Dimensions of WR650 waveguide for hydrogen.
For the neutral hydrogen line at 1420.4058 MHz, WR-650 is the standard rectangular waveguide size most commonly used. Its dimensions are:
| Dimension | Inches | Millimetres |
|---|---|---|
| Broad wall (a) | 6.500 in | 165.1 mm |
| Narrow wall (b) | 3.250 in | 82.55 mm |
These dimensions place the TE10 cutoff frequency at approximately 908 MHz, so 1420 MHz propagates comfortably within the single-mode operating region.
For hydrogen-line work:
- Operating frequency: 1420.4058 MHz
- Free-space wavelength: approximately 211.1 mm
- WR-650 recommended band: roughly 1.15–1.72 GHz
- Internal size: 165.1 mm × 82.55 mm
A useful formula for the dominant TE10 mode cutoff is:
where:
- fc = cutoff frequency
- c = speed of light
- a = broad-wall dimension
Using a = 0.1651 m gives fc ≈ 0.908 GHz.
For a 1420 MHz feed probe in WR-650, a typical starting point is:
- Probe length ≈ lambda/4 ≈ 52.8 mm
- Backshort spacing ≈ lambda_g/4 ≈ 68.6 mm
with final adjustment made using an antenna analyzer or VNA.
How well would 150mm x 50mm aluminium box section work.
A 150 mm × 50 mm aluminium box section can work as a hydrogen-line waveguide feed, but not nearly as well as a true WR-650.
Let’s compare the dimensions:
| Waveguide | Broad wall (a) | Narrow wall (b) |
|---|---|---|
| WR-650 | 165.1 mm | 82.6 mm |
| Aluminium box | 150 mm | 50 mm |
For your 150 mm × 50 mm section, the TE₁₀ cutoff frequency is:
Using mm gives:
- TE₁₀ cutoff ≈ 1000 MHz
At 1420 MHz the guide is above cutoff, so the dominant mode propagates normally.
The next mode cutoffs are approximately:
- TE₂₀ ≈ 2000 MHz
- TE₀₁ ≈ 3000 MHz
So 1420 MHz is safely in the single-mode region, which is good.
Advantages
- Readily available and inexpensive.
- Smaller cross-section may be mechanically easier to mount at the dish focus.
- Single-mode operation at 1420 MHz.
- Losses will be very low for the short lengths used in feed horns.
Disadvantages
- Aperture area is only about 55% of WR-650:
- WR-650 area ≈ 13,600 mm²
- 150 × 50 area = 7,500 mm²
- The narrower 50 mm dimension will produce a different illumination pattern than a WR-650 feed.
- Expect somewhat wider beamwidth in one plane and less efficient dish illumination.
- Feed dimensions from WR-650 designs won’t directly transfer.
For a dish feed
Many amateur hydrogen-line observers have successfully used waveguides ranging from about 140–170 mm wide. For a typical 2–4 m dish, a 150 mm × 50 mm box section with a probe feed and a simple flare horn should work reasonably well.
A good starting point would be:
- Probe length ≈ 53 mm
- Probe positioned on the centreline of the 150 mm wall
- Backshort about 65–70 mm behind the probe
- Add a pyramidal flare if possible
For hydrogen-line work, the difference between a carefully tuned 150 × 50 mm feed and an ideal WR-650 feed is likely to be only a few tenths of a decibel. If the box section is already available, it is certainly worth trying before sourcing genuine WR-650.
Are there any other standard available aluminium box section sizes that would work well to produce a homemade WR650 waveguide?
Yes. If you’re trying to approximate WR-650 (165.1 mm × 82.6 mm) using readily available aluminium rectangular tube, the most important dimension is the broad wall width. The narrow wall dimension affects the radiation pattern, but is less critical than the width.
Common metric box sections that would work reasonably well at 1420 MHz include:
| Box Section | TE10 Cutoff | Comments |
|---|---|---|
| 150 × 75 mm | 1000 MHz | Probably the closest commonly available size. |
| 150 × 100 mm | 1000 MHz | Slightly taller than WR-650 but works well. |
| 160 × 80 mm | 937 MHz | Excellent approximation to WR-650. |
| 160 × 60 mm | 937 MHz | Good if easier to source. |
| 175 × 75 mm | 857 MHz | Slightly oversized but perfectly usable. |
| 200 × 100 mm | 750 MHz | Larger than needed but still single-mode at 1420 MHz. |
The ideal homemade substitute would be 160 mm × 80 mm rectangular tube if you can find it. Compared with WR-650:
| Dimension | WR-650 | 160 × 80 |
|---|---|---|
| Width | 165.1 mm | 160 mm |
| Height | 82.6 mm | 80 mm |
That’s within about 3% in each dimension.
Fabricating a closer equivalent
Another approach is to use:
- 150 × 75 mm box section
- Cut along one side
- Add a 15 mm strip of aluminium
- Rivet or screw together
This produces approximately 165 × 75 mm, very close to WR-650.
What many amateurs actually use
For hydrogen-line work, many successful amateur feeds are not exact WR-650 dimensions at all. Examples include:
- 150 × 75 mm rectangular tube
- 160 × 80 mm tube
- Coffee-can and paint-tin feeds
- Cylindrical waveguides made from aluminium ducting
The key is obtaining a good match and an illumination pattern suited to the dish. A well-tuned 150 × 75 mm feed often performs nearly as well as a textbook WR-650 feed on a 2–5 m dish.
For your radio astronomy work, if you can source 160 × 80 mm aluminium box section, that would be my first choice as a homemade WR-650 equivalent. A 150 × 75 mm section would be my second choice because it is much more commonly stocked by metal suppliers.