Integrated Sliding-Short/Probe Tuner with Doorknob Transition for High-Q Cavities

TL;DR

This work delivers an integrated, cavity‑front impedance tuner that combines a sliding short, a doorknob transformer, and a movable coaxial probe to achieve near‑critical coupling for a high‑Q waveguide‑fed cavity. A closed‑form ABCD chain‑matrix model links four mechanical blocks to the electrical targets $\Gamma$, $\beta$, and $Q_L$, explicitly accounting for the fused‑silica feedthrough and the TM$^z_{011}$ load. Validation comes from full‑wave simulations and bench measurements, delivering $|S_{11}|\approx -30$ dB and $|S_{21}|\approx -0.7$ to $-0.8$ dB near 17.8–18.1 GHz, with robust performance under plasma loading as the impedance evolves. The design eliminates external stub boxes, remains scalable to other resonators and bands, and enables in‑situ tuning to maximize power absorption in plasma and propulsion applications.

Abstract

We present an integrated three-knob tuner that internalizes impedance matching inside the launch adapter of a waveguide-fed, high-$Q$ cavity. The tuner combines a waveguide sliding short, a doorknob transition, and a micrometer-driven adjustable coaxial probe. A transmission-line/ABCD model is derived that maps the three mechanical degrees of freedom to the electrical objectives $Γ\rightarrow 0$, $β$, and $Q_{\rm L}$, explicitly including the fused-silica feedthrough capacitance. The model yields closed-form matching conditions and predicts the critical-coupling set. Full-wave FEM simulations and bench measurements validate the approach: with $h \approx 0.55$~mm and backshort distance $\approx 0.80$~mm, the return loss reaches $|S_{11}| \approx -30$~dB near 17.8--18.1~GHz while sustaining peak electric fields of $\sim 1.8 \times 10^5$~V/m at the nozzle (normalized to 1~W). The measured through loss of the launch assembly is $|S_{21}| \approx 0.7$--$0.8$~dB at resonance. A parametric study shows that backshort lengths $L_{\rm {bs}} \geq 0.5 λ_{\rm g}$ excite a parasitic stub resonance, introducing a second $S_{11}$ minimum and localizing energy behind the doorknob; keeping $L_{\rm {bs}} \leq 0.4 λ_g$ avoids this. In helium plasma discharges at $P_{\rm {in}} = 10$~W, \textit{in-situ} retuning of the short and probe maintained a favorable match as the plasma impedance evolved, increasing absorbed power from $\sim 43\%$ to $\sim 76\%$ while increasing helium propellant flow rate from 25 to 351~sccm. The compact tuner eliminates external stub boxes and generalizes to other waveguide-coupled resonators and plasma sources.

Figures (16)

• Figure 1: The front view (left) and the side view (right) geometry of a doorknob protrusion with a variable antenna from left to right.
• Figure 2: Waveguide cavity mount with sliding short. Incoming microwaves enter from the right (red arrow) and are reflected by the short; translating the plunger tunes the series susceptance $X_{\rm s}(l_{\rm s})$.
• Figure 3: Cut-away of the antenna-sliding-short system. The micrometer drive sets the exposed probe length $h$; the plunger fixes $l_{\rm s}$. The doorknob post (right) realizes $L_{\rm d}^{\star}$ and $C_{\rm d}$.
• Figure 4: Metallic screen closing the micrometer cavity. Experimental evidence shows $>10\;\text{dB}$ suppression of spurious leakage at 17.8 GHz.
• Figure 5: Door-knob post viewed along the guide axis. Gap $g$ and post height $l_{\rm d}$ were set to the analytical optimum; mechanical slots permit $\pm 25$$\mu$m shimming without breaking vacuum.