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SNR Enhancement of an Electrically Small Antenna Using a Non-Foster Matching Circuit

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SNR Enhancement of an Electrically Small Antenna Using a Non-Foster Matching Circuit applied sciences Article SNR Enhancement of an Electrically Small Antenna Using a Non-Foster Matching Circuit Yong-Hyeok Lee 1 , Sung-yong Cho 2 and Jae-Young Chung 1,* 1 Department of Electrical and Information Engineering; SeoulTech, Seoul 01811, Korea; [email protected] 2 Department of Manufacturing Systems and Design Engineering; SeoulTech, Seoul 01811, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-2-970-6445 Received: 2 June 2020; Accepted: 24 June 2020; Published: 28 June 2020 Featured Application: Miniaturized broadband antennas for wireless communication devices Abstract: A non-Foster circuit (NFC) is known as an active broadband matching technique to improve the impedance matching bandwidth of an electrically small antenna (ESA). There has been a vast amount of papers that report the generation of negative impedance using an NFC and its effectiveness on broadband antenna matching. However, only a few discussed its impact on the signal-to-noise-ratio (SNR), which is one of the most important figures-of-merit for a wireless communication system. In this paper, the SNR enhancement due to an NFC was measured and discussed. An NFC was carefully designed to have a low dissipation loss and to meet the stability conditions. The optimized NFC design was fabricated and applied to an ESA length of λ⁄15 at a frequency range of 150 to 300 MHz. The measured results showed that the NFC enhanced the received power of the antenna system by more than 17 dB. However, due to the noise added by the NFC, the SNR enhancement was not guaranteed for some frequency points. Nevertheless, an average of 7.3 dB of SNR improvement over the frequency band of interest is possible based on the experiment result. Keywords: broadband impedance matching; electrically small antenna; non-Foster circuit; UHF; VHF 1. Introduction The demand for miniaturized communication systems has tremendously increased for many applications such as mobile devices, medical wearables, military radios and vehicles, etc. One of the key components that prohibit the miniaturization is an antenna because of its physical length (or area) requirement, depending on the frequency. For example, the ideal length of a quarter-wavelength antenna operating at 300 MHz is 25 cm. No matter how small the design of the circuit or chassis is, the size of the whole communication system is big due to the antenna. For the last decade, a vast amount of research has been conducted on the so-called electrically small antenna (ESA), which is defined by an antenna whose maximum dimension is less than λ/2π [1,2]. Although an ESA presents a small footprint, it usually accompanies low radiation resistance and high reactance, which makes it difficult to match to the system impedance (e.g., 50 W). To promote the impedance matching, often passive components, such as inductors (L) and capacitors (C), are used to remove the reactance at a designated frequency band. However, the matching bandwidth is extremely narrow, since the passive L, C, and the antenna itself follow the Foster’s reactance theorem [3]. The latter asserts that the reactance must increase as the frequency increases. Figure1a shows an example of adjusting the reactance of a capacitive ESA using an inductor. Both the C and L increase as the frequency increases, following Foster’s reactance theorem. The ESA’s capacitance is completely cancelled by the inductance at a single frequency, implying the matching Appl. Sci. 2020, 10, 4464; doi:10.3390/app10134464 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 10 Figure 1a shows an example of adjusting the reactance of a capacitive ESA using an inductor. Both the C and L increase as the frequency increases, following Foster’s reactance theorem. The Appl. Sci. 2020, 10, 4464 2 of 9 ESA’s capacitance is completely cancelled by the inductance at a single frequency, implying the matching bandwidth is narrow. To overcome the bandwidth limitation of passive impedance matching,bandwidth a is non-Foster narrow. To circuit overcome (NFC) the matching bandwidth technique limitation employing of passive active impedance components matching, (e.g. a transistor)non-Foster has circuit been (NFC) introduced matching [4,5]. technique The operation employing principle active componentsof an NFC is (e.g., simple, transistor) as depicted has been in Figureintroduced 1b. It [ 4generates,5]. The operation a negative principle C (or L), of which an NFC is against is simple, the asFoster’s depicted reactance in Figure theorem,1b. It generates and thus a broad negative reactance C (or L),cancellation which is againstbandwidth the can Foster’s be ac reactancehieved. Various theorem, non-Foster and thus circuit a broad designs reactance have beencancellation reported bandwidth for the canbroadband be achieved. matching Various of non-Foster small antennas circuit designs[6–11]. haveHowever, been reportedmost of forthem the presentedbroadband only matching the enhancement of small antennas in the [6 antenna’s–11]. However, reflection most coefficient of them presented ( ) after only applying the enhancement the NFC, whichin the antenna’sis not sufficient reflection to coevalidatefficient the (S 11system-w) after applyingide improvement. the NFC, which It is isimportant not suffi cientto examine to validate the signal-to-noisethe system-wide ratio improvement. (SNR) since It the is important active co tomponents examine employed the signal-to-noise in the NFC ratio might (SNR) introduce since the additionalactive components noise into employed the system in the[12,13]. NFC might introduce additional noise into the system [12,13]. (a) (b) Figure 1. AntennaAntenna reactance reactance cancellation using using ( a)) passive components which follow the Foster’s reactance theorem. ( (b)) Negative Negative reactance generated by the no non-Fostern-Foster circuit matching technique. In this paper, an NFC design with improved matching bandwidth and and stability stability is is presented. presented. Compared toto the the previously previously designed designed NFC [NFC6], this [6], work this has work a much has smaller a much footprint smaller by shorteningfootprint theby shorteningmicrostrip line,the andmicrostrip thus the line, overall and lossthus and the parasitic overall elossffect and originating parasitic from effect the originating NFC can be from reduced. the NFCMore can importantly, be reduced. not onlyMore the importantly, impedance enhancement, not only the butimpedance the received enhancement, power andSNR but enhancementthe received powerafter applying and SNR the enhancement NFC, is presented after andapplying discussed. the NF TheC, NFCis presented was carefully and discussed. designed, focusingThe NFC on was the carefullyreduction designed, of internal focusing loss, resulting on the inreduction an average of internal of 17.3 dBloss, of resulting received in power an average improvement of 17.3 indB the of receivedbandwidth power of 150 improvement to 300 MHz. Toin improvethe bandwidth the stability of 150 of to the 300 NFC, MHz. a two-step To improve validation the stability check strategy of the NFC,of observing a two-step the stability validation condition check instrategy both frequency of observing and time the domainsstability wascondition applied. in Theboth NFC frequency design andis described time domains in detail was in applied. Section2 The. The NFC noise design power is described added by thein detail implemented in Section NFC 2. The was noise measured power addedand the by overall the implemented SNR enhancement NFC was was measured calculated. an Thed the results overall show SNR that enhancement the proposed was NFC calculated. provides Thean average results ofshow 7.3 dB that of enhancementthe proposed ofNFC SNR provides despite ofan the average introduced of 7.3 noise. dB of These enhancement measured of results SNR despiteare discussed of the inintroduced Section3. noise. These measured results are discussed in Section 3. 2. Non-Foster Non-Foster Circuit Circuit Design Design This section first first introduces the ESA used in this study. This is important since the NFC design strongly depends on the impedance of the ESA conne connectedcted with it. Next, the proposed NFC design is discussed together with the stability validationvalidation methods and their simulation results. 2.1. Characteristics of Electrically Small Antenna 2.1. Characteristics of Electrically Small Antenna A short monopole was selected as the load. Figure2 shows its measured resistance and reactance A short monopole was selected as the load. Figure 2 shows its measured resistance and together with a picture of the monopole. The length of the monopole is 8.4 cm, corresponding to the reactance together with a picture of the monopole. The length of the monopole is 8.4 cm, quarter-wavelength of 893 MHz. Thus, it can be considered as an ESA at the frequency of interest corresponding to the quarter-wavelength of 893 MHz. Thus, it can be considered as an ESA at the (i.e., 150–300 MHz). The measured resistance and reactance in Figure2a,b, respectively, clearly display impedance characteristics of an ESA: the resistance is low and reactance is high. One can easily notice Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 10 frequency of interest (i.e., 150–300 MHz). The measured resistance and reactance in Figure 2a,b, respectively, clearly display impedance characteristics of an ESA: the resistance is low and reactance is high. One can easily notice that the reactance behavior is similar to Figure 1a: the Appl.negative Sci. 2020 reactance, 10, 4464 reduces as the frequency increases in a rational manner. This means that as3 of the 9 frequency increases, the negative value of the capacitive reactance decreases (i.e., ). The purpose of the NFC is to generate a negative capacitance complimentary to the antenna’s thatpositive the reactance capacitance, behavior and istherefore, similar tothe Figure input1 a:re theactance negative can reactancebe 0 over reducesa wide asbandwidth.
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