WR-650 already covers 1.12–1.70 GHz, so 1.420 GHz sits comfortably in-band—you don’t need to resize the guide. What you do need is a simple, well-tuned coax-to-waveguide probe with a backshort (or a short straight section into a horn).

Here’s a practical recipe that works well at the H I line (1420.4058 MHz):

1) Key dimensions & checks

• WR-650 internal size: a = 165.1 mm (6.500″), b = 82.55 mm (3.250″)
• TE₁₀ cutoff: f_c ≈ 0.908 GHz → only TE₁₀ propagates at 1.420 GHz (TE₂₀/TE₀₁ ≈ 1.816 GHz).
• Free-space wavelength at 1420.406 MHz: λ₀ ≈ 211.1 mm
• Guide wavelength (TE₁₀) ca. 274.4mm.

• Quarter-guide-wave: λg/4 ≈ 68.6 mm → this is your starting backshort spacing.

2) Build a coax probe + backshort transition

Layout (along the guide x-axis): [short-circuit plate] —— (λg/4) —— [probe] → open guide/horn

Placement across the guide:

• Drill through the broad wall (the large face).
• Put the probe on the centerline across the width (midway along b), where the TE₁₀ E-field is maximum.

Backshort distance (critical):

• From the shorting plate (or a solid end-plate) to the center of the probe: start at 68.5 mm.
• Provide ±10 mm of mechanical adjustment (slot holes or removable shim rings) for tuning.

Probe hardware:

• Use a 50 Ω N-female bulkhead (rugged) or SMA (if you prefer smaller hardware).
• Inner conductor becomes the probe: copper/brass rod or the connector’s pin extended.
• Diameter: 3–6 mm works well; keep it smooth and rigid.

Probe length (starting value):

• Start with L_probe ≈ 22–28 mm protrusion into the guide (from the inside of the broad wall).
  • This is a safe, slightly-short starting point at L-band; you’ll lengthen/trim during tuning.
  • Make it adjustable if possible (threaded boss + locknut, or a set-screw “trombone” tip).

Keep-out around probe:

• Leave ≥1 probe diameter clearance to the nearest edges/features.
• Deburr the hole; ensure good electrical contact of the connector flange to the guide (star washers or silver-paint if needed).

3) Straight section & horn (optional but recommended)

• Provide ≥ λg/2 ≈ 137 mm of straight guide from probe toward the aperture (if not immediately into a horn).
• A short pyramidal horn improves pattern and spillover. A simple flare that reaches an aperture ~0.8–1.0 λ₀ across (~170–210 mm) is already helpful; bigger is better if you’re not on a dish.

4) Tuning for S₁₁ and band-centering

Use a VNA (or an SDR + noise source + bridge if needed):

1. Backshort first:
   • Sweep S₁₁ around 1400–1450 MHz.
   • Slide the backshort to bring the deepest return-loss dip near 1420.4 MHz.
2. Probe length next:
   • If the dip is above 1420 MHz → lengthen the probe slightly.
   • If it’s below 1420 MHz → shorten it slightly.
   • Aim for S₁₁ < −15 dB at 1420 MHz (−20 dB is very achievable).
3. Fine match:
   • Small tweaks (±1–2 mm) in backshort and probe length interact—iterate a couple of times.
   • If you want more bandwidth or a deeper null, add a tiny capacitive hat (a small round washer) on the probe tip or use a second tiny tuning screw a few cm away on the broad wall.

5) LNA and system hookup (radio astronomy-friendly)

• Bolt a low-noise LNA (NF ≤ 0.5–0.7 dB at L-band) right at the probe port (or as close as mechanically possible) to keep system noise down.
• Weather-seal the connector and seams (EMI gasket + butyl tape).
• Use good coax to the receiver (short, low-loss; e.g., LMR-400 or 1/2″ Heliax if long).

6) Sanity checks & numbers you can trust

• With a = 165.1 mm, TE₁₀ cutoff is ~0.908 GHz; higher modes ~1.816 GHz → single-mode at 1420 MHz.
• λg/4 ≈ 68.6 mm is a robust starting backshort spacing.
• Probe length 22–28 mm (3–6 mm Ø) gets you close; expect ±5 mm of final tweak.
• Provide mechanical adjustment: slots/shims for the backshort, threaded probe if you can.

7) If you’re feeding a dish vs. a stand-alone radiator

• On a dish: Add a scalar ring or use a choked horn to reduce spillover and improve illumination efficiency; set phase center near the dish focus.
• Stand-alone: A pyramidal horn sized to ~1.5–2.0 λ₀ aperture gives ~12–16 dBi with decent beamwidth for sky surveys.

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If you want, I can sketch a quick dimensioned drawing (probe, backshort, hole positions) and a tuning worksheet for logging S₁₁ vs. adjustments.