A low-cost mechanism to reconfigure the operating frequency band of a Vivaldi antenna for cognitive radio and spectrum monitoring applications

A frequency agile Vivaldi antenna whose operating frequency band can be switched between two selected bands is proposed in this paper for spectrum monitoring and cognitive radio applications. A RF switch is introduced into the back-slot of a Vivaldi antenna to allow switching of the operational band. The realised gains of the antenna are 10.5 dBi in the low band around 3.1 GHz, and 12 dBi in high band around 4.1 GHz. The radiation pattern is stable and its direction is consistent across the two bands. This design can be applied to multiple reconfigurable bands by using more RF switches to tune the desired operating frequency. A set of reliable design equations has been provided as well. This reconfigurable antenna offers improved gain and isolation over multiple, wideband and multiband antennas without increasing the cost and size when compared to those designs reported.


Introduction
Spectrum monitoring applications require antennas with wide bandwidth characteristic [1].Multi-resonant antennas are not suitable because they operate in different modes and produce different radiation patterns, thus, the main lobe may not point at the desirable direction when tuning the band.
Wideband spectrum sensing is critical for efficiently finding spectral opportunities in cognitive radio as well [2].Therefore, a wideband antenna that maintains a constant radiation pattern across its bands is required [3].
Not many antennas can achieve very wideband operation while maintaining high gain, high isolation between bands and a consistent radiation pattern.Several antennas have been proposed in [4][5][6] but they sacrifice the gain for tuning the operating frequency.Other existing designs suffer from variations in the radiation pattern, particularly the direction of the main lobe [7].High isolation between frequency bands is also required to reject signals from adjacent bands therefore achieving higher signal-tonoise ratio and jamming resistance [8].Although some antennas have been reported for wideband tuning and high gain, they do not provide high rejection for close bands [6,9,10].Also, other designs provide isolation, but they compromise the high gain as reported in [11][12][13][14][15][16].
To date, the reports of such antennas that can meet all the requirements for spectrum monitoring and cognitive radio in the literature are rare; therefore, a novel design is needed.In this study, a simple and effective wideband frequency reconfigurable antenna with a consistent radiation pattern, high gain and high isolation is proposed.A Vivaldi antenna is utilised as the base for the proposed reconfigurable antenna because of its consistent radiation characteristics in its whole wide frequency band [17].
To avoid the gain deterioration, this study proposes a microstrip line fed reconfigurable Vivaldi antenna which is optimised for the frequencies of interest and can reject the undesirable frequency bands.By controlling the resonant length of the back-slot of the Vivaldi antenna by the mean of a radiofrequency (RF) switch and the length of the open-circuited stub of the microstrip feed line, the undesirable frequency band can be rejected while a preferred band can be selected.As the RF switch determines the length of the signal path for the operating frequency band, therefore the desirable high-frequency band can be simply adjusted by selecting the proper location of the RF switch based on the design equations provided in Section 3.

Antenna geometry and design
The proposed antenna is designed on a single microwave substrate as shown in Fig. 1.The dielectric substrate used is Taconic RF-43 with ε r = 4.3 and thickness 0.762 mm.The top layer presents an open-circuited microstrip line.The stub-to-slot intersection is formed by a quarter guided wavelength λ 0 /(4 ε eff ) microstrip stub and a three-quarter free-space wavelength (3λ 0 /4) back-slot at the low-frequency band.The intersection is highlighted in red in the top layer of Fig. 1.For maximum coupling, the stub length (L stub ) of the microstrip feedline should be a quarter wavelength or odd multiple of a quarter wavelength of the guided wavelength ((2n + 1)λ g /4 where n ∈ ℕ) and the back-slot length should be a quarter wavelength or odd multiple of three-quarter free space wavelength ((2n + 1)λ 0 /4 where n ∈ ℕ) at the operating frequency band.Any other operating frequency that does not meet either condition will be rejected.Three-quarter wavelength at the lowfrequency band is chosen for the back-slot, so enough space is available for tuning to a higher frequency band.
A RF switch is introduced on the back-slot to switch to a highfrequency band.The RF switch controls the signal path while setting maximum coupling in the stub-to-slot intersection to produce a higher operating band.To operate in the lower band (switch is OFF), the switch enables the signal to use the full length of the back-slot.In the high band (switch is ON), the length of the slot is shorter, therefore it only excites the desirable higher frequency band.Outside the operating frequency band, either the length of the open-circuited microstrip line or the back-slot is not optimum, therefore the signal is rejected.Particularly, when both conditions are not met, it provides high isolation.
To support the monitoring of those common wireless systems, a Vivaldi antenna is designed to operate from 1 to 6 GHz following the equations in [18].A low-insertion-loss RF switch (Hittite HMC550AE SPST [19]) is used as it provides adequate RF performances from direct current (DC) to 6 GHz.

Technical analysis
To support the research community, a set of reliable empirical equations is presented to calculate the antenna dimensions and switch positions.The method also calculates the rejected frequencies with at least 14 dB isolation for each band with a minimum accuracy of 96.6% when compared with a commercial full-wave electromagnetic simulation.To extract these equations both transmission line theory and empirical experience are applied.
The design parameter equations for the passbands are presented in ( 1)-( 4), in reference to those shown in Fig. 1.This method assumes a basic Vivaldi antenna is already designed and it covers the whole operating frequency bands of interest.
λ LB represents the wavelength of the desired operating frequency in the low band and λ HB represents the wavelength of the desired operating frequency in the high band.For the stub length (L stub ) and the stub position (P stub ) a trade-off is made between the two frequency bands by using the wavelength of the middle frequency between the low band and the high band, which is defined as Stub length(L stub ) = 0.79 Stub position(P stub ) = 0.76 Switch position(P switch ) = 0.75 3λ HB 4 . ( Maximum transmission for the passband can be achieved when the slot length (L slot ) is about three-quarters of the free-space wavelength, the stub length (L stub ) is a quarter of the guided wavelength and the stub position (P stub ) is a quarter of the average free-space wavelength at the desired operating frequency bands.
Based on the results from the intensive simulations, correction factors, between 0.75 and 0.8, are introduced in ( 1)-( 4) to improve the accuracy of the equations.A RF switch is introduced in the back-slot at the position calculated using (4) to evaluate the P switch as presented in Fig. 1.
The wavelengths of the rejected frequencies (λ rf1 , λ rf2 , λ rf3 , λ rf4 ) can be defined depending on the slot length (L slot ) and stub length (L stub ) for each design as well.When the slot length is equal to a half-wavelength (see Fig. 2a), a full-wavelength (see Fig. 2b) or multiples of these (nλ 0 /2) (see Fig. 2c) for a particular frequency the resulting gain of the Vivaldi antenna in the direction of interest drops, as the signal is resonating in the slot.These are presented in ( 5) and ( 6).The stub length also determines rejected frequencies, as shown in (7) and (8).This occurs when the stub is a multiple of the half guided wavelength (nλ g /2) (see Fig. 2d) Several models have been designed by using ( 1)-( 8) and simulated using CST Microwave Studio [20].The measured Sparameters of the RF switch are embedded in the simulation to obtain more accurate results in all the four bands.Fig. 3 presents the simulated results for four frequency bands: 2, 3, 4 and 5 GHz.The antenna design parameters were first evaluated by using the eight equations presented and then were further optimised by the commercial software.In the simulation of all the four frequency bands, the 2 GHz band showed the most deviation, therefore to keep the clarity of the figures, The 2 GHz band calculated by the formulae with no optimisation, which is shown in dashed lines, is selected for comparison.The yellow arrow in Fig. 3b marks an isolation of 15 dB to the adjacent band with a difference of 0.3 dB between the 2 GHz band by formulae (in pink) and the 2 GHz band optimised (in red).When comparing the calculated results with the results from the commercial software, they presented a 96.6% accuracy calculated from the relative standard deviation (RSD) for the design parameters presented in the equations as shown in Fig. 4. When evaluating the accuracy of one parameter, the other parameters were kept unchanged.To further verify the accuracy of the design, a prototype based on (1)-( 8) is fabricated.The design parameters and their values are indicated in Table 1.

Fig. 3 Simulated results for 4 frequency bands calculated using equations (1)-(8) and, then, optimized. As a comparison the 2 GHz band calculated using the formulas with no optimization (in pink dashed lines) is presented as well
For the switch to operate properly, a cut is required to separate the Vivaldi antenna into two halves for proper DC biasing of the switch.The width of the cut is 0.2 mm and the length is a quarterwavelength at the low-frequency band, to transform an open circuit into a short circuit at the end of the back-slot.
Other switch requirements are: a control voltage between 0.0 and 5.0 V and a 100 pF DC blocking capacitor, which sets the lower cut-off frequency of the RF switch to 1 GHz, connected to each RF port of the switch.

Analysis of surface current distributions
The current distributions are presented for the low-and highfrequency bands in Fig. 5. In-band operation occurs when the stub and slot are matched.The signal is propagated to the exponential flare of the antenna and radiated, as shown in Figs.5a and d.The Vivaldi antenna is operating as a travelling-wave antenna and radiating.In out-band operation, the signal is resonating and stays in the slot instead and, thus the antenna is not radiating as shown in Figs.5b and c.In these cases, the stub and back-slot are not matched causing the Vivaldi antenna to reject the undesirable band.

Simulation and measurement results
Fig. 6a shows the bottom layer with the exponential flare of the two-band reconfigurable Vivaldi antenna and Fig. 6b shows the top layer with the microstrip feed line and the soldered RF switch of the prototype.
The RF switch is soldered in the top layer but connected to the bottom layer through vias, as the usable space on the bottom layer is limited.This RF switch requires two pins connected to ground, one pin connected to positive supply voltage (Vdd), one pin connected to the controlling voltage and two pins connected to the RF signals to be switched.The footprint of the RF switch does not fit inside the back-slot of the bottom layer and would otherwise overlap and disturb the RF signals propagating in the back-slot.Thus, the RF switch is soldered in the top layer and the RF pins connected through vias to the bottom layer.
The small board on the top-left corner is connected to the RF switch in the top layer to switch the operating band of the antenna.It can be connected to a microcontroller that switches the bands automatically to suit cognitive radio applications, for example.
Table 2 presents the specifications of a selection of the most significant antenna designs compared with the design proposed in this study in terms of antenna requirements.'Bands' indicate the whole frequencies the antenna can operate at.To achieve a greater bandwidth most of the designs sacrifice the gain and isolation between bands [11,12].Although a half-Vivaldi with a tuneable stop band can achieve an isolation of up to 30 dB in some bands [9], the gain provided by the antenna is only 2 to 7 dBi, which makes it not suitable for high-band applications such as spectrum monitoring and cognitive radio.Most of the proposed designs present the gain with high fluctuations [6,9,12].
Fig. 7a shows a comparison between measured (solid lines) and simulated (dashed lines) reflection coefficients for the two-band reconfigurable Vivaldi antenna.The measured gain is compared with the simulated gain in Fig. 7b.Measured results are in good agreement with simulated results.The differences can be caused by the poor switch isolation between ON and OFF states, as the measured RF switch isolation between ON and OFF states is only 9 dB in the high-frequency band.Table 3 summarises the results measured for the two bands.
Fig. 8 compares the measured and simulated radiation patterns for the E-plane and H-plane of the low band and the high band.In both frequency bands, measured results agree with simulated results.

Conclusion
A two-band reconfigurable Vivaldi antenna that provides a consistent radiation pattern, high gain and high isolation is proposed for spectrum monitoring and cognitive radio applications.The antenna can switch between two frequency bands by using a low-insertion-loss RF switch.A prototype of the proposed antenna is fabricated and verified in an anechoic chamber achieving good agreement with the simulation results.

Fig. 1 Fig. 2
Fig. 1 Bottom layer (left) and top layer (right) of the two-band reconfigurable Vivaldi antenna geometry

Fig. 4
Fig. 4 RSD calculated from the simulated results for the design parameters

Fig. 7
Fig. 7 Measured (solid lines) and simulated (dashed lines) (a) Reflection coefficient, (b) Gain for the two-band reconfigurable Vivaldi antenna

Fig. 8
Fig. 8 Measured (red solid lines) and simulated (blue dashed lines) (a) E-plane, (b) H-plane radiation pattern for the low band, (c) E-plane, (d) H-plane for the high band

Table 1 Design
parameters for the two-band Vivaldi

Table 3
Measured results for the two-band Vivaldi antenna