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Affordable Radar Antenna with Electronic Elevation Scan and Multiple Beams



OBJECTIVE: Develop a new antenna for the AN/SPS-49 radar that incorporates electronic beam steering in elevation, provides for multiple elevation beams, and incorporates the means for shaping of both transmit and receive beams to improve high elevation radar coverage. 

DESCRIPTION: The AN/SPS-49 is a venerable radar deployed widely throughout the Fleet. In such legacy systems, life-cycle cost reduction is a constant goal and maintenance cost is the key driver. Being a rotating radar, periodic overhauls of the antenna are required to replace worn or weathered parts, repair physical damage, re-seal, and re-paint. Little can be done to avoid this. However, the SPS-49 antenna incorporates one feature that might be simplified by the introduction of innovative technology. The SPS-49 antenna is a parabolic reflector fed by a H-plane sectoral horn. The reflector is asymmetric with the wide dimension aligned horizontally and the H plane of the horn aligned vertically with the narrow dimension of the reflector. Elevation scan in the SPS-49 antenna is accomplished through mechanical drives, powered by electrical motors that “rock” the entire antenna assembly to compensate for ship motion (roll and pitch). This mechanical assembly adds weight, is prone to wear, and requires robust electrical controllers located below deck. Furthermore, the antenna rotary joint must pass DC electrical power in addition to the radio frequency (RF) transmit power. Repair and replacement of these components contribute greatly to the overall SPS-49 sustainment cost. If the antenna elevation could be varied through electronic means, electro-mechanical parts would be eliminated, weight could be reduced, and the life-cycle cost of the radar would decrease, even though the antenna would still need to rotate. The Navy seeks an innovative rotating antenna technology, compatible with the SPS-49 radar that provides simple, non-mechanical elevation scan over a limited range. The “antenna” in this case is considered only that (rotating) portion above the pedestal and rotary joint that forms and transmits the beam. The most mechanically and electrically simple, lightweight, and affordable solution that meets the performance requirements is desired. In addition to meeting the existing SPS-49 elevation requirement, a desired solution would be for the electronic elevation scan technique to also permit implementation of multiple elevation beams. A minimum of two elevation beams are required to allow elevation estimation against low-to-medium altitude targets, and appropriate beam shaping will be needed to achieve the required cosec2 coverage. As a goal, more than two elevation beams are desired if this can be achieved while meeting the requirements for performance, beamforming, size, and weight described below. It is understood that, in meeting these objectives, the addition of duplexers and other beamforming electronics (as part of the antenna) may be necessary. However, if incorporating active elements, the antenna should not introduce harmonics or inter-modulation products in the transmitted signal. Examples of antennas that could enable electronic steering in the elevation plane (and potential implementation of multiple elevation beams) include the use of a vertical array of “row-boards” with individual phase control (by row) and corporate feed, phased array feeds with a main reflector surface, reflective printed-element arrays (“reflect arrays”) with element-level electronic phase shifting illuminated by a primary feed horn, and transmissive printed-element arrays (“transmit arrays” or “array lenses”) with element-level electronic phase shifting illuminated by a primary feed horn. While examples of these antenna types have been demonstrated before, the sheer size and power of the SPS-49 antenna and its requirements for beam shape and elevation scan represent a significant technical challenge, especially in light of the desire for a lightweight, rugged, and yet affordable design. The current SPS-49 antenna reflector is approximately 24 feet wide and 8 feet tall. The weight of the rotating assembly (reflector, feed, and supporting structure) is approximately 2000 pounds. Due to ship structural considerations, i weight and overall size cannot be exceeded. At a minimum, the desired antenna must transmit across the band 850-950 MHz with a total elevation scan of ±25 degrees. The peak power supplied to the antenna at the output of the rotary joint is 300 kW maximum (at 4% duty cycle) and the desired aperture efficiency (relative to the power supplied at the rotary joint) is 65% minimum. The transmitted beam should have a 3 dB beam width in the azimuthal direction of no more than 3.5°. In the elevation plane the combined transmit and receive patterns shall provide cosec2 coverage to 30 degrees. The antenna gain shall be at least 28 dB (measured relative to the power supplied by the rotary joint). Azimuthal side lobes shall not exceed -30dB (relative to the peak antenna gain) in the region of 10° on either side of the main beam. Beyond 10° from the main beam, side lobes shall not exceed -15 dBi (relative to isotropic). The interface to the antenna is through a rotary joint, which is not considered part of this effort. Proposed designs should assume a waveguide feed and an available communications path (analog or digital) to control the elevation scan. If the proposed technology will incorporate electronics integrated within the antenna assembly, low voltage power (nominally 24 V maximum) can also be assumed available through the rotary joint. However, active liquid cooling is unavailable. A prototype antenna is desired and, should a reflect array, transmit array, or similar type antenna be selected, the feed is considered an integral part of the design. However, as a full-size prototype will likely be prohibitively expensive, a partially populated antenna array is acceptable, provided that the full antenna performance can be determined through extrapolation (by analysis, modelling, and simulation) of measured prototype data. Likewise, cost, size, and weight shall be extrapolated from the partially populated prototype. The prototype need not be subjected to environmental testing (which is also prohibitively expensive), but the prototype design shall anticipate the need for environmental enclosures (radomes, gaskets, seals, etc.) and structural strengthening for shipboard operation and rotation at 12 rpm when determining final size, weight, and cost. Estimates of weight shall include a mechanical structure capable of withstanding high winds (90 knots operational and 120 knots without damage) and icing in accordance with MIL E 16400 (and without sustaining damage with ice loading of seven pounds per square foot of antenna surface). 

PHASE I: Propose a concept for an affordable and lightweight antenna meeting the objectives and performance parameters described above. Demonstrate feasibility through a combination of analysis, modelling, and simulation. The feasibility analysis shall include predictions of performance parameters, size, weight, and cost described in the Description. The Phase I Option, if exercised, will include development of initial design requirements, performance specifications, and a capabilities description to build a prototype solution in Phase II. 

PHASE II: Develop and deliver a prototype (or partially populated array prototype) that meets the requirements defined above. Ensure that the prototype should be sufficiently complete (populated) such that measured data is meaningful and can be extrapolated (using analysis, modelling, and simulation) to predict the performance of a full prototype antenna. The size, weight, and cost of a full, qualified (deployable) antenna shall also be extrapolated from the data obtained from the prototype design. At the conclusion of Phase II, the prototype antenna (and supporting data) will be delivered to the Government for additional testing, design analysis, and to facilitate future systems integration. 

PHASE III: Support the Navy in transitioning the technology for Government use. This is expected to entail the finalization of specifications, completion of a final design, production of a drawing package, selection of materials, testing, and support during system and ship integration. The final antenna will be tested according to the SPS-49 system specification and applicable military specifications for shipboard equipment. The final product will therefore be a complete antenna, suitable and qualified for replacement of the existing SPS-49 antenna. The technology should also find additional applications for other surface shipboard radar systems and possibly land-based military radars. Potential commercial applications include weather and air traffic control systems. 


1. “AN/SPS-49(V) Radar Set.” United States Navy Fact File, 20 September 2018.

KEYWORDS: Reflect Arrays; Transmit Arrays; Array Lenses; Phased Array; Electronic Elevation Scan; Electronic Phase Shifting 

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