Here’s a ready-to-use block diagram plus a practical minimal parts list and quick build/test checklist so you can turn your WR-650 waveguide-to-coax adapter into a full waveguide feed for 1420 MHz H-I work.

Block diagram (ASCII)

[Receiver / SDR with 50 Ω coax]
            |
     (50 Ω coax)
            |
[Waveguide-to-coax adapter (50Ω → WR650)]
            |
    [WR650 straight section] — (× n)
            |
   [WR650 straight / bend / filter]
            |
     [WR650 standard-gain horn]
            |
   (radiation toward reflector / sky)

Minimal parts list (what to buy / spec)

1. Waveguide-to-coax adapter — you already have this (WR650 flange on the waveguide side; confirm its coax connector type: N or SMA).
   • Purpose: transition coax ↔ WR650 (probe inside).
2. WR650 straight waveguide section(s) — choose one or more straight sections to give the physical run you need (common lengths: 100 mm, 300 mm, 500 mm, 1 m). Use WR650 / WG6 straight sections with the same flange family as the adapter (BJ14 / R14 / UDR14 / FDP14 are cross-references).
   • Example product family: WR650 straight sections (cover ~1.13–1.73 GHz).
3. WR650 standard-gain horn (with WR650 flange) — a factory horn sized for ~1.13–1.73 GHz (WR650 flange) is the simplest feed to use. Typical gain ~12–15 dBi and a WR650 flange (CPR650F / FDP14 etc.).
   • Example: WR-650 standard gain horn (1.14–1.73 GHz, ~15 dBi).
4. Waveguide flanges & gasket kit — matching flange type and gaskets (silicone/gasket kit) for WR650 (BJ14 / FDP14 / UDR14 etc.). Flange hardware: bolts, nuts, alignment pins if needed. Flange cross-reference shows BJ14 ↔ WR650.
5. Mechanical support / clamps / mast mounting — to relieve stress on flanges (U-bolts, saddle clamps, small pole brackets). (Common workshop hardware; ensure waveguide faces supported near joints.)
6. Optional: WR650 bends / H-plane / E-plane components — if you need to route around obstacles, buy WR650 90° or custom bends or H-plane tees.
7. Test gear
   • NanoVNA or VNA (handheld NanoVNA OK for return-loss/VSWR at 1.42 GHz) to check S11 and ensure low reflections.
   • Torque wrench / spanner set for flange bolts; electrical contact grease (if required) and cleaning cloth.

Recommended specific shopping hints (examples)

• WR650 straight section, 100–300 mm length, UDR14/FDP14 flange (low loss, VSWR ~1.03). Good to stock 1×300 mm or 2×100 mm pieces.
• WR650 standard gain horn, WR650 flange, 1.13–1.73 GHz, ~15 dBi (Pasternack / FT-RF / other RF vendors carry these).
• Flange gaskets kit for WR650 (silicone rubber gasket sized FDM14 / PDR14).

Quick build checklist (step-by-step)

1. Inspect adapter flange and confirm flange type (BJ14 / UDR14 / FDP14 — adapter should mate to WR650 straights). Use the flange cross-reference if unsure.
2. Bolt the first straight section to the adapter flange using the correct bolts and gasket. Ensure faces clean and parallel; use alignment pins if provided.
3. Add additional straight sections as needed; avoid many short joints if you can (each joint is a potential reflection). Keep run mechanically supported.
4. Bolt horn to the final WR650 flange. Align the horn so the aperture pointing properly at the reflector/sky.
5. Test with NanoVNA/VNA at ~1.420 GHz: measure S11/return-loss. Aim for good match (low return loss); if you see high SWR, check flange contacts, gasket seating, and internal probe seating in adapter.
6. If you need a different pattern (narrower beam), consider larger horn or reflector + feed; for simple H-I work a standard WR650 horn works well as a compact feed.

Notes & caveats

• The adapter contains the small probe/transition — it only provides a short stub of waveguide. You must bolt genuine WR650 sections (same internal cross-section) to make a continuous WR650 chain.
• Use as few flange joints as practical and ensure good mechanical support to avoid misalignment or poor RF contact.
• If your adapter uses a particular coax connector (N vs SMA), ensure your receiver/SDR coax uses the same connector or use a high-quality connector adapter (keep in mind added losses).

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