How do I design a feedhorn antenna for terahertz frequency operation?
Terahertz Feedhorn Antenna Design and Manufacturing
The feedhorn antenna is the interface between free space and the guided-wave circuit in a terahertz receiver or source. Its performance directly impacts beam efficiency, aperture illumination of downstream optics, and ultimately the system sensitivity. At terahertz frequencies, achieving the required dimensional precision is the primary engineering challenge.
Design Requirements
A well-designed terahertz feedhorn should produce a Gaussian beam pattern with beam efficiency above 95%, return loss better than 15 dB across the operating band, and cross-polarization below -25 dB. The corrugated conical horn is the standard choice because it naturally produces a hybrid HE11 mode pattern that closely approximates a Gaussian beam, enabling efficient coupling to lenses, mirrors, and quasi-optical components throughout the terahertz system.
Corrugated Horn Geometry
The corrugation design follows established rules: the corrugation depth transitions from approximately half-wavelength at the throat to quarter-wavelength in the flare section. The corrugation period should be less than one-third wavelength, and the slot width should be about half the corrugation period. These dimensions become extraordinarily small at terahertz frequencies: at 1 THz, the quarter-wave corrugation depth is 75 micrometers, and the corrugation period is about 100 micrometers.
Manufacturing Approaches
- Split-block CNC machining: The horn is machined in two halves that bolt together. Practical to about 800 GHz-1 THz with 5-axis CNC mills holding 5-10 micrometer tolerances
- Electroforming: A precisely machined aluminum mandrel is electroplated with copper, then the mandrel is chemically dissolved. Produces seamless horns to 1.5 THz
- Silicon DRIE: Deep reactive ion etching of silicon wafers creates corrugated horn structures with sub-micrometer precision. Suitable for frequencies above 1 THz and for horn arrays
- 3D printing (SLM/DMLS): Direct metal laser sintering produces horn geometries not possible with traditional machining. Surface roughness (currently 5-20 micrometers Ra) limits use to below about 500 GHz without post-processing
Beam width (corrugated horn): theta_3dB ~ 1.18 x lambda / D_aperture [rad]
Gaussian beam waist: w0 ~ 0.6435 x R_aperture (for optimally coupled corrugated horn)
Frequently Asked Questions
Can I use a smooth-walled horn at terahertz frequencies instead of corrugated?
Smooth-walled horns are simpler to manufacture and acceptable for some applications, but they produce asymmetric E and H-plane beam patterns with higher sidelobes and cross-polarization than corrugated horns. Diagonal horns (Potter horns) provide improved symmetry without corrugations and are sometimes used as a compromise at terahertz frequencies where corrugation machining is impractical.
What is the typical gain of a terahertz feedhorn?
Terahertz feedhorns typically have gains of 20-28 dBi, depending on aperture size and flare angle. The aperture is chosen to match the f/D ratio of the downstream quasi-optical system, not to maximize gain. Over-sized horns produce inefficient illumination of the coupling optics.
How do I test a feedhorn at terahertz frequencies?
Far-field antenna pattern measurements at terahertz frequencies use a combination of terahertz sources (multiplier chains or QCLs) and detectors mounted on a precision positioner. Near-field scanning with terahertz probes is also possible. Return loss is measured using a terahertz VNA or standing-wave technique with a calibrated detector.