensitivity set-up noise floor at L-band) test methods were adopted. Several PIM orders and power densities were applied at different mesh tensions and mesh finishing. Also vibrations and limited temperature excursions over the samples were performed in order to assess PIM susceptibility of the mesh versus the plating process, the mesh tension and the impact of micro-vibrations that may be induced under spacecraft manoeuvres

Fig. 1-LDA antenna geometry An important step is the correlation between the measured PIM values on the samples and the PIM expected on the real antenna. To this end, any contact point of the mesh may be considered as a potential PIM source that provide either a phase-correlated PIM field density over the antenna feed-array plane or, adversely, all the infinitesimal PIM contributions can be phase-uncorrelated. The first situation is consistent with the assumption that any contact point of the mesh has identical PIM susceptibility since the tension and the finishing of the mesh is uniform. On any given point over the feed-array focal plane, the PIM field density will be the coherent sum of all the mesh nodes contributions. Each node of the mesh provide a field level according to the primary field intensity impressed at the reflector mesh at that point by the illuminating feed with a relative phase dependent by the reflector geometry and the primary feed phase pattern. Under these conditions, since the reflector has focusing properties, a very low level of the PIM field at any point of the feed-array plane can be expected, provided that the antenna has adequate clearance, as in this case. Conversely, the uncorrelated case appears to be a worst condition. In this case, power adding of all infinitesimal PIM contributions due to the mesh nodes, at any point of the feed-array plane is performed. Being the PIM sources assumed independent and phase uncorrelated, the PIM power level at any point of the reflector focal plane depends upon the impinging primary field intensity over the surface and the PIM roll-off versus power according to the PIM order. Assuming the feed aperture in the far-field, the received PIM power is: ( ) ( / 4 ) * ( , )* Pr ( , )* ( Pr , ) 1 2 PIM refl R G efl efl PIMorder i i Ntot i å lPIM p i R Ji ji d J j s d = = (1) where: GR(qi,ji) is the feed gain at (qi,ji) at the PIM frequency; dPrefl(qi,ji)=Pc*GT(qi,ji)/(4pRi2 ) is the carrier power density at point mesh (qi,ji) where Ri is the feed distance to the point mesh, Pc is the carriers power and GT(qi,ji) is the feed gain at the carrier frequencies gain; s(dPrefl,PIMorder) is a scalar factor which accounts for the isotropically re-irradiated PIM power level by the junction, which depends by the carrier power density dPrefl(qi,ji) incident on the junction, the PIM order and the mesh quality; Ntot = total number of mesh nodes = Arefl*dnodes where Arefl is the curved reflector area and dnodes is the number of nodes per unit area. For a plane sample with area Asample, placed in the far-field of the illuminating feed, the same equation holds, allowing to correlate the PIM level measured on the sample with that expected at the antenna level, once the geometry of the setup and the feed used for the tests are known

A disadvantage of the radiated test set-up is the limited sensitivity of the bench. Typically the sample must be illuminated at very high power densities not representative of the real carrier power densities as seen by the reflector antenna. In the conducted measurements case, based on a contactless waveguide set-up, this problem can be highly reduced because the attenuation factor due to the relative feed-sample distance does not apply. Additionally, the noise floor of the bench is lower because the mesh itself filters the carriers. The incident power density law distribution of the carriers on the sample can be easily evaluated on the basis of the TE10 fundamental propagating mode. Assuming a symmetrical scattering of the PIM (valid if the mesh thickness can be neglected), the received PIM power, scattered by the sample, is only a function of the carriers power density on the sample w.r.t. the reflector mesh. Therefore, the correlation of the PIM level of the sample with that on the feed placed in the reflector focal plane is proportional to the two areas ratio, properly weighted by a scalar factor which accounts for the impinging field intensity distribution over the two surfaces, the PIM roll-off Vs power and the set-up test type/geometry that correlate the PIM sources distance w.r.t the PIM detector. MESH DESCRIPTION The mesh of the Large Deployable Antenna (LDA) reflector is made of three wires of tungsten (twisted together) with the individual core diameter of 15-microns. Overcoat covering each micro-wire is made of gold (thickness 0.25- microns) lay over an intermediate layer of nickel (thickness 0.1/0.2-microns). The mesh installed on the reflector will have a nominal tension of 5/6 grams per cm along the piece and 7/8 grams per cm across the piece. A specially constructed mesh tension frame enabled measurements to be taken, as a function of tension, ranging from 1.0 to 60 grams per cm. The complex mesh structure suggested a number of potential mechanisms for PIM product generation: relative movement of the mesh wires, the variety and uncertainty of electrical contact points and their oxidation were potential PIM sources. So measurements were made under simulated environmental conditions including low-frequency vibrations, temperature variation and various tensions. Fig. 2 gives an expanded view of the mesh sample. Fig. 2-Mesh sample expanded view