Magic-T Applications: E-H Tuner and Microwave Mixer

The magic-T, also known as a hybrid tee, is formed by combining an E-plane tee and an H-plane tee. When all four ports are terminated with matched loads, the magic-T exhibits the following key characteristics:

• Signal Combining and Isolation: Equal amplitude waves fed into ports 1 and 2 result in zero output at port 3 (E-arm) and a combined output (sum of port 1 and port 2) at port 4 (H-arm).
• E-arm Input: A wave input at port 3 divides equally into ports 1 and 2, but with opposite phases. There is no coupling to port 4 (H-arm).
• H-arm Input: A wave input at port 4 divides equally into ports 1 and 2 in phase. There is no coupling to port 3 (E-arm).
• Collinear Port Isolation: A wave fed into either port 1 or port 2 will not appear at the other port (port 2 or port 1, respectively). This isolates the two collinear ports from each other.

The magic-T finds use in various applications, including:

• E-H tuner for impedance matching
• Balanced mixer in microwave receivers (to balance out LO noise at the input of the IF amplifier)
• Power combiners
• Duplexer circuits

E-H Tuner Application

In an E-H tuner, both the E and H arms are terminated by movable shorts, functioning as stubs. The E-arm shorts are referred to as E-plane stubs, and the H-arm shorts are referred to as H-plane stubs.

The position of these shorts can be adjusted to match a broad range of load impedances, effectively reducing the VSWR (Voltage Standing Wave Ratio) of the waveguide system.

Balanced Mixer Application

In a balanced mixer configuration, the incoming signal is fed to the E-arm, and the Local Oscillator (LO) signal is fed to the H-arm. These two signals enter the collinear arms and are mixed by crystal diodes placed here to produce the Intermediate Frequency (IF) signal.