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Waveguide Tee That Is Employed In Balanced Mixers

The combination of E-plane tee and H-plane tee is referred as magic-T or hybrid tee.

The magic-tee waveguide has the following features when all four ports are terminated with a matched load.

1.Two equal amplitude waves fed at port 1 and port 2, produce zero at port 3(E-arm) and sum of port 1 and port 2 at port 4(H-arm).

2.A wave input at port-3 divides equally at port 1 and port-2,but opposite in phase with no coupling at port-4(H-arm)

3.A wave input at port-4 divides equally at port-1 and port-2 in phase with no coupling to port-3(E-arm)

4.A wave fed into one port-1 or port-2 will not appear in the other port-2 or port-1. Hence these two collinear ports 1 and 2 are isolated to each other.

A mixer can be formed between the two collinear ports when an antenna is connected to the E plane port of the magic T and a local oscillator is connected to the H plane port, terminating one of the collinear ports, and a mixer circuit is connected to the other collinear port.

In a balanced mixer configuration, an incoming signal is fed to E-arm and LO signal is fed to H-arm. These two signals enter the collinear arms and beat with crystal diodes placed here to produce IF signal.

In practice, there are a variety of limitations on the different degrees of match, balance and isolation of the magic tee in microwave. Such restrictions mainly involve the following three aspects: 

1. the tuning and impedance matching mechanism is fundamental to the normal operation of Magic T, however, it is due to this mechanism that the corresponding frequency range limits, insertion loss, E-H port isolation limits, and collinear port isolation limits. 

2. the matching required by E-plane port and H-plane port has an impact on the matching degree of collinear port. 

3. the assembly quality also affects the performance of the Magic T, since the E-plane ports and H-plane ports must be perfectly orthogonal to maintain a high degree of isolation.

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