Waveguide-to-Coax Adapter
Understanding Waveguide-to-Coax Adapters
A massive Megawatt radar transmitter outputs its power through a WR-90 waveguide flange. However, you cannot plug a waveguide pipe directly into an SMA or N-Type port on a spectrum analyzer. The two systems operate on completely different physics (Waveguide $TE_{10}$ vs. Coaxial $TEM$).
| Characteristic | 24 GHz | 77 GHz | 79 GHz |
|---|---|---|---|
| Bandwidth | 250 MHz | 1 GHz | 4 GHz |
| Range Resolution | 60 cm | 15 cm | 3.75 cm |
| Antenna Size | Moderate | Small | Small |
| Regulation | ISM (global) | Licensed | Licensed (UWB) |
To bridge this gap, engineers use a Waveguide-to-Coax Adapter.
The Probe Coupling Architecture
The most common design for this transition is the Orthogonal Probe. It operates exactly like a tiny antenna broadcasting inside a metal room.
- The coaxial connector (e.g., an SMA female) is bolted to the broad wall of the waveguide.
- The center pin of the coaxial connector protrudes directly down into the hollow cavity of the waveguide. This pin acts as a monopole antenna.
- As the RF wave travels down the waveguide, the vertical Electric Field ($E$-field) slams into the vertical metal pin.
- The energy induces a current in the pin, which travels straight up into the coaxial cable, perfectly converting the $TE_{10}$ wave into a coaxial $TEM$ wave.
The Importance of the Backshort
If you just stick a pin into a pipe, half the energy goes up the pin, but the other half keeps traveling down the pipe and is lost. To force 100% of the energy into the coaxial pin, the adapter uses a Backshort.
The waveguide cavity does not continue past the pin; it hits a solid metal wall. This wall is placed exactly one-quarter wavelength ($\lambda_g / 4$) behind the pin.
- The wave hits the pin (some energy goes up).
- The rest of the wave travels past the pin, hits the solid back wall, and reflects 180 degrees backwards.
- Because the wall is $\lambda_g / 4$ away, the round-trip distance is exactly $\lambda_g / 2$. This perfectly phase-aligns the reflected wave with the incoming wave.
- The two waves constructively interfere exactly at the pin's location, forcing 100% of the energy up into the coaxial cable with near-zero VSWR.
Frequently Asked Questions
Can you put Megawatts of power through a coax adapter?
Absolutely not. This is the primary bottleneck. The waveguide might be able to handle 1 Megawatt of peak power, but the moment the energy is funneled into the tiny Teflon-filled coaxial connector (like an SMA or N-Type), the voltage will arc across the center pin and instantly vaporize it. Coax adapters are strictly for low-power ( $< 500$ Watts) receiver chains or test equipment.
What is an End-Launch adapter?
A standard probe adapter (Right-Angle) places the coaxial connector on the broad wall, perpendicular to the waveguide. An End-Launch adapter places the coax connector directly on the flat end of the waveguide, parallel with the flow. This requires a complex 'stepped ridge' inside the cavity to gradually transform the impedance, but it allows for straight-line routing.
Why do some adapters have a teardrop-shaped pin?
To increase bandwidth. A straight cylindrical pin is highly resonant at a single frequency. By machining the probe pin into a teardrop, cone, or doorknob shape, the capacitance is spread out, drastically widening the usable frequency band of the adapter and lowering the VSWR across the entire range.