How can I use a WR-650 coaxial cable to waveguide adapter as part of a hydrogen line receiving system?
Good questions. First, to be clear — by “convert … to a full waveguide” I assume you mean: you have a (a waveguide‑to‑coax adapter) and you want to use it as part of a full waveguide feed (i.e. have a continuous waveguide path, not just coax → waveguide → coax). Here’s how that works — and what you need to do / check.
✅ What the adapter does (and what it is)
- A “waveguide‑to‑coax adapter” is a transition device: it couples electromagnetic energy from a 50 Ω coax line (TEM mode) into the hollow rectangular waveguide (TE₁₀ mode), or vice versa. Internally it has a small probe (or loop) that sits in the waveguide cavity and picks up (or launches) the waveguide mode.
- The adapter’s waveguide end has a standard flange — in this case for WR650 (flange “BJ14 / R14”) — so it mates to other WR650-compatible waveguide hardware (straight sections, horns, loads, transitions, etc.).
So the adapter itself is a partial waveguide: it includes a short piece of WR650 waveguide (enough to support the probe) and the coax connector.
🔧 What you need to convert (or extend) it into a full waveguide feed
If you want a continuous waveguide feed (e.g. from your receiver down to a horn or another waveguide section), you need to attach one or more WR650 straight‑waveguide sections (or other WR650 compatible components) to the waveguide end flange of the adapter.
Here’s what you need / check:
| What | Why / What to check |
|---|---|
| A WR650 straight waveguide section (or other WR650‑flange component) | To provide the full-length waveguide beyond the adapter’s short section |
| Correct flange match (BJ14 / R14 on both pieces) | To ensure physical and RF connection aligns properly — adapter’s flange must mate to the section’s flange |
| Secure mechanical and RF seal (gasket or proper flange torque) | To prevent leakage/reflections at the joint — waveguide joints must maintain metal continuity and airtightness (or proper shield) |
| Awareness of mode — adapter launches TE₁₀ mode, so waveguide section must support that mode at your frequency (1.420 GHz) | All parts must be dimensioned for WR650 (i.e. correct internal cross‑section) to ensure single‑mode, low‑loss propagation at 1.4 GHz |
In short: the adapter provides one end of a WR650 waveguide. To make a “full waveguide,” you simply bolt on standard WR650 waveguide sections — making sure the flange type and internal dimensions are correct.
⚠️ What it isn’t — limitations and caveats
- The adapter does not replace a full waveguide; it only provides a transition + a short waveguide stub. To build a longer feed you need additional waveguide sections.
- The internal probe inside the adapter is a coupling structure. You must ensure that the waveguide section attached does not disturb the probe or misalign the mode (i.e. flange alignment and internal cleanliness are important).
- If you intend to go from coax → waveguide → coax again (e.g. coax‑feed‑waveguide‑horn), you may need two adapters (one at each transition, unless the horn or waveguide assembly already has a waveguide flange).
- Any joints must be well-constructed so that reflections (VSWR) remain low; otherwise you could degrade sensitivity — especially critical in radio astronomy (like H I 1420 MHz work).
🛠️ Practical Steps (for your hydrogen‑line antenna setup)
Given your goal (amateur radio astronomy at ~1420 MHz), here’s how you could incorporate the adapter in a “full waveguide + feed” chain:
- Get one (or more) WR650 straight‑waveguide sections of suitable length (e.g. 300 mm, 500 mm, 1 m — depending on your physical layout).
- Bolt the straight section(s) to the WR650 flange on the adapter, using proper gaskets / O‑rings or flanged waveguide seals.
- On the other end of the waveguide run, attach your antenna feed: e.g. a horn, reflector, or custom feed structure that also provides a WR650 flange.
- Ensure mechanical support (especially if you mount the waveguide at angles / on a mast), to avoid stress on the flange joints.
- Test the resulting waveguide + adapter assembly using a return‑loss / VSWR meter (or a network analyzer) at ~1420 MHz to confirm the installation is good and reflections are low.