Pyramidal horn antenna

A pyramidal horn starts as a short length of rectangular waveguide with a probe in it, then flares out in both directions to a larger rectangular mouth. The flare makes a gradual transition from the waveguide to free space, so the horn is well matched over a wide band and has a clean, predictable beam with low sidelobes. Those are exactly the properties you want when you don’t yet know what your noise floor looks like.

For a given gain there’s an optimum-gain design: the flare length and mouth dimensions that reach that gain with the shortest horn. It ends up with an aperture efficiency of about 51%. Our dish and horn calculator solves the standard design equations (Balanis, Antenna Theory) for any target gain and throat size.

Worked example: 17 dBi from a WR-650 throat

WR-650 is the standard waveguide size for L-band: 165.1 × 82.55 mm inside. For a 17 dBi horn at 1420.406 MHz the solver gives:

• H-plane aperture (wide side): 675 mm
• E-plane aperture: 514 mm
• flare length from throat to mouth: 480 mm
• check gain at those dimensions: 16.99 dBi

That’s about the size of a carry-on suitcase, light enough for a camera tripod. Its gain is about 4 dB short of a 1 m dish (see antenna gain) and its beam is roughly 20 to 25 degrees wide.

Why start with a horn

The hydrogen line from the Galactic plane fills even a wide beam, so a horn sees it at nearly full strength. The horn needs no dish, no feed alignment and no mount beyond a tripod, and it works in a drift scan with no tracking. LAB-008 builds one into a complete station, and LAB-009 takes it out for first light.

Building notes

Foam insulation board lined with aluminium foil tape works well at 21 cm; overlap the tape at seams and keep the inside surfaces flat. Thin aluminium sheet is sturdier. The throat section is a short box of WR-650 inside dimensions, closed at the back, with the probe a quarter guide-wavelength from the back wall, the same principle as a cantenna. Mount the LNA right at the probe connector.