Standard Gain UWB Planar Horn Antennas Hans G. Schantz

Next-RF, Inc. Huntsville, AL, USA

h.schantz@q-track.com

Jae Jeon Lawrence Livermore National Laboratory

Livermore, CA, USA jeon2@llnl.gov

Abstract—Horns are an important class of UWB antenna. This paper presents a compact planar horn design well-suited to serve as a gain reference or UWB measurement instrument. The design was made both compact and well-matched by introducing a right angle turn in a tapered balun feed structure. This paper describes some of the design challenges in translating numerical modeling into a successful implementation. Finally, we implemented the design in two different scale factors: a 30cm x 30cm Model 860A horn for use from 800MHz to beyond 6GHz and a 10cm x 10 cm Model 310C horn optimized for use from 3-10GHz.

Keywords-antenna, horn, planar, UWB

I. INTRODUCTION

Invented by J.C. Bose in the 1890s, then rediscovered in the 1930s and 1940s, horns are an important and useful subset of antennas [1]. As early as 1951, Coleman J. Miller employed a planar ridge element inside a horn antenna [2], and ridged horns were implemented by Dean K. Yearout and Harvey L. Jergins before 1960 [3]. By the 1980’s similar planar antenna elements began to emerge from William H. Nester and others [4]. Fig. 1 shows these three pioneering designs.

Horns offer particular advantages for UWB applications. First, they tend to be non-dispersive. Unlike “frequency-independent” antennas such as conical spirals or log periodics, horns maintain a constant phase center [5]. Thus, they can transmit and receive impulse signals with minimal phase distortion. Second, they can be relatively compact. They tend to have higher aperture efficiency than comparable reflector antennas and thus can offer more gain and directivity for a given physical dimension. Ridged horns are often used for UWB measurements and as gain references. However these antennas tend to be both bulky and expensive. This paper describes a compact planar horn design well-suited to replace a ridged horn in precision UWB applications.

First, we present the challenges inherent to planar horn design. Then, we describe the novel compact architecture of this planar horn design, and some of the difficulties in transitioning from a numerical model to a PCB prototype. We detail the performance of the Model 310C planar horn antenna. Finally, we explain the process of scaling the successful Model 310C by a factor of three to yield the Model 860A planar horn antenna. Both antennas offer low dispersion directive UWB performance. The Model 310C operates beyond 2.2GHz. The Model 860A is optimized for operation from 800MHz up to at least 6GHz .

Fig. 1 Miller’s planar ridge horn inset (top) [2], Yearout’s dual-ridge antenna (bottom left) [3], and Nester’s planar horn (bottom right) [3].

II. PLANAR HORN ARCHITECTURE

A. Planar Horn Design Challenges A planar horn must transition from a feedline to a slotline.

A stripline may be coupled to a slotline, however a discontinuous transition is tricky. One classic technique is to extend the slotline, the stripline, or both lines by an additional quarter wavelength at a desired operational frequency with an appropriate open or short termination, as seen in Fig. 2. Then, any reflection is returned in phase. This not only extends the dimensions of the antenna but can result in dispersion.

B. Implementing a Compact Horn Architecture By implementing a direct micro-strip to slot line transition,

one can avoid bulky quarter-wavelength scale transitions. In the preferred architecture, a taper balun [6] couples an unbalanced coaxial connector to a balanced stripline before coupling to the horn slot line. The taper balun is scaled to be about one-half wavelength at the lowest operating frequency. By further incorporating a right angle transition, the planar horn becomes quite compact. Even at frequencies where the antenna is less than one wavelength in dimension, this design exhibits a +3dBi or better gain. Fig. 2 shows this compact planar horn architecture.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

978-1-4577-2032-1/12/$26.00 © 2012 IEEE ICUWB 2012370

Fig. 2 A classic planar horn interface may employ quarter wavelength matching structures (top). A direct microstrip to slotline transition including a right angle transition yields a compact planar horn form factor (bottom).

III. TRANSITION FROM NUMERICAL MODEL

This compact planar horn illustrates some of the challenges involved in translating an optimized numerical model to a finished product. The inspiration for this elliptically tapered was prior success in obtaining excellent matching for an elliptically tapered dipole antenna [7]. An optimized HFSS model appeared to yield excellent results. The initial implementation, the Model 310A antenna, yielded respectable results that nevertheless fell somewhat short of performance expectations. The problem arose from the way in which HFSS automatically approximated an ellipse as a multi-faceted polygon. In the critical feed region, the intended elliptical taper was really a linear taper for purpose of the numerical design. Fig. 3 shows this subtle difference, as well as a matched pair of the Model 310C UWB Planar Horn Antennas.

A revised implementation in which the physical antenna incorporated the linear taper of the optimized numerical design yielded improved results. The 310A exhibited VSWR blips above 2:1 around 3.2GHz and 5.8GHz. The 310C maintains a VSWR under 2:1 above 2.2GHz. The 310C also exhibits gain about +2dBi better than the 310A for frequencies beyond 5GHz. Fig. 4 compares the matching and realized gain of the 310A and 310C UWB Planar Horn Antennas.

Fig. 3 Detail from an HFSS model showing the elliptical taper the designer thought he’d implemented (black line) and the piecewise linear approximation that the design software actually assumed (solid magenta area) (top), and matched pair of Model 310C UWB Planar Horn antennas.

Fig. 4 Matching of the 310A and the 310C shows the 310C yielding improved VSWR, better than 2:1 for frequencies greater than 2.2GHz (top). Boresight for the 310C shows gain improved by about +2dBi beyond 5GHz (bottom).

371

Fig. 5 The 310C is well matched with VSWR 2:1 or better for frequencies beyond 2.2GHz.

IV. MODEL 310C HORN ANTENNA RESULTS

This section takes a more detailed look at the performance of the Model 310C horn antenna from a matched pair measurement. The 310C exhibits a 2:1 or better VSWR for frequencies greater than about 2.2GHz, as shown in Fig. 5.

Motohiso Kanda established that the principal of reciprocity is violated to the extent of picking up an extra 90 degrees phase shift in the frequency domain (which is generally ignored in practice) [8]. Thus, the basic rule is that an antenna’s transient receive response is the time derivative of the transient transmit response. Ideal "Constant Gain" antennas radiate a faithful copy of a voltage signal and receive an integrated version of a free space signal. Ideal "Constant Aperture" antennas radiate a differentiated copy of a voltage signal and receive a faithful copy of a free space signal. Of course, there exists a certain amount of time dispersion due to the finite size of the antenna smearing out the response.

Fig. 6 The 310C approximates a constant aperture antenna between about 3-5GHz with phase linear to ±20 degrees. Thus the 310C can receive transient UWB waveforms with high fidelity.

Fig. 7 The 310C exhibits a directional pattern as seen in this HFSS generated 3-D pattern at 4GHz.

The realized gain of the Model 310C UWB Planar Horn Antenna is greater than +3dBi for frequencies greater than 2.5GHz. Phase is linear to within ±20 degrees and approximated a constant aperture between about 3-5GHz. Thus, this antenna will be able to receive transient UWB waveforms in this band with high fidelity. The time domain response of the received voltage signal will closely replicate the time domain response of the incident 3-5GHz signals. Fig. 6 presents this measurement. The pattern is directive, as shown in the HFSS 3-D pattern plot of Fig. 7, and narrows with increasing frequency.

Measurements of the matched pair of 310Cs were limited by the 6GHz bandwidth of the HP 8753D network analyzer. A scaled-up version of the design provides further insight into the design’s potential performance.

V. SCALING THE UWB PLANAR HORN ANTENNA

The spectral response of an antenna depends on the antenna scale. An antenna scaled-up by a factor of three will exhibit the same spectral response, shifted down to one-third the original frequency. Scaling a planar antenna is not a simple matter of scaling the entire PCB layout, because typically the PCB thickness remains constant. Thus, impedance critical dimensions must remain unscaled to maintain the appropriate performance. Fig. 8 shows the 30cm x 30cm Model 860A UWB Planar Horn Antenna beside a matched pair of 10cm x 10 cm Model 310Cs. Note for instance, how the width of the taper balun is constant to maintain impedance, and only the length is scaled.

Fig. 8 A matched pair of 10cm x 10cm Model 310C UWB Planar Horn Antennas (left) and a 30cm x 30cm Model 860A UWB Planar Horn Antenna (right).

372

Fig. 9 Scaling the spectral response of the Model 310C UWB Planar Horn Antenna by one-third yields an accurate prediction of the performace of the Model 860A, a Model 310C scaled up by a factor of three.

Fig. 9 presents the actual measurements of the 860A and 310C results, scaled by a factor of three to account for the size difference. The scaled prediction yielded a close match to the realized performance.

VI. MODEL 860A UWB PLANAR HORN ANTENNA RESULTS

This section takes a closer look at the performance of the Model 810A. The gain exceeds +3dBi for frequencies greater than about 750MHz. The gain curve exhibits constant aperture behavior up to about 2 GHz, where it transitions to a relatively constant +9dBi gain up to about 3.5GHz. Then the gain increases to about +12dBi around 5GHz. Phase in a matched pair measurement is quite linear to within ±30 degrees. Figure 10 presents the gain and phase measurements. Matching of the 860A blips above a 2:1 VSWR just past 1GHz, so there is some room for improvement. Still, matching is otherwise better than 2:1 from 800MHz to above 6GHz. Figure 11 shows the matching results.

Fig. 10 Gain and phase response of the Model 860A UWB Planar Horn Antenna.

VII. MODEL 860A UWB PLANAR HORN ANTENNA SIMULATION

Determination of the gain from a matched pair measurement at relatively short range (about 2m) requires an assumption as to the phase center of the antenna. Assuming the feedpoint is the phase center yields an artificially high estimate of range and therefore an artificially high estimate of gain. Assuming the extreme end of the physical horn structure yields an artificially low estimate of range and therefore an artificially low estimate of gain. In translating the matched pair S21 measurement into a gain measurement, we assumed the physical center of the 30cm x 30 cm antenna was the phase center. Comparison of a CST numerical simulation to measured results for the Model 860A UWB Planar Horn Antenna validates this assumption.

Fig. 11 Matching of the Model 860A UWB Planar Horn Antenna.

373

Fig. 12 Measured gain of the Model 860A UWB Planar Horn Antenna compared to a CST simulation.

Fig. 13 Measured matching of the Model 860A UWB Planar Horn Antenna compared to a CST simulation.

Fig. 12 compares the simulated gain to the measured gain. We obtained simulated gain data from the far-field calculation using CST. We assumed an open space boundary in all three dimensions around the antenna, along which a bounding box was delineated at eight wavelengths away at the lowest frequency of interest to define field calculation domain. Subsequently, these calculated fields yield far-field vectors. The simulated gain data shown in Fig. 12 is the realized gain, which includes impedance mismatch loss as well as conduction and dielectric loss. The measured gain shows some spread and variation since the data was taken in an open lab environment. Otherwise, we achieved excellent agreement.

Fig. 13 compares the simulated matching to the measured matching. For accurate simulated impedance matching result, mesh generation on the antenna model was carefully devised. Mesh lines around the balun area, which features smallest mesh dimension, were selected to provide adequate resolution width-wise. Fixed-points were created at critical curvature vertices on the radiating elements of the planar horn for smooth mesh line

dimension transitions for higher accuracy. Here again, agreement is excellent, although the measured data exhibited slightly better matching than the simulation predicted.

VIII. CONCLUSION

Planar UWB horn antennas have the potential to offer excellent directive performance with relatively linear phase response. The Model 310C and Model 860A UWB Planar Horn Antennas are well-suited for use as standard gain horns in UWB applications.

ACKNOWLEDGMENT

The authors wish to thank Gary W. Johnson for his assistance.

REFERENCES

[1] H. G. Schantz, “Three Centuries of Ultra-Wideband Antenna Development,” ICUWB 2012.

[2] C.J. Miller, “Feed horn,” U.S. Patent 2,691,731, October 12, 1954. [3] D. K. Yearout and H. L. Jergins, “Dual-ridge antenna,” U.S. Patent

2,944,258, July 5, 1960. [4] William Nester, “Microstrip notch antenna,” U.S. Patent 4,500,887,

February 19, 1985. [5] H.G. Schantz, “Dispersion and UWB antennas,” 2004 International

Workshop on UWB Systems and Technology, 18–21 May, 2004, pp. 161–165.

[6] J.W. Duncan and V.P. Minerva, “100:1 Bandwidth Balun Transformer,” Proc. IRE, Vol. 48, 1960, pp. 156–164.

[7] H.G. Schantz, “Planar elliptical element ultra-wideband dipole antennas, 2002 IEEE Antennas and Propagation Symposium, 16-21 June 2002, vol. 3, pp. 44–47.

[8] Motohiso Kanda, “Time Domain Sensors for Radiated Impulsive Measurements,” IEEE Transactions on Antennas and Propagation, Vol. 31, No. 3, pp. 438–444.

374