How does a Blass Matrix differ from a Butler Matrix?

A Butler Matrix uses hybrid couplers and fixed phase shifters in a log2(N) stage network, producing orthogonal beams at fixed angles. It requires N to be a power of 2 and beam angles cannot be independently chosen. A Blass Matrix uses a traveling-wave coupler grid where beam directions are set by the line lengths between couplers, allowing arbitrary beam angles and any number of beams. The Blass Matrix is more flexible but physically larger and lossier due to the coupler grid.

What are the disadvantages?

Blass Matrices are physically large (N*M couplers for N elements and M beams), have insertion loss proportional to the number of coupling stages (typically 3-6 dB), and require matched terminations on all unused ports. The traveling-wave nature means each beam's power is progressively reduced as it couples to successive elements. Modern digital beamforming has largely replaced Blass Matrices in new designs.

Where are Blass Matrices still used?

They remain in legacy radar systems, some satellite ground terminals with fixed multi-beam requirements, and specialized EW systems where analog beamforming provides instantaneous wideband coverage without the latency of digital processing. They are also used in educational settings to teach beamforming fundamentals.

RF System Architecture

Blass Matrix

A Blass Matrix is a traveling-wave analog beamforming network (BFN) that uses a grid of directional couplers and transmission lines to form multiple simultaneous beams from a phased array antenna. Each beam port feeds a horizontal transmission line that crosses vertical lines connected to array elements. Couplers at each intersection sample a fraction of the signal, and the progressive phase shift created by the transmission line lengths between couplers sets the beam direction. Unlike the Butler Matrix, the Blass Matrix allows arbitrary beam angles and non-power-of-2 element counts.

Category: RF System Architecture

Type: Analog multi-beam BFN

Understanding the Blass Matrix

The Blass Matrix arranges M beam lines horizontally and N element lines vertically. At each of the M×N intersections, a directional coupler transfers a controlled fraction of power between the beam line and the element line. The phase of each coupled signal depends on the electrical length of the transmission line from the beam port to that coupler, creating the progressive phase taper needed for beam steering.

Each beam direction is set by choosing the appropriate line lengths at design time. Multiple beams form simultaneously because each beam line operates independently. All unused coupler ports must be terminated in matched loads to prevent reflections.

Blass Matrix Beam Steering

Progressive phase between elements:
Δφ = k·d·sin(θ)
k = 2π/λ, d = element spacing

Coupler grid: M beams × N elements = M×N couplers
Total loss: Typically 3-6 dB + coupling loss

Analog BFN Comparison

Network	Beams	Flexibility	Size	Complexity
Blass Matrix	Arbitrary	Any angles	M×N couplers	High
Butler Matrix	2ⁿ	Fixed orthogonal	N/2·log₂N hybrids	Medium
Rotman Lens	Arbitrary	True time delay	Lens + ports	Medium
Digital BF	Unlimited	Full control	N ADCs + DSP	Very high

Common Questions

Frequently Asked Questions

Blass vs Butler Matrix?

Butler uses hybrids in log₂(N) stages for fixed orthogonal beams (power-of-2 only). Blass uses M×N couplers allowing arbitrary beam angles and any element count. Blass is more flexible but larger and lossier.

Disadvantages?

Large size (M×N couplers), 3-6 dB insertion loss, requires matched terminations. Modern digital beamforming has largely replaced Blass in new designs.

Where still used?

Legacy radar, fixed multi-beam satellite terminals, wideband EW systems where analog provides instantaneous coverage without digital latency.

Related Terms

← Blade Antenna BLAST →