Cross-Guide Coupler
Crossed-Slot Coupling Geometry
The cross-guide coupler, sometimes called the crossed-guide or Moreno coupler after the engineer who characterized the two-slot version, joins a primary waveguide carrying the main signal to a secondary waveguide that crosses it at 90 degrees on the broad-wall side. Coupling occurs entirely through apertures cut in the wall that the two guides share. In the dominant TE10 mode, the broad wall carries both a longitudinal magnetic field that peaks at the side walls and a transverse component near the center, so a slot placed and angled correctly excites the crossing guide through a controlled mix of magnetic and electric coupling. By using two slots arranged as a cross (or an X), the designer arranges the coupled waves to reinforce toward the intended coupled port and subtract toward the opposite, isolated port, which is what produces directivity.
Slot length sets the resonant frequency and therefore the center of the coupling band, while slot width and the angle of the cross control the coupling magnitude and the balance between the forward and backward contributions. Because the slots are small relative to the broad-wall dimension, the perturbation to the main line is slight, and the main-arm match stays good with VSWR typically below 1.15 to 1. This is fundamentally why cross-guide couplers are built as loose couplers; tightening the coupling below about 15 dB would require slots large enough to noticeably load the main guide and spoil both the match and the directivity.
Directivity is the figure of merit that separates a usable coupler from a simple power tap. It is the ratio, in decibels, of the power at the coupled port to the unwanted power that leaks to the isolated port for a given main-line excitation. High directivity means the coupled-port reading faithfully represents the forward wave and is not contaminated by the reflected wave, which is essential when the coupler feeds a reflectometer or a power monitor. Practical two-slot cross-guide couplers hold 20 to 35 dB of directivity across the operating band, with the best values near the slot resonance and a gradual fall-off toward the band edges.
Governing Coupling Relationships
C = −20 log10 |S31| dB
Directivity:
D = 20 log10 (|S31| / |S41|) = C − I dB
Isolation:
I = −20 log10 |S41| = C + D dB
Coupled power from incident power:
Pc = Pin × 10(−C/10)
Where ports 1 and 2 are the main-line input and output, port 3 is the coupled port, and port 4 is the isolated port. Example: for C = 30 dB and a Pin of 1 kW, Pc ≈ 1 W; if directivity D = 25 dB, the isolated-port leakage I ≈ 55 dB, so ≈ 3.2 μW reaches port 4.
Cross-Guide Coupler vs. Other Broad-Wall Couplers
| Coupler Type | Slot/aperture count | Typical coupling | Directivity | Bandwidth | Best use |
|---|---|---|---|---|---|
| Cross-guide (two-slot) | 2 crossed slots | 20 to 40 dB | 20 to 35 dB | 10 to 15% | High-power monitoring tap |
| Bethe-hole | 1 circular hole | 20 to 30 dB | 20 to 30 dB | 2 to 5% | Single-frequency sampling |
| Multi-hole (Bethe array) | N holes (5 to 20+) | 3 to 30 dB | 30 to 45 dB | 20 to 40% | Broadband, tight coupling |
| Branch-guide | 2 to 4 branches | 3 to 10 dB | 20 to 30 dB | 10 to 20% | Tight coupling, power split |
| Short-slot (Riblet) | Common-wall slot | 3 dB (hybrid) | 20 to 30 dB | 10 to 20% | 3 dB quadrature hybrid |
Frequently Asked Questions
How does a cross-guide coupler differ from a Bethe-hole coupler?
Both are broad-wall couplers, but a Bethe-hole coupler uses a single circular aperture and cancels the backward wave through hole offset or guide tilt, giving good directivity only near one frequency. A cross-guide coupler crosses the guides at 90 degrees and uses two crossed slots that couple through both electric and magnetic fields, allowing a wider 10 to 15 percent band and an adjustable coupling-versus-directivity trade. Typical cross-guide performance is 20 to 30 dB coupling with 20 to 35 dB directivity, while a Bethe-hole is usually a single-frequency device.
What coupling values and directivity are realistic for a cross-guide coupler?
These are loose monitoring couplers, so 20, 30, or 40 dB coupling is standard; 10 dB or tighter is impractical because the slots would load the main line and spoil the match. Directivity for a well-machined two-slot design is 20 to 35 dB over the band, falling toward the edges. Main-arm insertion loss is usually under 0.2 dB and VSWR below 1.10 to 1.15 to 1. At 30 dB coupling, 1 kW in the main line appears as roughly 1 W at the coupled port.
Why is a cross-guide coupler attractive for high-power waveguide systems?
The main signal stays inside a continuous full-size waveguide with no center conductor, dielectric, or narrow gap to break down, so peak power handling can reach hundreds of kilowatts at X-band when the line is pressurized or evacuated. The slots remove only a known small fraction of the energy, keeping insertion loss and dissipation negligible. Because the secondary guide crosses at a right angle, the coupled port exits sideways, which is mechanically convenient for tapping a waveguide run to feed a power monitor, frequency counter, or reflectometer.