Ultra-Wideband Antenna Performance Comparison William Coburn and Seth McCormick US Army Research Laboratory Adelphi, USA
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Abstract—For ground penetrating radar applications antennas with bandwidth typically 3-to-1 or greater are required. Many designs can be found in the literature but performance tends to suffer when extending the bandwidth to lower frequencies. Here the nominal frequency range is 0.3–2.3 GHz (154%) and the TEM horn is used as a reference for performance comparisons. Planar antenna designs have many advantages such as low-profile, light-weight, and ease of fabrication and integration. Various planar monopole and a slottype antennas are compared where the results show that increasing the size can extend the bandwidth to lower frequencies but introduce radiation pattern instability at high frequency. This paper summarizes a numerical investigation using FEKO to provide antenna alternatives for multistatic radar systems. The results serve to guide the selection of broadband antenna elements for ground penetrating radar system developers. Keywords—monopole antenna, planar antennas, slot antenna, TEM horn, ultra-wideband, Vivaldi antennas.
I. INTRODUCTION This paper presents a numerical analysis of UltraWideband (UWB) antennas using FEKO (www.feko.info) software using the Method of Moments. The TEM horn [1] is used as a 3D reference antenna for comparing various designs as it exceeds the impedance bandwidth (IBW) requirement. UWB antennas are often compared according to the electrical size at some frequency and performance such as peak gain, fractional IBW, etc. The performance vs. frequency can be shown but a combined figure-of-merit (FOM) is more desirable as a performance comparison and can be tabulated at critical frequencies. Here we choose a FOM based on the gain, G, normalized to the antenna VSWR and electrical size, L, at each frequency. So FOM = G/(VSWR x L) is used to compare performance as a function of frequency. The largest antenna dimension, D, is used to calculate the electrical size, L = D/λ at each frequency. The linear gain in the desired direction is used in the FOM for antennas radiating in both endfire and. other directions. The antenna width in the operational configuration may be more important since it determines how many transmit/receive (TX/RX) pairs can be used in the cross-range direction. The objective is to improve the low frequency performance compared to existing alternatives to allow ground penetrating radar (GPR) system developers to conduct trade studies on size, weight, and power plus cost (SWAP-C) vs. performance for downward-looking multistatic GPR systems in the frequency range 0.3–2.3 GHz (154% fractional IBW).
Various antenna simulation and measured results will be compared using this frequency dependent FOM. The planar antennas are microstrip or coplanar waveguide fed on Rogers low-loss substrates having εr = 2.33 or 3.5 and FR4 having εr = 4.4 and tanδ = .02 with maximum thickness 1.574 mm (62-mils). In some cases microwave absorber loading is used to extend the IBW to lower frequencies [2]. A metal ground plane can also be used to increase directivity, but the height must be larger than about 0.2 wavelengths and causes variations in the gain vs. frequency. For downwardlooking GPR applications many TX/RX pairs of small antennas are often used such as the cavity backed spiral or planar slot antennas [3]. Here the objective is to increase the boresight (i.e., at nadir) gain at low frequency in the most compact size possible without reducing the high frequency performance. For multistatic radar systems it may be possible to reduce the number of TX/RX pairs by using larger elements having improved gain without sacrificing the overall system performance. The results show that increasing the antenna size can extend the IBW but at high frequency transitions to a traveling wave type antenna with poor pattern stability in the desired direction. II. ANTENNA ALTERNATIVES A planar antipodal Vivaldi antenna can be obtained by careful design of the tapered slot with a common implementation shown in Fig. 1 (left) [4]. A TEM horn is used as a 3D performance references for the IBW and pattern stability in a relatively large size as shown in Fig. 1 (right). A Vivaldi monopole radiating at 54° above the ground plane is also used as a performance reference. Planar antenna alternatives radiating at broadside are compared to these reference antennas. The antennas were simulated with FEKO over the desired frequency range where the FOM is reduced by the VSWR and if the boresight gain does not increase as expected with increasing electrical size. At low frequency (LF) the realized gain at 0.3 GHz or the frequency where VSWR = 3 is used in the FOMLF. The mid-band performance at 1 GHz, FOMMF, and the high-band, FOMHF, are also summarized. The numerical results have similar frequency dependence with most designs exhibiting the onset of beam tilting or splitting at high frequency. Those designs that have sufficient IBW are normalized to the antenna electrical size and VSWR to provide a frequency dependent FOM. In this summary the FOMLF, FOMMF, and FOMHF are tabulated for comparison. An analysis of antenna alternatives is done primarily by simulation with the
GPR system developers down selecting the antenna options based on the overall system performance and SWAP-C considerations.
shown in Fig. 4 (left) at 2 GHz for an absorber loaded asymmetric hourglass monopole and in Fig. 4 (right) for a planar dual slot antenna both with a one-inch air gap. A summary of the FOM calculated at three frequencies is shown in Table I.
Fig. 1. Endfire antennas: (left) antipodal Vivaldi and (right) TEM horn.
III. RESULTS The simulated radiation pattern for the Vivaldi monopole is shown in Fig. 2 at 1 and 2 GHz where the pattern is stable vs. frequency so the antenna would be installed at the appropriate angle to obtain the required coverage. For planar antenna designs radiating at broadside the peak gain often tilts from zenith at higher frequencies which is captured in the FOM. An example is shown in Fig. 3 (left) at 2 GHz for a planar sleeve monopole and in Fig. 3 (right) for a planar slot with additional resonant slots to better control this beam tilt. Additional results for planar antenna designs with the available measured data will be included in the presentation.
Fig. 2. A 305 mm (12-inch) tall Vivaldi monopole radiation pattern at: (left) 1 GHz and (right) 2 GHz.
Fig. 4. Absorber loaded antennas: (left) hourglass monopole and (right) dual slot radiation patterns at 2 GHz. TABLE I.
FOM AT THREE FREQUENCIES FOR VARIOUS ANTENNAS D (mm) LF (GHz) FOMLF FOMMF FOMHF
Antenna Type TEM Horn
590
0.3
3.2
4.1
2.3
Antipodal Vivaldi
437
0.67
0.48
1.1
2.6
Vivaldi Blade
254
0.3
1.7
4.3
3
Vivaldi Blade
305
0.3
2.1
2.7
3.4
Asymmetric Hourglass
254
0.34
1.4
1.3
0.9
Asymmetric Hourglass (w/ABS)
254
0.3
1
0.7
1.2
Asymmetric Hourglass
305
0.3
2.2
1.2
1.1
Asymmetric Hourglass (w/ABS)
305
0.3
0.8
0.9
1.4
Sleeve Monopole (FR4)
263
0.6
1.52
1.16
0.56
Sleeve Monopole (Duroid)
300
0.4
0.42
1.36
0.31
Planar Dual Slot
254
0.4
1.03
0.66
0.77
Planar Dual Slot (w/ABS)
254
0.37
1.14
1.37
0.58
Slotted Planar Slot
200
0.45
1.56
2.46
1.07
Slotted Planar Slot
300
0.33
1.2
1.2
0.46
Large Planar Slot
300
0.46
1.5
0.64
0.5
Large Planar Slot (w/ABS)
300
0.43
0.64
0.6
0.66
IV. CONCLUSION The objective to improve the low frequency gain requires larger antennas with some trade-offs in performance. A frequency dependent FOM is proposed that captures these trade-offs and allows a comparison of alternatives based on size and performance. Various antennas will be compared using this FOM for both simulated and measured results. REFERENCES
Fig. 3. Bulbous sleeve monopole: (left) and slotted planar slot antenna (right).
The results indicate that for planar antennas the slot-type antenna is better matched at low frequency. The antenna transient excitation and radiated field indicate some late time ringing which can be mitigated by absorber loading. This loading requires a tradeoff between impedance matching at low frequencies and antenna efficiency and should be measured to determine adequate radar performance [2]. An example is
[1] [2] [3] [4]
C. Grosvenor, et al., “TEM Horn Antenna Design Principles,” Nat. Inst. of Stand. and Tech., NIST TN-1544, January 2007. W. Coburn, "An ultra-wideband absorber backed planar slot antenna," App. Comp. Electro. Express Journal, vol. 1, no. 1, January 2016. C. Ly, et al., “Detection of Buried IEDs Using a Novel Planar Slot Antenna Array for Downward-Looking Radar,” Proc. Tri-Service Radar Symposium, Springfield VA, July 2017. S. McCormick, “Modeling, Simulation, and Measurement of Balanced Antipodal Vivaldi Antennas,” US Army Research Lab., ARL-TR-8111, August 2017.
