Applications & Considerations for Double-Ridge Guide Horn Antennas
A general trend in telecommunications, radar and other sensing applications is higher operating frequencies, into the mmWave bands. Though the bandwidth/throughput, spectrum congestion and resolution are attractive at mmWave frequencies, mainstream applications at these higher frequencies can interfere with critical military, government, weather sensing, satellite sensing, satellite communications and other scientific research. Ensuring minimum interference for critical applications in the mmWave spectrum will be a burden on the electromagnetic compliance (EMC) certification and electromagnetic interference (EMI) authorities, as well as anyone working in applications vulnerable to unintended -- or intentional -- interference.
A key antenna technology for testing and EMI/RFI monitoring/surveillance is the double-ridge guide horn (DRGH) antenna. This article explores the trends influencing the utility of DRGH antennas, their performance and technology developments.
Pyramidal horn antennas are widely used, classic aperture antenna configurations, where a rectangular waveguide is gradually transitioned into a horn, which provides an impedance-matched path from the waveguide to free space. These linearly polarized waveguide antennas have been a popular choice for many high gain, point-to-point radio links at mmWave frequencies. Closed-form equations enable a designer to closely match the antenna’s gain to its dimensions, and the radiation pattern can be closely approximated. However, these antennas typically perform over a relatively narrow bandwidth, which limits their use for certain testing scenarios. With EMI testing, for example, the immunity and emissions tests typically measure noise containing frequency components spanning a much wider bandwidth.
DRGH antennas introduce ridges - internal ridged arches attached to the edges of the pyramid on the E-plane - adding capacitance effects that lower the cut-off frequency of the dominant TE10 propagation mode, which extends the single-mode bandwidth. This eliminates the need for additional antennas to cover the required bandwidth, with their respective connections and potential paths for additional EMI.
The main DRGH antenna radiation pattern occurs on the main axis, i.e., at 0 degrees. At high frequencies, the main beam becomes narrower, and the side lobes increase in power. At high frequencies, i.e., >12 GHz, the main lobe splits into four off-axis lobes, where the amplitudes of these four lobes eventually surpass the power of the on-axis lobe.1
The components of a conventional DRGH involve the coax-to-waveguide feed, a standard pyramidal horn structure and two central ridges (i.e., top and bottom), as shown in Figure 1. Classic iterations of the DRGH antenna use dielectric sidewalls instead of the standard flared aperture found in horn antennas. The ridges are tapered to vary the impedance from the 50 Ω feed point to the impedance of free space at the aperture of the horn antenna, i.e., 377 Ω. The shielding ground of the coaxial feed is connected to the first ridge, the second ridge is connected to the center pin through an extension of the coaxial inner conductor. The cavity at the base of the structure is connected to the flared walls of the horn through a secondary metallic box, which creates a shorting plate that reduces the return loss in the waveguide transition.
Several design modifications have attempted to overcome the deteriorating radiation pattern at high frequencies.2–4 These include:
- Adjusting the ridges to reduce edge diffraction
- Modifying the side flares with a metallic grid, dielectric sidewalls or removing them entirely
Each of these approaches improves the bandwidth of the antenna by increasing the frequency where the characteristic deterioration of the radiation pattern occurs, extending it to Ka-Band.
In some DRGH antenna designs, the ridges have a “fast” opening or rapid transition from the waveguide to free space. The downside is particularly severe at the aperture: higher mismatch and unwanted loss. The sharp edges within the ridges can also cause diffraction affecting the radiation pattern at higher frequencies. To optimize this impedance transition, various ridge designs have been used: linear, exponential, circular or other mathematical functions - even sinusoidal. Some impedance tapers use a combination of linear (near the feed), exponential (in the middle) and circular (near the aperture) to improve the antenna’s performance.3
Modifying the side plates within the pyramidal horn structure includes completely removing the H-plane flares; adding dielectric H-plane flares; or adding thin metallic strips to form a bridge between the E-plane plates, which improves the structural integrity of the antenna (see Figure 2). The conventional DRGH has one ridge positively charged, the other negatively charged, which support an E-field between the ridges that propagates toward the aperture of the horn. With this design, a high E-field occurs between the ridges and a relatively low E-field between the ridges and the flares orthogonal to them, i.e., the flares that do not have ridges. This property makes these H-plane flares extraneous. They can be removed to reduce weight, with the additional benefit of increasing the half-power beamwidth at high frequencies.3
Using a dielectric sidewall causes the degrading gain reduction to occur on-axis at the upper frequency limit of the DRGH, likely due to higher-order modes (e.g., TE20, TE30). However, removing the sidewalls entirely degrades the low frequency performance: the on-axis beamwidth increases and the gain decreases.2 For this reason, a metallic grid is often used; it has a less severe gain dip at the upper frequencies without degrading the low frequency performance of the antenna.
EMC MEASUREMENT METHODS
Electromagnetic (EM) emissions emanate from many sources, including pulse generators, oscillators, digital logic circuits, switching power supplies, motors and converters - typical components found in electronic equipment. For standards and testing, the EMI from these components is typically classified in two categories: conducted and radiated emissions. Radiated testing involves emissions testing of near-field and far-field radiation, as well as immunity testing, i.e., determining the susceptibility of a device to emissions from surrounding devices. Conducted emissions are related to the EM signals from nearby or interconnected circuits. Standards for EMC testing include:
- CISPR 16-1-4
- EN 55022/55011
- ANSI-C 63.2/63.4/63.7
- DEF STAN 59-41
Typically, EMI tests occur within an open-area test site (OATS), test cell or screened room (i.e., a reverberating or anechoic chamber). Test setups use a table, where the antenna is placed at a specified distance above the ground, e.g., 0.8 m, and a determined distance from the equipment under test (EUT), typically from 3 to 30 m. The height of the antenna varies between 1 and 6 m and the EUT is rotated until the maximum emissions are observed. The tests are conducted in both horizontal and vertical polarizations, using various approaches to speed testing: an automated test setup with the EUT on a turntable, antenna on a mechanical mast, software-controlled measurement equipment and broadband antennas.
Several types of antennas are used for EMI testing (see Table 1). Typically, active antennas - antennas that include components such as integral amplifiers, preamplifiers and other active devices to amplify the signal - are effective for low frequency measurements, such as active loop and active rod (monopole) antennas. E-field measurements typically use monopole antennas, while low frequency H-field measurements use loop antennas. Broadband antennas, such as the biconical antenna, can be used for the 30 to 300 MHz range, with a log-periodic antenna for 300 MHz to 1 GHz and broadband horn antenna from 1 to 40 GHz.
For most EMC standards, the antenna is the primary measurement transducer coupling the measured variable in the antenna’s radiated EM field into the measuring receiver, so understanding its behavior in the test environment is critical. The E-field strength is specified in units of V/m at a specified distance from the EUT, while the test receiver is calibrated in voltage across a 50 Ω impedance. The antenna is calibrated to the voltage at the 50 Ω input of the receiver for a given electric field strength (V/m) at a given frequency. This calibration is known as the antenna factor (AF), the incident E-field divided by the voltage measured at the receiver and specified in dB/m. While the typical parameters for measuring antenna performance are gain, VSWR and directivity, these are not as important for EMC emissions measurements as the AF. The AF gives a more accurate assessment of the antenna’s performance, ensuring the antenna is oriented for maximum response. For a typical test setup (see Figure 3), the field strength at the antenna (μV/m in dB) is equal to the sum of the AF, the attenuation of the interconnecting cable (in dB/m) and the voltage at the receiver (μV in dB):5
The AF must be known to adequately measure test site parameters such as the site VSWR and normalized site attenuation (NSA). Determining the NSA involves measuring the insertion loss between the terminals of the transmit and receive antennas and including the AF of each antenna:
where VDIRECT is the voltage measured at the terminal of the transmit antenna, VSITE is the voltage at the receive antenna and AFR and AFT are the AFs for the receive and transmit antennas, respectively.
The NSA of the test site is compared to the loss of an ideal site, based on a list of theoretical values over frequency, with the differences known to be deviations caused by test site imperfections. The CISPR standard for a test environment requires the site be within ±1 dB of the ideal test site, which requires a test environment to have very tight tolerances. As the calibration accuracy depends upon the site, these tolerances are necessary for measurement accuracy. Two forms of calibrations are typically performed: a tuned dipole in an OATS and the standard site method. Using a tuned dipole as a reference antenna requires retuning at each frequency and depends on the integrity of the antenna and test site. The standard site method bypasses these conditions, instead using three uncalibrated antennas that are calibrated using three measurements between pairs of antennas. However, this approach also requires a high-quality test site.
DRGH BENEFITS for EMC TESTING
EMC testing requires a balance between the main lobe beamwidth and gain of an antenna. The broader the beamwidth, the more energy can be collected by an antenna over an area; on the other hand, the higher the gain, the better the system noise performance. Highly directive antennas offer less sensitivity to off-axis reflections, making the imperfections of the test site less relevant. However, less area is covered at a distance, so larger objects cannot be tested. In this case, EUTs are measured in consecutive sweeps with the antenna aimed at different parts. Broader beamwidth often correlates with lower gain, so more power is required. In this case, a balun is often used at the antenna’s feed to prevent damage to the antenna, particularly for biconical and log-periodic antennas, where power can accumulate in the core and windings of the antenna structure. The addition of a balun limits the input power that can be applied, which subsequently increases the VSWR and reduces the total power being radiated and the overall efficiency of the test system.
Practically, waveguide generally dissipates excess heat from high transient powers efficiently because of the large metallic surface area of the guide. For this reason, they are often used in the antenna feeds for high frequency radars, where high-power handling is required. The DRGH antenna has inherently higher frequency performance and higher power handling -- to hundreds of Watts -- compared to other broadband antennas used for EMC emissions testing, so a balun is typically not necessary when using DRGH antennas.
In an EMC test environment, the DRGH aperture dimensions are specified by many standards, defined by the distance between the antenna and the EUT. For instance, the ANSI standards require the aperture dimension of a horn antenna to be small enough so the measurement distance is equal to, or greater than, the far-field distance.6 The far-field distance, also known as the Rayleigh distance (Rm), is given by:
where D is the largest dimension of the antenna aperture and λ the free space wavelength.
The DRGH antenna design modifies the classic pyramidal horn antenna to extend the bandwidth. While this leads to design challenges associated with the DRGH’s radiation pattern at higher frequencies, structural adjustments can address them, such as tailoring the ridges and replacing the H-plane flares with a grid-type metallic structure. Also, the high frequency operation of the DRGH antenna can be extended without compromising its low frequency performance. The DRGH is specified in EMC emissions testing standards because of its broadband performance and coverage above 1 GHz. It is a desirable alternative to the log-periodic and biconical antennas for EMC applications.
- C. Bruns, P. Leuchtmann and R. Vahldieck, “Analysis and Simulation of a 1-18-GHz Broadband Double-ridged Horn Antenna,” IEEE Transactions on Electromagnetic Compatibility, Vol. 45, No. 1, Feb. 2003, pp. 55–60.
- B. Jacobs, J. W. Odendaal and J. Joubert, “An Improved Design for a 1–18 GHz Double-Ridged Guide Horn Antenna,” IEEE Transactions on Antennas and Propagation, Vol. 60, No. 9, Sept. 2012, pp. 4110–4118.
- M. Abbas-Azimi, F. Arazm and J. Rashed-Mohassel, “Design of a New Broadband EMC Double Ridged Guide Horn Antenna,” 2006 First European Conference on Antennas and Propagation, Nice, 2006, pp. 1–5.
- M. Botello-Perez, H. Jardon-Aguilar and I. G. Ruiz, “Design and Simulation of a 1 to 14 GHz Broadband Electromagnetic Compatibility DRGH Antenna,” 2005 2nd International Conference on Electrical and Electronics Engineering, 2005, pp. 118–121.
- T. Williams, “EMC for Product Designers,” Newnes, 2017.
- “American National Standard for Electromagnetic Compatibility–Radiated Emission Measurements in Electromagnetic Interference (EMI) Control–Calibration and Qualification of Antennas (9 kHz to 40 GHz),” ANSI C63.5-2017 (Revision of ANSI C63.5-2005), May 2017, pp.1–114.