# Low-Profile Ultra-Broadband Log-Period Monopole End-Fire Antenna

Authors: Liang Ma; Rui Xu; Hongchen Wen; Zhenbing Li; Jian Li; Yongjun Huang
Journal: International Journal of Antennas and Propagation
 (2018)

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## Abstract

This paper proposes an ultra-broadband (2–13 GHz) and low-profile log-period monopole end-fire antenna for the flush-mounted applications. 24 monopoles with a log-period rule are used to cover the whole operating frequency band, and those monopoles are printed on both sides of a low-loss dielectric layer vertically placed over a slot feeding line with wideband microstrip-to-slotline transition. The low profile is realized by bending the parts of the long monopoles so that the overall antenna size is obtained as 40 mm × 100 mm × 13.6 mm. The proposed antenna is fabricated, and the measured results agree with the simulated results very well. The measured results indicate that the proposed antenna can work at the whole 2–13 GHz band with very good end-fire radiation patterns and stable gain performances.


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## Body

## 1. Introduction

In recent years, a wideband and low-profile antenna with an end-fire radiation pattern has attracted much attention because of the high demand in modern wireless communication and radar areas [1–5]. Many high-performance end-fire antennas have been reported, especially the newest designs published in the past several years, like the monocone quasi-Yagi antenna [6, 7], log-periodic slot antenna [8], SIW H-plane ridged antenna [9–11], Vivaldi antenna [12, 13], surface wave antenna [14, 15], and low-profile log-period monopole antenna [16, 17]. Most of those end-fire antennas have realized the ultra-wideband and low-profile features. However, those designs suffer from the complex feeding networks, when the ultra-wide impedance matching bandwidth is needed. Specifically, for the log-period monopoles, each monopole need 180° phase delay and the regular microstrip feeding line will occupy a large space. Recently, researchers proposed a novel slot feeding line which can provide naturally 180° phase delay for each monopole [17]; however, the bandwidth is limited by the microstrip-to-slot transition.In this paper, by optimizing the microstrip-to-slot transition, an ultra-wideband (which can cover the six-octave frequency band) and low-profile log-period monopole end-fire antenna is proposed. Compared with the previously reported end-fire antenna [17], the antenna proposed in this paper indicates a wider impedance matching bandwidth covering from 2 GHz to 13 GHz. And most importantly, the bended monopoles, which strongly reduce the vertical size, are printed on a single dielectric layer with a very simple configuration. The proposed antenna can be easily flush-mounted on the high-speed moving objects for conformal applications.


## 2. Antenna Structure

The geometry of the proposed antenna is shown in Figure1(a). In such a design, 24 monopoles with a log-period rule [16, 17] are used to cover the 2–13 GHz operating frequency band and those monopoles are printed on both sides [18] of a low-loss dielectric layer (Ro 4350, εr=3.66, and tanδ = 0.004) vertically placed over a slot feeding line with wideband microstrip-to-slotline transition. As shown in Figures 1(b) and 1(c), the low-profile is realized by bending some of the long monopoles so that the overall antenna size is obtained as 40 mm (w) × 100 mm (l) × 13.6 mm (h). The used dual-fan-shape microstrip-to-slotline transition structure [19], as shown in Figure 1(d), provides the ultra-wideband balance input for the 24 monopoles. Such microstrip-to-slotline transition structure has a characteristic impedance of 50 Ω, whereas the slotline has a characteristic impedance of approximately 80 Ω. A microstrip stub and slot stub are inserted in the transition structure to improve the impedance matching, as shown in Figure 1(d).Figure 1
(a) Three-dimensional antenna geometry: (b) side view, (c) top view, and (d) bottom view.
(a)
(b)
(c)
(d)


## 3. Design of Wideband Transition

Because we use the slot feeding line technique to provide the naturally 180° phase delay for each monopole, the wideband microstrip-to-slotline transition should be characterized firstly, by numerical simulation and optimizations with the FEM-based commercial software (HFSS). Here, a 40 mm (w) × 100 mm (l) × 0.635 mm Rogers RO 3210 substrate with a dielectric constant εr of 10.2 and a loss tangent of 0.003 is used as the substrate with a ground plane and a slotline. Such high dielectric layer used in this paper can reduce the antenna dimension, and the end-fire performance will not be destroyed. As shown in Figure 2(a), the back-to-back transition configuration is provided to optimize the dimensional size defined in Figure 2(b). Based on the analysis reported in [19], the radii of the fan-shape microstrip stub and slot stub determine the higher side and lower side resonance frequencies, respectively, and the relative positions for the two stubs decide the impedance matching performance.Figure 2
(a) Top and bottom views of the microstrip-to-slotline transition, (b) zoomed in plot for the detail of the transition part with parameter definition, and (c) the numerical simulation results for the wideband transition in the frequency range of 2–13 GHz.After numerical simulation and optimization with HFSS, the dimensional parameters are obtained as follows:a = 8.5 mm, c = 5.288 mm, s = 0.5 mm, t = 0.775 mm, r1 = 3.7 mm, and r2 = 5.7 mm. Based on the optimized structure parameters, the obtained numerical transmission and reflection performances are provided in Figure 2(c). As can be seen, the proposed transition structure can work properly at the whole frequency band from 2 to 13 GHz. The impedance matching performances at lower band (close to 2 GHz) and higher band (close to 13 GHz) are not perfect. This will be solved when the 24 monopole loadings are added in the transition.Moreover, to take deep understanding the slot feeding line which can provide the naturally 180° phase difference, the current distributions at several frequency points are plotted at Figure3. It is obviously shown that the currents on the two sides of the slot at all the frequencies are out of phase. This feature is the main advantage for the slot feeding line to support the log-period monopole end-fire antenna, compared with the regular microstrip feeding line [15, 16]. Next, we use the obtained slot feeding line and the wideband microstrip-to-slotline transition to design the ultra-wideband monopole end-fire antenna.Figure 3
Surface current distributions at the several frequency points for the wideband microstrip-to-slotline transition.


## 4. Design of Low-Profile End-Fire Antenna

Firstly, the 24 monopoles obeying the log-period rule are demonstrated based on the theory [15–17]:
(1)tanθ=HnRn,τ=RnRn+1=HnHn+1=WnWn+1,where θ is the outline angle; τ is the geometric ratio; and Rn, Hn, and Wn are, respectively, the distance from the start point to the position of monopole and the length and width of the monopole. Considering that our end-fire antenna is worked from 2 GHz, the largest monopole has the dimensional size of H24=31.214 mm and W24=4.624 mm. Based on the largest monopole size and the optimized outline angle of θ=23.3 deg and τ=0.865, all the 24 monopoles can be calculated based on (1).The obtained original wideband log-period end-fire antenna is shown in the top plot of Figure4(a) and the corresponding port reflection performance is plotted in the bottom of Figure 4(a). It can be seen that the proposed end-fire antenna has an impedance matching bandwidth (S11<−7dB) starting from 2 GHz to the modelled 13 GHz, and mostly, the S11 is below −10 dB. The simulated far-field E-plane and H-plane radiation patterns at several selected frequencies are concluded in Figure 5(a). It clearly shows very good end-fire radiation performances with low cross-polarizations at all the frequencies obtained. In the E-plane radiation patterns, a 30-degree titling angle is presented, due to the limited ground plane size [6–17].Figure 4
Structure and port reflections of (a) original and (b) low-profile wideband log-period monopole end-fire antennas.
(a)
(b)Figure 5
Simulated far-field E-plane and H-plane radiation patterns at several elected frequencies for (a) original and (b) low-profile wideband log-period monopole end-fire antennas.
(a)
(b)As mentioned in the beginning of this paper, we are aiming to design a low-profile log-period end-fire antenna for the flush-mounted applications. The original end-fire antenna occupies a relatively large space size, especially on the vertical direction. So we bend parts of the longer monopoles to the horizontal plane. This is realized by firstly cutting the longer monopoles (length larger than 10 mm) and then extending them with an additional holding layer with the same dielectric substrate (Ro 4350). So for each monopole, the length and width are not changed and all the monopoles still satisfy the log-period rule. The obtained low-profile log-period end-fire antenna is shown in the top of Figure4(b), and also the corresponding simulated port reflection is shown in the bottom of Figure 4(b). Compared with that of the original antenna, the impedance matching performance of the bended monopole antenna is not deteriorated too much. All the reflection is below −7 dB in the range of 2–13 GHz. Simultaneously, the simulated far-field E-plane and H-plane radiation patterns are parallelly shown in Figure 5(b) compared with the original antenna. Very similar end-fire radiation performances are obtained for all the frequencies. This indicates that the proposed low-profile technique is valid for the log-period end-fire antenna. Based on the numerical optimized design, next, we experimentally demonstrate the proposed end-fire antenna by fabricating the antenna prototype and measuring all the related performances.


## 5. Experimental Demonstrations

To experimentally demonstrate the impedance matching property and the radiation performances for the designed low-profile log-period end-fire antenna, each antenna function layer is firstly fabricated by standard printed circuit board technique. The fabricated antenna components, including the slot feeding line with microstrip-to-slotline transition, 24 monopoles on the dielectric substrate, and the top layer with the bended monopoles, are shown in Figures6(a)–6(c), respectively. Then the assembled antenna prototype with the soldered SMA connector can be found in Figure 6(d). The port reflection for the fabricated antenna is measured by a vector network analyzer (Agilent N5230A), and the result is shown in Figure 7. The corresponding simulated port reflection is also duplicated in Figure 7 for easy comparisons. From Figure 7, it can be seen that the measurement and simulation results are matched to each other very well, especially at the lower frequency region. The slight difference at higher frequency is resulting from the instability of the used substrate RO 3210 and also the fabrication and assembling process. But the impedance matching performance is still acceptable (S11<−7dB) at the whole working bandwidth.Figure 6
(a–c) The fabricated antenna components and (d) the assembled antenna.
(a)
(b)
(c)
(d)Figure 7
Simulated and measured port reflection for the assembled antenna.The far-field radiation patterns are measured in a 25 m × 15 m × 15 m microwave chamber (as shown in Figure8), and the far-field E-plane and H-plane radiation patterns at 3, 6, 9, and 12 GHz are given in Figure 9. The corresponding numerical far-field radiation patterns are duplicated in Figure 9 as well for easy comparisons. It can be seen that a good agreement between simulations and measurements (including the co-polarization and cross-polarization) is obtained as well. The measured results indicate the good end-fire performances with quasi end-fire radiation (E-plane), a 30-deg beam tilting from the ground. As explained before, this beam tilting is because of the finite ground plane used in measurements.Figure 8
The far-field radiation pattern measurement environment.
(a)
(b)Figure 9
Simulated and measured far-field E-plane and H-plane radiation patterns at several elected frequencies for the low-profile wideband log-period monopole end-fire antenna.The simulated and measured real radiation gains for the peak gain direction from 2 to 13 GHz are concluded in Figure10. It shows that, over the whole 2–13 GHz, the measured peak gain is averaged above 6 dBi. This gain performance is more stable than most of the previously reported end-fire antennas [6–17]. Table 1 concludes the 6–18 GHz wideband end-fire antennas reported recently. As can be seen, our design has shown the compared antenna performances.Figure 10
Simulated and measured port reflection for the assembled antenna.Table 1
Units for magnetic properties.
Works
Bandwidth (GHz)
Gain range (dBi)a
Dimensions (mm3)
[6]
6.2–18 (S11<−10dB)
1.5–4.2
150 × 90 × 3.5
[8]
6–18 (S11<−7dB)
4.3–11
90 × 50 × 11.3
[9]
6.6–18 (S11<−7dB)
3–14.2
71.3 × 56 × 3.175
[10]
6.1–19 (S11<−7dB)
4.8–13
78 × 55.8 × 3.175
[11]
6–18 (S11<−7dB)
7–11.5
160 × 86 × 4
[14]
6.1–18 (S11<−10dB)
4–12
80.3 × 29.1 × 3
[15]
3.78–18.54 (S11<−7dB)
0–13
233 × 100 × 8.5
[16]
2–18.3 (S11<−10dB)
7.3–9.2
290 × 100 × 8
[17]
1.5–6.8 (S11<−8dB)
4–10
420 × 240 × 8
This work
2–13 (S11<−7dB)
5–10
100 × 40 × 13.6


## 6. Conclusions

In this paper, an ultra-wideband low-profile log-period end-fire antenna was designed and experimentally demonstrated. The proposed antenna exhibits a measured impedance matching bandwidth (S11<−7dB) of 2–13 GHz, with very good end-fire radiation pattern and gain performances. The low-profile performance makes the designed antenna flush-mounted on the high-speed moving objects for the conformal applications.



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*Source: 7483719-2018-12-23.xml*