(As yet unbuilt. I am encouraged by Jason Burnfield’s folding 3m dish currently under construction. I do not have room for permanent mounting of a dish of this size but a folding, transportable version could be useful.)
For amateur 21 cm hydrogen-line work, a folding 3 m dish is practical if you accept a mesh reflector and a segmented frame. At 1420 MHz, the wavelength is 21.1 cm, so the reflector surface only needs to be accurate to about lambda/16 (~13 mm RMS) for good efficiency. This makes a collapsible design feasible.
Basic specifications
- Diameter: 3.0 m
- Frequency: 1420.4058 MHz
- Wavelength: 0.211 m
- Focal ratio (F/D): 0.35-0.40
- Recommended F/D: 0.38
- Focal length: F = 0.38 × 3.0 = 1.14 m
- Reflector depth: d = D^2/(16F) = 9/(16 × 1.14) ≈ 0.49 m
Thus the dish is about 49 cm deep.
Reflector
Use six “umbrella” sectors.
Each sector consists of:
- Two aluminium radial ribs:
- 25 mm × 25 mm square tube
- Length 1.5 m
- Three circumferential hoops:
- 20 mm aluminium tube
- Galvanised welded mesh:
- 12 mm × 12 mm openings
- 1 mm wire
The mesh requirement is much less stringent than optical telescopes:
- Rule of thumb: mesh opening < lambda/10
- For hydrogen: lambda/10 ≈ 21 mm
Therefore 12 mm mesh is excellent.
Folding mechanism
Construct the dish like a giant umbrella.
Centre hub
- Steel or aluminium hub.
- Six hinged radial arms.
- Arms fold backwards alongside the support mast.
Folded dimensions:
- Length: ~1.7 m
- Width: ~0.5 m
- Thickness: ~0.4 m
One person can transport this in a large estate car.
Feed support
Use three removable feed legs.
Each:
- 20 mm aluminium tube.
- Length about 1.25 m.
- Quick-release clevis pins.
The feed assembly detaches separately.
Feed
For hydrogen work, a simple feed is sufficient.
Options:
Best
- Dual-mode circular feed horn, such as the Brian Coleman dual-mode feed described elsewhere on this website.
- Gives good illumination and low spillover.
- Works well as long as F/D ratio is sufficiently large.
- Otherwise a cantenna would work well and is easy to construct (see other posts on making cantennas on this website).
Easier
- Cylindrical coffee-can feed.
- Diameter about 160 mm.
- Probe feed.
- Quarter-wave backshort.
Experimental
- Turnstile feed.
- Dual dipole feed.
Place the phase centre approximately 1.14 m above the dish vertex.
Mount
An altitude-azimuth mount is simplest.
Azimuth
- Trailer hub bearing.
- Steel pier.
Elevation
- Two pillow-block bearings.
- Handwheel or small gearmotor.
For drift scans, manual adjustment is adequate.
Expected performance
Dish area:
A = pi(D/2)^2
≈ 7.07 m^2
Assuming 60% efficiency:
Effective area:
Ae ≈ 4.2 m^2
Gain:
G = eta (pi D/lambda)^2
≈ 1200
≈ 30.8 dBi
Beamwidth:
HPBW ≈ 70 lambda/D
≈ 70 × 0.211/3
≈ 4.9 degrees
System performance:
- Hydrogen from the Galactic plane: easily detectable.
- Strong regions such as Cygnus and Cassiopeia: excellent.
- Galactic rotation studies: possible.
- Long integrations with SDR: very effective.
Receiver chain
A practical system would be:
Feed ↓ Low-noise amplifier at feed (0.5-0.8 dB NF) ↓ 1420 MHz band-pass filter ↓ Bias tee ↓ SDRplay RSPduo or Airspy ↓ Easy Radio Astronomy / GNU Radio ↓ Hydrogen spectroscopy software
Approximate weight
- Reflector frame: 12-15 kg
- Mesh: 4 kg
- Feed support: 2 kg
- Mount head: 10-15 kg
Total portable system:
Approximately 30-35 kg.
This design gives nearly the performance of a permanent 3 m hydrogen telescope while folding into a transportable package suitable for field use or storage in a garage.