Covering a frequency range spanning from 7 GHz to 70 GHz in standard configurations – while maintaining consistent gain, a predictable radiation pattern, and acceptable VSWR is a challenge that no single-mode rectangular waveguide horn can address. Standard gain horns are band-specific: a WR-28 Ka-band horn covers 26.5 to 40 GHz, and a WR-15 V-band horn covers 50 to 75 GHz, but neither can operate across both bands simultaneously, or below 26.5 GHz. For test engineers who need to sweep continuously from 7 GHz to 70 GHz, a set of seven or more standard horns, each with its own cable, mount, and calibration dataset, would otherwise be required – a logistical burden and a significant source of measurement uncertainty at every hardware changeover.

This article explains how a double ridged horn antenna achieves its wide bandwidth, what the ridge loading mechanism does to the waveguide’s electromagnetic properties, and where this antenna type delivers performance that no narrowband alternative can match in test and measurement, EMC, and broadband RF system applications.

The Problem with Standard Rectangular Horn Antennas

A standard pyramidal horn antenna is fed from a rectangular waveguide whose dominant TE10 mode propagates above a cutoff frequency determined by the waveguide’s wide inner dimension. Below this cutoff, the waveguide is evanescent – it cannot support the propagating mode – and the horn ceases to function as a useful radiator. For WR-15 waveguide, the TE10 cutoff is approximately 39.9 GHz. The WR-15 horn cannot radiate below this frequency; it requires a separate, physically larger waveguide and horn to cover the frequency range below it.

This fundamental constraint means that covering a wide frequency range with standard rectangular horns requires multiple antennas, each dedicated to a specific waveguide band. For a test system that must cover 7 to 70 GHz, seven separate horn antennas with seven separate waveguide interfaces would be necessary. Each changeover introduces connector repeatability errors, requires recalibration, and consumes test time. The double ridged horn was developed specifically to overcome this constraint.

How a Double Ridged Horn Antenna Works

The Ridge Loading Mechanism

A double ridged horn antenna modifies the standard rectangular waveguide cross-section by introducing two conductive ridges – one projecting inward from the top wall, one from the bottom wall – that run the length of the feed waveguide section and continue into the flared horn body. These ridges are oriented parallel to the E-field of the dominant TE10 mode, and they are shaped in cross-section as step profiles or tapered ramps that progressively narrow the gap between the ridge tips as the ridges approach the centre of the waveguide.

The narrow gap between the ridge tips introduces a concentrated capacitive loading – the top and bottom ridge faces, separated by a small air gap, form a distributed capacitor in the transverse plane of the waveguide. This capacitive loading lowers the cutoff frequency of the dominant TE10 mode well below the cutoff of the unloaded rectangular waveguide with the same outer dimensions. In a well-optimised double ridged horn, the ridge geometry reduces the TE10 cutoff frequency by a factor of three to five, allowing a horn with an outer cross-section comparable to a WR-137 waveguide to operate with useful radiation efficiency from below 7 GHz all the way to 70 GHz.

Higher-Mode Suppression

An equally important function of the ridges is the suppression of higher-order waveguide modes. In an unloaded rectangular waveguide, the TE20, TE01, and higher modes begin to propagate as frequency increases above their respective cutoffs, potentially causing pattern distortion, gain ripple, and polarisation impurity. The ridge geometry raises the cutoff frequencies of these higher-order modes relative to the dominant TE10 mode, maintaining single-mode propagation across a significantly wider bandwidth than would be achievable without ridges. Single-mode propagation over a decade of bandwidth is what makes the double ridged horn suitable for precision measurement applications where pattern consistency and gain predictability across frequency are required.

Coaxial Feed Transition

Unlike narrowband waveguide horns that accept a standard waveguide flange, most double ridged horn antennas for broadband test use incorporate a coaxial-to-ridge waveguide transition at the base of the feed section. This transition – typically designed around a 1.85mm connector – converts the unbalanced coaxial TEM mode to the ridge-loaded TE10 mode with minimum reflected power across the full operating bandwidth. The transition quality determines the antenna’s VSWR performance: a well-designed transition achieves VSWR below 2.0:1 from 7 to 70 GHz, while a poorly designed transition introduces impedance resonances that cause gain ripple and pattern anomalies at specific frequencies within the band.

Key Performance Parameters

Gain and Gain Flatness

Double ridged horn antennas produce gain that increases with frequency across the operating band – typically 3 to 6 dBi at 7 GHz, rising to 10 to 14 dBi at 70 GHz. This frequency-dependent gain reflects the increasing electrical aperture size as wavelength decreases. For EMC measurements requiring a known field strength at the test point, this frequency-dependent gain must be corrected using a calibrated antenna factor – a frequency-specific table that translates between received power at the antenna port and electric field strength at the antenna aperture.

Mi-Wave characterises the gain and antenna factor of its double ridged horn antennas across the full specified operating bandwidth, providing the calibration data needed for quantitative field strength measurements. Gain flatness – the peak-to-peak variation across the band – is held to within ±3 to ±4 dB through careful ridge profile optimisation, ensuring that gain variations do not introduce large correction factors that amplify measurement uncertainty.

VSWR and Return Loss

VSWR determines how much incident power is actually radiated versus reflected back to the source. A VSWR of 2.0:1, corresponding to a reflection coefficient of -9.5 dB, means that approximately 11% of incident power is reflected rather than radiated – acceptable for most test and measurement applications. Premium designs achieve VSWR below 1.5:1 across 7 to 70 GHz, enabling direct connection to measurement receivers without requiring isolators to protect sensitive input ports from the reflected wave.

Radiation Pattern and Cross-Polarisation

The E-plane and H-plane radiation patterns of the double ridged horn differ because the ridges concentrate the E-field in the plane of the ridge gap, narrowing the E-plane beamwidth relative to the H-plane. Typical H-plane beamwidth at mid-band is 50 to 70 degrees; E-plane beamwidth is narrower. Cross-polarisation isolation – the ratio of co-polarised to cross-polarised gain at boresight – is typically better than 20 dB in a quality double ridged horn, and must be verified for any application involving polarimetric measurements. Mi-Wave’s full range of antenna products includes pattern and polarisation measurement data to support antenna selection for demanding measurement applications where cross-polarisation performance is critical.

Applications of the Double Ridged Horn Antenna

EMC and EMI Testing

The primary commercial application of the double ridged horn is radiated emissions testing and immunity testing per CISPR 16, MIL-STD-461, IEC 61000, and related standards. In a semi-anechoic or fully anechoic test chamber, the antenna illuminates the equipment under test from multiple azimuth positions while a spectrum analyser or receiver measures the radiated signal at the antenna port. The calibrated antenna factor converts the received power to radiated field strength, enabling direct comparison against regulatory emission limits.

The double ridged horn’s coverage from 7 GHz to 70 GHz – or 1 to 18 GHz in lower-frequency variants – eliminates the need to change antennas mid-sweep, reducing test time and eliminating the connector repeatability errors and recalibration steps that affect multi-antenna measurement setups. In production EMC test facilities where throughput directly affects commercial capacity, this time saving has direct economic value.

Antenna Gain Measurement

In antenna test ranges and compact range facilities, the double ridged horn serves as a broadband standard gain antenna for characterising unknown antennas by the gain comparison or substitution method. Because its gain is known and calibrated across a wide frequency range, a single standard configuration supports gain measurements from 7 to 70 GHz without changing the reference antenna, eliminating the systematic error introduced by antenna-to-antenna changeover uncertainty.

Radar Cross Section Measurement

RCS measurement ranges require broadband transmit and receive antennas that maintain consistent, well-characterised radiation patterns across the measurement frequency range. Double ridged horn antennas used in RCS ranges must meet stringent sidelobe specifications to prevent multipath reflections from the chamber walls and absorber imperfections from being mistaken for target RCS contributions. Mi-Wave works with RCS measurement facility operators to specify double ridged horn antennas with the sidelobe performance required for accurate low-RCS target characterisation.

Electronic Warfare and Signal Intelligence

Broadband monitoring antennas for EW receivers and SIGINT collection systems must cover wide frequency ranges without antenna hardware changes that would create coverage gaps. The double ridged horn’s decade bandwidth makes it suitable for direction-finding, signal intercept, and spectrum monitoring applications across the microwave and lower millimeter wave spectrum. Its robust all-metal construction provides the mechanical durability and environmental resistance needed for field-deployed collection systems.

Comparison with Alternative Broadband Antennas

Below 6 GHz, log-periodic dipole arrays (LPDAs) and biconical antennas compete with and often outperform double ridged horns in gain flatness and sidelobe performance. Above 18 GHz, however, the double ridged horn has no practical broadband competitor offering comparable gain, directivity, and pattern stability in a compact, mechanically robust form factor. Planar Vivaldi arrays and open-ended waveguide probes offer alternatives in specific frequency segments but cannot match the double ridged horn’s decade bandwidth at V-band and above. The machined metal construction of a precision double ridged horn also offers inherent advantages in field-deployed and outdoor applications where the mechanical fragility of planar array designs would be a reliability concern.

Conclusion

The double ridged horn antenna achieves what no conventional single-band waveguide horn can: reliable, calibrated antenna performance across a frequency range spanning nearly a decade. The ridge loading mechanism that makes this possible – reducing the TE10 cutoff frequency through capacitive loading while simultaneously suppressing higher-order modes – is elegant in concept but demands precision engineering in execution to deliver the gain flatness, pattern consistency, and VSWR performance required for quantitative measurement applications.

For test engineers specifying antennas for broadband RF test systems, the double ridged horn offers a compelling reduction in system complexity, calibration overhead, and hardware cost compared to sets of narrowband alternatives. Its combination of wide bandwidth, consistent radiation patterns, and robust all-metal construction has made it the standard tool for EMC testing, antenna characterisation, and broadband receiver systems operating from the microwave through the lower millimeter wave frequency range.

Mi-Wave manufactures double ridged horn antennas in the Series 265 configuration, covering 7 to 70 GHz with calibrated antenna factor data and full pattern measurement support. Engineers specifying broadband antennas for test, measurement, or operational RF system applications are welcome to contact Mi-Wave for product specifications and application engineering assistance.