E-Plane Tee (Series Tee)
Understanding the E-Plane Tee
If you need to split an RF signal into two separate paths inside a waveguide system, you must use a junction. The E-Plane Tee is one of the two fundamental ways to build a T-junction (the other being the H-Plane Tee). It is called the "E-Plane" Tee because the new branch is attached parallel to the Electric (E) field lines of the dominant TE₁₀ waveguide mode.
The 180-Degree Phase Split
The most important engineering characteristic of the E-Plane Tee is what happens to the signal's phase. Imagine feeding a radar pulse down the vertical arm of the "T" (Port 3). When the wave hits the bottom wall of the main waveguide, the electric field is forced to split and bend outward in opposite directions toward Port 1 and Port 2.
Because the wave physically bends in geometrically opposite directions (one left, one right), the resulting electric fields in the two output arms are pointing in opposite directions. Mathematically, this means the two output signals are identical in power, but exactly 180 degrees out of phase.
S₂₃ = -S₁₃
What this means:
If you input 100 Watts at Port 3 (the vertical arm):
- Port 1 outputs 50 Watts at 0° phase.
- Port 2 outputs 50 Watts at 180° phase (indicated by the negative sign in the matrix).
The Difference Port (Subtraction)
Because passive RF components are reciprocal, you can run them backwards. What happens if you pump two signals into the side arms (Port 1 and Port 2) and extract the result from the vertical arm (Port 3)?
- If the two input signals are perfectly in-phase, the 180-degree geometry of the tee will cause them to perfectly cancel each other out at the junction. The output at Port 3 will be zero.
- If the two input signals are 180-degrees out of phase, the geometry flips one of them, they constructively combine, and all the power exits Port 3.
For this reason, when an E-Plane Tee is used as a combiner, it acts as a subtractor. Port 3 is often referred to as the Difference Port or the Delta ($\Delta$) port in a monopulse radar system.
E-Plane vs. H-Plane Summary
| Feature | E-Plane Tee (Series) | H-Plane Tee (Shunt) |
|---|---|---|
| Attachment Wall | Broad Wall | Narrow Wall |
| Power Split (from Branch) | Equal (50% / 50%) | Equal (50% / 50%) |
| Phase Shift | 180° (Out of phase) | 0° (In-phase) |
| Combining Action | Subtracts signals (Difference Port) | Adds signals (Sum Port) |
Frequently Asked Questions
Why is it called an E-Plane Tee?
It is named based on its geometry relative to the electromagnetic field. The auxiliary branch (the vertical arm of the "T") is attached to the broad, wide wall of the waveguide, which is parallel to the Electric (E) field lines of the dominant TE10 mode.
Why do the outputs have a 180-degree phase shift?
Because the input wave enters the perpendicular arm and splits in two opposite directions down the main waveguide. The electric field lines are forced to bend left and right. If you trace the arrows of the E-field, the wave traveling left is "pointing up" while the wave traveling right is "pointing down"—this geometric opposition creates the 180-degree phase difference.
What happens if I feed signals into both colinear arms at the same time?
If you feed two identical, in-phase signals into the two horizontal arms, they will meet in the middle and completely cancel each other out at the vertical E-arm because of the 180-degree phase property. The E-arm becomes a "difference" port.