Specify the Right Waveguide Horns for An Application

Waveguide horn antennas represent technology that is old but still invaluable, especially for sending and receiving signals well into the millimeter-wave frequency range. Electronic communications and radar systems are reaching higher in frequency as the numbers of users continue to increase. For applications like Fifth Generation (5G) cellular wireless communications, self-driving autonomous vehicles, and military radar systems, signals well into the millimeter-wave frequency range (30 GHz and above) are becoming more common in daily use, requiring rugged components supporting the transmission and reception of those signals, including different types of waveguide horn antennas. Fortunately, Impulse Technologies (www.impulse-tech.com) handles a variety of different waveguide horns covering many different millimeter-wave frequency bands, from leading manufacturers such as Spanish supplier Anteral (www.anteral.com). Selecting the best waveguide horn antenna for an application is a matter of understanding the mechanical and electrical parameters of different types of waveguide horns and how they can be made to fit in different systems.

Essential waveguide horn parameters to consider for a high-frequency application include frequency range (which will be determined by the antenna waveguide size), bandwidth, gain, directivity, impedance, impedance, input return loss or VSWR, beam width and height, radiation pattern, power-handling capability, size, weight, and type of transmission-line interface, such as waveguide or coaxial connector. For example, a waveguide horn with waveguide feedline and input interconnection can typically handle higher power levels for a given frequency than a horn antenna with coaxial cable feedline requiring a coaxial-to-waveguide adapter.

Waveguide horns are based on rectangular or circular (conical) waveguide. They are directional in nature, sending and receiving focused electromagnetic (EM) beams using relatively compact mechanical structures with dimensions that shrink with increasing frequency. They are passive components with negligible loss, so the directivity of a waveguide horn is usually almost the same value as the gain for a given frequency. The excellent directivity of waveguide horns makes them well suited for applications requiring the use of focused EM energy, such as in radar systems for emerging autonomous vehicular applications and radar guns for police authorities.

The capability to send and receive signals at millimeter-wave frequencies where power is limited make waveguide horn antennas viable candidates for satellite communications (satcom) systems and for test and measurement applications, even for measuring the radiation patterns of other high-frequency antennas. Waveguide horns are often used in satcom systems to direct signal energy towards a second antenna, such as a reflector or Cassegrain antenna, with a wider radiation pattern for increased beam coverage. They also provide high phase accuracy for microwave/millimeter-wave measurement applications, such as plotting the radiation patterns of an antenna under test (AUT). Waveguide horns have no resonant elements and are capable of wide instantaneous bandwidths in support of complex modulated communications signals.

A waveguide horn antenna is considered a form of aperture antenna, in which a waveguide transmission line feeds a precisely formed opening in the waveguide structure. A horn antenna can be formed from rectangular waveguide by flaring the opening into a defined opening, such as the shape of a pyramid, to control the impedance match between the waveguide and the free space into which it is radiating. A gradual flare yields a good impedance match between the waveguide and free space, although gradual flares typically require longer length for a waveguide horn antenna. A well-controlled impedance match between the two transmission media minimizes any reflections of electromagnetic (EM) waves from the waveguide to the free space, resulting in high antenna efficiency and low return loss or VSWR.

The flare angle will have a great deal to do with minimizing reflections from a waveguide horn for a given operating frequency and length of the antenna; achieving minimal reflections will contribute to maximizing gain for that flare angle. Horns with small apertures and small flare angles will provide low gain compared to wide horns with large flare angles and large apertures. One tradeoff for higher gain in waveguide horn antennas is the need for a longer horn length to accommodate a larger flare angle (approaching a maximum of 90°).

Achieving optimum performance with a waveguide horn antenna requires a good impedance match between the waveguide feed line and the waveguide input port of the antenna. The physical dimensions of a waveguide horn antenna and the size of the waveguide matched to the input port of the antenna, such as WR-19 (40 to 60 GHz), WR-15 (50 to 75 GHz), and WR-10 (75 to 110 GHz), will determine the frequency range of a connected waveguide horn antenna. The closest match possible to a 50-Ω environment at the antenna interconnection will result in the lowest possible VSWR for maximum transfer of energy from the input waveguide to the waveguide horn antenna.

Highlighting Horns

Waveguide horn antennas come in many shapes and sizes, including as standard gain horns, high gain horn antennas, lens horn antennas (LHAs), and high gain lens horn antennas (HGLHAs). All provide frequency coverage throughout the millimeter-wave range with performance levels meant to satisfy a wide range of applications. Even what are considered standard gain horn antennas (Fig. 1) are capable of impressive gain within the millimeter-wave frequency range. For example, standard gain horns developed by Anteral and available from Impulse Technologies are designed to cover a total frequency range of 8.2 to 500 GHz in 19 bands, with 26-dBi typical mid-band gain.

In contrast, HGLHAs designed and manufactured by Anteral and available from Impulse Technologies are capable of nominal gain above 40 dBi at millimeter-wave frequencies, with antennas available in seven frequency bands from 110 to 600 GHz. They add a plano-complex polytetrafluoroethylene (PTFE) or Teflon lens to a rectangular waveguide feeder and standard rectangular waveguide flange. The lens provides phase correction to the waveguide horn for increased gain with low sidelobes at minimum waveguide size. HGLHAs from Impulse and Anteral are usually protected by a radome or some other novel protective housing, such as an aluminum cylinder around the outside of the antenna. Well suited for communications, test, and radar applications, HGLHAs are capable of excellent impedance match to their waveguide feedlines, with typical VSWR of less than 1.30:1.

In a different shape, LHAs (Fig. 2) are based on circular waveguide, with a plano-convex PTFE lens added in the aperture to achieve phase correction. LHAs developed by Anteral and available from Impulse Technologies cover 8 to 170 GHz in 11 frequency bands with nominal midband gain of 30 dBi and with linear or circular polarization. The antennas achieve the high gain with low sidelobes and with low VSWR, less than 1.30:1. Yet another type of conical horn antenna, the focusing lens horn antenna (FLHA), incorporates a double-convex PTFE lens in the aperture for phase correction and enhanced focusing, as might be used in precise testing of radiation patterns, radiation leakage, and material characterization. Available from Impulse Technologies and Anteral in 13 frequency bands from 5.85 to 220.0 GHz, they feature VSWRs of less than 1.30:1 within those microwave and millimeter-wave bands.

For the optimum performance benefits of any kind of waveguide horn antenna, the waveguide feed lines themselves should provide good performance in terms of such parameters as insertion loss, power-handling capability, return loss or VSWR, and isolation. Waveguide transmission lines are often used in radar systems because of their low loss and much higher power-handling capabilities than coaxial cables at microwave and millimeter-wave frequencies. In cases where signals are being fed from coaxial cable assemblies, coaxial-to-waveguide adapters will be required as part of the interconnection of the waveguide horn antenna to the system infrastructure.

 

Figure captions

1. Standard gain horn antennas developed by Anteral and available from Impulse Technologies are cover a total frequency range of 8.2 to 500 GHz in 19 bands, with 26-dBi typical mid-band gain.

2. LHAs use circular waveguide and a Teflon or PTFE lens in the aperture to provide phase correction and enhanced beam focusing across 11 frequency bands from 8 to 170 GHz with nominal midband gain of 30 dBi.