Waveguide Engineering

Corporate Feed

Q: How does a corporate feed differ from a series feed?

In a corporate feed the input splits through a symmetric binary tree of power dividers, so every element sits at the end of an identical path length. Because all paths are equal, the electrical phase to each element is independent of frequency, and the array stays at broadside across a wide bandwidth. A series feed instead taps elements off a single transmission line one after another, so the inter-element phase changes with frequency and the beam squints as you tune away from the design frequency. The cost of the corporate feed is area and loss: an 8 by 8 array needs 63 two-way dividers and the longest meandered line, while a series feed uses far less line. Corporate feeds are preferred for fixed-beam broadside arrays and active phased arrays where each element has its own phase shifter; series feeds are used for low-cost slotted-waveguide and traveling-wave arrays where a small designed beam squint is acceptable.

Q: How much insertion loss does a corporate feed network add?

Loss scales with the number of divider stages, which is log2(N) for N elements. Each ideal lossless reactive split removes 3 dB per branch as a normal consequence of dividing power, so a 16-element microstrip corporate feed has 4 stages and an inherent 12 dB split per output, plus real dissipative loss. On RF-grade laminate at X-band that dissipative loss runs roughly 0.1 to 0.3 dB per centimeter, and the longest meandered branch in a large planar tree can be 10 to 20 cm, giving 1 to 4 dB of extra one-way loss. Wilkinson dividers add isolation resistors that dissipate reflected and unbalanced power, while reactive T-junction splitters avoid the resistor loss but give no isolation. Waveguide and air-stripline corporate feeds cut the dissipative loss to a small fraction of a dB and are used where transmit efficiency or receive noise figure is critical.

Q: Can a corporate feed apply an amplitude taper for sidelobe control?

Yes. By setting unequal split ratios at each divider, the corporate feed delivers a chosen amplitude distribution (Taylor, Chebyshev, or binomial) across the aperture to lower sidelobes. A uniform feed gives roughly -13 dB first sidelobes; a 25 to 30 dB Taylor n-bar taper requires divider ratios that taper the edge elements to about 10 to 30 percent of the center power. The split ratio of a microstrip divider is set by the relative line widths or by quarter-wave transformer impedances at the junction, and unequal Wilkinson dividers use a specific resistor value and arm impedances to keep ports matched. The trade-off is a wider main beam and lower aperture efficiency, since tapering the edges reduces the effective aperture area.

/KOR-puh-rit feed/

Used to drive the elements of an antenna array, this feed splits the input through a symmetric binary tree of power dividers so that the path length from the common port to every element is identical. Equal path length means the phase delivered to each element is the same and does not change with frequency, which keeps a fixed-beam array pointed at broadside over a wide band. The architecture is the standard choice for planar microstrip arrays and active phased arrays, where unequal split ratios at each junction also impose a Taylor or Chebyshev amplitude taper for sidelobe control. The trade-off versus a series feed is more board area, longer meandered transmission line, and higher dissipative loss as the array grows.

Category: Waveguide Engineering

Divider Stages: log₂(N)

Beam Behavior: Squint-free broadside

How a Corporate Feed Distributes Power

A corporate feed earns its name from the way it branches like a corporate organization chart: one common input port splits into two, each of those into two more, and so on, until the final branches reach the radiating elements. For an N-element array (with N a power of two) the tree has log₂(N) stages and N minus 1 two-way dividers. An 8 by 8 planar array of 64 elements therefore needs 63 dividers arranged in 6 stages. Because the splitting is fully parallel, the signal reaches every element at the same time; the geometry guarantees equal path length without relying on any frequency-dependent line lengths.

That equal path length is the defining advantage. Since each element sees the same electrical delay, the inter-element phase is zero regardless of frequency, so the array radiates at broadside and does not squint as the operating frequency is tuned. This makes the corporate feed broadband in a way a traveling-wave or series feed cannot match. The price is physical: the dividers and the meandered interconnect consume aperture area and add dissipative loss that grows with array size, and on lossy laminate that loss can degrade transmit efficiency and receive noise figure for large apertures.

Designers exploit the same tree to shape the aperture illumination. By making each junction an unequal divider, the feed delivers more power to the center elements and less to the edges, synthesizing a Taylor or Chebyshev distribution that suppresses sidelobes well below the uniform-illumination level. The cost is a slightly wider main beam and reduced aperture efficiency, a classic trade managed during the array's electrical design.

Equal Path Length and Loss

Number of two-way dividers (N elements):
N_div = N − 1, Stages = log₂(N)

Ideal split power per output (lossless):
P_out / P_in = 1 / N → Split (dB) = 10·log₁₀(N)

Total one-way feed loss:
L_feed ≈ 10·log₁₀(N) + (α × 𝓁_branch) dB

Where N = element count, α = line attenuation (≈ 0.1 to 0.3 dB/cm on X-band laminate), 𝓁_branch = longest meandered path length. Example: N = 16, 𝓁_branch = 12 cm, α = 0.2 dB/cm → 12 dB split + ≈ 2.4 dB dissipative loss.

Corporate Feed vs. Series Feed

Attribute	Corporate Feed	Series Feed
Topology	Parallel binary divider tree	Elements tapped along one line
Path length to elements	Equal (by design)	Progressive, increases down line
Beam squint vs. frequency	None (broadside, broadband)	Squints; narrowband or designed squint
Dividers / lines needed	N − 1 dividers, long routing	One line, minimal routing
Amplitude taper	Set by unequal split ratios	Set by coupling per tap
Typical use	Phased arrays, fixed broadside arrays	Slotted-waveguide, low-cost arrays
Relative loss (large N)	Higher (long meander)	Lower (short line)

Common Questions

Frequently Asked Questions

How does a corporate feed differ from a series feed?

A corporate feed splits the input through a symmetric binary tree so every element has identical path length; the phase is frequency-independent and the array stays at broadside across a wide band. A series feed taps elements off one line, so inter-element phase changes with frequency and the beam squints. The corporate feed costs area and loss: an 8 by 8 array needs 63 two-way dividers, while a series feed uses far less line. Corporate feeds suit fixed-beam and active phased arrays; series feeds suit low-cost slotted-waveguide and traveling-wave arrays where a small designed squint is acceptable.

How much insertion loss does a corporate feed network add?

Loss scales with log₂(N) divider stages. The ideal reactive split removes 10·log₁₀(N) dB as a consequence of dividing power, plus real dissipative loss. On RF laminate at X-band that dissipative loss runs about 0.1 to 0.3 dB/cm, and the longest meandered branch in a large planar tree can be 10 to 20 cm, adding 1 to 4 dB one-way. Wilkinson dividers add isolation resistors that dissipate unbalanced power; reactive T-junctions avoid that loss but give no isolation. Waveguide and air-stripline feeds cut dissipative loss to a fraction of a dB.

Can a corporate feed apply an amplitude taper for sidelobe control?

Yes. Setting unequal split ratios at each divider delivers a Taylor, Chebyshev, or binomial distribution across the aperture. A uniform feed gives about -13 dB first sidelobes; a 25 to 30 dB Taylor taper drives the edge elements to roughly 10 to 30 percent of the center power. The split ratio is set by relative microstrip line widths, quarter-wave transformer impedances, or, for an unequal Wilkinson, a specific resistor and arm impedances. The trade-off is a wider main beam and lower aperture efficiency.

Related Terms

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