Beamformers - General Information
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Beamformers are complex networks used to precisely control the phase and amplitude of RF energy passing through them. They are typically used in two complementary modes.
I) In RF transmitting systems such as radars, beamformers are employed between the RF signal source and the antenna radiating elements to "shape" The resulting radiated electromagnetic field in terms of its instantaneous field intensity in three dimensional space. The result is to "illuminate" a target or region of space with a precisely contoured beam’ of RF energy whose characteristics are known and to varying degrees can be controlled.
2) In receiving systems, beamformers are employed between the antenna arrays and the receiver to affect (shape) the relative spatial sensitivity of the system to RF signals originating in its field of view. The result is to effectively "focus" the receive system on a specific target or region of space.
Beamformer configurations vary widely from just a few basic building blocks up to tens of thousands of them depending on system performance requirements. Nevertheless, it will be shown that just a few building blocks can be combined in a few ways to meet even complex performance demands requiring thousands of’ replications of these basic building blocks.
There are two basic ways to shape an RF beam.
- Passive elements
- Active elements
The simplest beam forming method uses passive reflectors or parasitic elements to affect the near field region surrounding the radiating element. Conductive surfaces shaped into carefully planned geometric shapes such as paraboloids are used to create a pattern of constructive and destructive interference in the vicinity of the radiating element. When properly designed and implemented. The result is a precisely shaped beam of RF energy focused in the desired direction with a shape and extent tailored to the application.
Antennas based on parabolic reflectors have found use at frequencies as low as 45MHz in very large systems. However as a practical matter, parabolic antennas find their greatest use in wavelengths less than I meter; above 300 MHz. Additionally, below about 10Hz, arrangements of linear parasitic elements in planar arrays are used extensively to form carefully contoured beam patterns.
Using passive antenna elements such as paraboloid or linear parasitic reflectors as beamformers has the advantage of simplicity and low cost. However, die disadvantages are many. For example, due to the massive structure involved. arrays based on passive reflectors cannot move as rapidly as may required in systems that mist track rapidly moving objects or must track many objects "simultaneously" by quickly moving the beam from one target to another. Even when the antenna can be made small and of lightweight materials for use on satellites, rapidly moving an antenna array to focus on diverse regions in sequence would impart highly undesirable momentum to the overall spacecraft resulting in an unstable platform requiring expensive dampening provisions.
Active elements accomplish Beamforming by substituting actively radiating antenna elements for passive ones to create the desired field. Precisely controlled RF currents are fed to the active antenna elements which create the desired beam shape from the controlled constructive and destructive interference patterns established in the near-field region of the array. Thus, there is no need to physically move the antenna to focus the beam. on various targets. The beam can be moved arbitrarily. Moreover, the shape or The resulting beam can be dynamically altered from a broad "floodlight" illumination of a region to a "spotlight" small beam focused closely on a target of interest. The ability to dynamically alter the shape of a radar team is of great importance in tracking missiles and satellites.
As mentioned above, "beamformers" work by carefully controlling the amplitude and phase of RF energy conveyed to the radiating elements of an antenna array. Manipulating the amplitude and phase of the RF energy can be accomplished at various pouts in the path between, RF signal generation and its ultimate radiation as an RF field. However, maintaining a precise phase and amplitude relationship through an arbitrary number of active stages is difficult. When system requirements include a broad range of fre4lueneies as well, the technical challenges mount.
As a result, standard practice is to use passive networks operating at both the transmit and receive frequencies.
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