Microstrip Antennas Experiments for Digital Television Reception

Victor Santos, Kathia Garcia, and Fernando Lopes Departamento de Engenharia Electrotécnica ISEC /IPC - Instituto Politécnico de Coimbra Rua Pedro Nunes - Quinta da Nora 3030-199 Coimbra, Portugal [email protected]; [email protected]

Abstract-This paper presents three microstrip antennas, Among all the possible structures the most common are: the which were designed specifically for the DTT (Digital Terrestrial microstrip, the stripline and the coplanar waveguide. Television) system. The design of the antennas was firstly carried out analytically, then simulations have been performed using the Microstrip transmission line widespread use in recent years is student version of the Ansoft Designer software tool. Finally, field due especially to its simple construction (using the same tests have been performed using the implemented antennas in procedure as the conventional printed circuit boards) and the different locations around Coimbra. wide range of applications, not only as transmission lines, but also as filters, antennas, couplers, etc. I. INTRODUCTION In Portugal, digital television broadcast began on April 29, 2009, with coverage gradually extended until the end of 2010. Between 2009 and 2012 there is a period of simulcast via analog and digital standards. From 2012 the terrestrial Fig. 2. Microstrip transmission lines: electrical field. television broadcast service will no longer be available via analogue and all emissions will be conducted through the Fig. 2 shows electric field lines in the microstrip structure. digital standard. As the top of the structure is in contact with air, some field TDT (Televisão Digital Terrestre) is received either via a lines close on the ground plane passing through the air and the digital set-top box, or an integrated receiving device in the TV, dielectric. We can thus treat the structure as being immersed in that decodes the signal received via a standard aerial antenna. a homogeneous medium and relative dielectric constant εref. However, due to network and frequency planning issues, an This is a relative dielectric constant between the average antenna capable of receiving a different channel group may be relative dielectric constant of the substrate and the air (unit). In required. Indoor antennas are even more likely to be affected other words, εref is the value of a relative dielectric constant so by these issues and possibly need replacing [1]. that the actual structure immersed in an inhomogeneous In order to overcome this problem, this paper aims to medium has the same behavior as the structure immersed in a design, implement and test antennas for the DTT system. homogeneous medium of dielectric constant given by εref. This Microstrip technology was selected to implement the antennas, concept is illustrated in Fig. 3. taking into account its advantages, namely: light weight and low volume; can be easily adapted to a host surface due to the low planar profile; low fabrication costs; can be easily integrated with microwave integrated circuits (MICs).

II. MICROSTRIP TRANSMISSION LINE Fig. 3. Relative dielectric constant εref representation. A microstrip transmission line is a layered structure with two parallel conductors separated by a thin dielectric substrate For low frequency values, and up to values near 10 GHz, that allow propagation of guided electromagnetic energy. The the value of εref is almost constant and is given by [2] lower conductor acts as a ground plane. The upper conductor 1 is a long narrow strip of width equal to W [2]. The detailed − ε +1 ε −1 ≈ h ’ 2 geometric structure of a microstrip line is shown in Fig 1. ε ≈ r + r ⋅∆1+12⋅ ÷ (1) ref 2 2 « W ◊

where
r stands for the dielectric constant of the substrate, h represents the height of the dielectric substrate, and W refers to the width of the microstrip line. The characteristic impedance of a microstrip transmission line can be calculated depending on the line parameters W and Fig. 1. Microstrip transmission line layout. h, by using the following expression [3]:

60 ≈ 8h W ’ 126 Other width values different from the obtained in (1) can be Z 0 = ⋅ ln∆ + ÷ > selected. However, for smaller widths, the efficiency of the ε « W 4h ◊ ε ref ref (2) radiator is lower, while for larger widths, the efficiency is 120π 126 Z = < higher but due to the occurrence of higher order modes, 0 ≈W ≈W ’’ ∆ ÷ ε ref distortion can occur. ε ref ⋅ ∆ +1.393 + 0.667 ⋅ ln∆ +1.444÷÷ « h « h ◊◊ In order to design a compact microstrip patch antenna, high dielectric constants must be used which are however less efficient and result in narrower bandwidth. Hence, a compromise must be reached between antenna dimensions and III. MICROSTRIP ANTENNAS antenna performance. Microstrip patch antennas are the most common form of The next step is the effective dielectric constant
ref printed antennas. The patch is generally made of conducting evaluation, according to Eq. (1), in order to account for the material such as copper or gold and can take any possible fringing fields around the periphery of the patch. shape (e.g. circle, square, ellipse, ring and rectangle). In this Once the width of the patch, the effective dielectric constant part of the conducted work only the rectangular patch has been of the substrate and ∆l are known, it is possible to calculate the analyzed. The radiating patch and the feed lines are usually actual length L of the patch for a given resonance frequency f0, photo etched on the dielectric substrate. Figure 4 shows the using the following formula [6]: geometry of a microstrip patch antenna.

L= c − 2 ⋅ ∆l (4) 2 ⋅ f0 ⋅ ε ref

In Eq. (4), ∆l is the growth of the actual size of the antenna due to the leakage field, which can be obtained on the basis of other variables through [6]:

≈W ’ (ε ref + 0.3)⋅ ∆ + 0.264÷ « h ◊ ∆l=0.412 ⋅ h ⋅ (5) ≈W ’ (ε ref − 0.258)⋅ ∆ + 0.8÷ Fig. 4. Microstrip patch antenna layout. « h ◊ Microstrip patch antennas radiate mainly because of the For a rectangular microstrip patch antenna, the resonance fringing fields between the patch edge and the ground plane. A frequency for any TM mode is given by [7] thick dielectric substrate having a low dielectric constant is nm desirable for good antenna performance, since this provides 2 2 better radiation efficiency and larger bandwidth [4]. Such a ≈ m ’ ≈ n ’ f = c ⋅ ∆ ÷ + ∆ ÷ (6) configuration leads, however, to a larger antenna size. 0 « ◊ « ◊ 2 ⋅ ε ref L W This type of antenna presents many advantages such us light weight, small volume and low fabrication cost. However, where m and n are modes along L and W, respectively. microstrip patch antennas present also some disadvantages, where the most limiting are: a narrow bandwidth; low As the transmission line model is applicable only to infinite efficiency and gain; radiation from feed and junction and low ground planes, some assumptions are made in order to have a power handling capacity. finite ground plane. However, similar results for finite and

infinite ground plane can be obtained if the size of the ground A. Rectangular Patch Antenna plane is greater than the patch dimensions by approximately Generally, the overall project objective is to achieve certain six times the substrate thickness all around the periphery. In performance characteristics for a given operating frequency the implemented microstrip patch antenna, presented in Fig 5 a based on an appropriate antenna geometry. A rectangular larger ground plane has been considered. patch antenna can be designed using the following procedure. TABLE I Firstly the patch width W is evaluated. For efficient radiation, MICROSTRIP PATCH ANTENAS the width value W is given as a function of the substrate Antenna W L Resonant ε relative permittivity value and resonance frequency [5]: patch shape (mm) (mm) ref freq. (MHz) Rectangular 108.42 84.76 4.273 842.0 1 − ≈ ε +1’ 2 Square 85.0 85.0 4.242 842.74 W= c ⋅∆ r ÷ (3) 2 ⋅ f0 « 2 ◊

The values obtained from previous formulas led to a B. Folded Dipole rectangular patch with the dimensions shown in Table I. The folded dipole antenna is very popular in receiving TV However, for simplicity of construction and usage, a square broadcast signals. It has essentially the same characteristics as patch with 85.0 mm side and a resonant frequency equal to a λ/2 dipole; it has an input impedance four times larger. Its 842.72 MHz was selected. microstrip equivalent is presented in Fig. 7. All microstrip antennas were implemented in a PCB (Printed Circuit Board) with a FR4 epoxy dielectric with a thickness of 60mils (1.524 mm) and relative permittivity equal to 4.4. The ground plane, patch and feed lines are made of cooper 0.5 oz.

Fig. 7. Microstrip folded dipole. The total current supply to the folded dipole is given by

I V V ≈ 2Z 2Z ’ Fig. 5 Implemented square microstrip patch antenna. a ∆ d + t ÷ Iin = It + = + = V ⋅∆ ÷ (8) The final step in the microstrip patch antenna design is the 2 2Zt 4Zd « 4Zt Zd ◊ feeding network selection. The most used feed techniques are the microstrip line, coaxial probe (both contacting schemes), The input impedance of the folded dipole is given by aperture coupling and proximity coupling (both non-contacting schemes). V 2Z ⋅ 4Z 4Z Z Z = = t d = t d (9) Microstrip line feed is done at the center of the square with in Iin 2Zt + 4Z d 2Zd + 2Zt the help of a quarter wave section for proper impedance matching with a 50  system. The input impedance at the Based on Eq. (8), it can be shown that the folded dipole mentioned point is equal to 251 . behaves in the same way as the system shown in Fig. 8. The input impedance of the λ/2 folded dipole is four times that of ZT = Z0 ⋅ Zin = 50⋅ 251.5 = 112.13Ω (7) an isolated dipole λ/2.

The /4 transformer microstrip line has the following size: 51.65 mm length and 0.4835 mm width corresponding to characteristic impedance equal to 112.13  as in Eq. (7).

Fig. 8. Equivalent circuit of the folded dipole consists of two elements The folded dipole λ/2 presents an input impedance near 300. As the coaxial cables are usually 50 or 75, it is necessary to convert the impedance of 300 for the folded Fig. 6. Microstrip /4 transformer and feed. dipole to be adapted to the cable. This is possible using a BALUN (BALance to UNbalance) as shown in Fig. 9. The square patch was the first microstrip antenna to be implemented in this project. The aim was to validate the simulations with measurements made by a microwave network analyzer. The selection of the 50 impedance instead of the 75, typically used in TV distribution systems, was made taking into account the need to perform the earlier mentioned experiments in conjunction with the antenna radiation diagram assessment in an anechoic chamber. Thus, in terms of the Fig. 9. Voltage balun (4:1) build with a ferrite core. practical usage this antenna presents an impedance mismatch being the VSWR equal to 1.5.

The balun must ensure impedance matching between the antenna and the power circuit for the entire bandwidth [2]. In this project we used a 4:1 voltage Balun which consists of two coils connected to one another according to Fig. 9. The current flowing in the bottom coil induces an equal and opposite voltage in the top coil. The primary circuit of the coil contains N turns and the secondary 2N, causing the input impedance being equal to

2 ≈ N ’ 1 Z = Z ⋅∆ ÷ = Z (10) Fig. 11. Microstrip spiral antenna. in' L « 2N ◊ 4 L

IV. RESULTS This section presents the results obtained by simulations and C. Spiral Antenna measurements for the earlier presented microstrip antennas: Spiral antennas, a type of frequency independent antennas, square patch; folded dipole and spiral antenna provide uniform electrical characteristics over a wide The square patch antenna receives a signal power level, at frequency band. A spiral antenna [7] is defined only by the interest frequency in channel 67, around 71.1dBV, at the angular geometric dimensions. Its electromagnetic behavior is third floor of the Electrical Engineering Department building consequently frequency independent. Thus, its impedance, in ISEC campus from the Coimbra (Penedo da Saudade) TDT radiation pattern and polarization remain invariant. This arises transmitter. In the same location the folded dipole antenna from the fact that this antenna presents a high impedance collects 74.1dBV of power and the spiral square antenna a matching in the band and a circular polarization. Such little less power of 66.3 V as presented in Table II. antennas typically have broad radiation patterns and low gain, what is not suitable for many applications. TABLE II Theoretical development of frequency independent antennas MICROSTRIP ANTENAS was carried out initially by V. Rumsey [7] unifying concepts Antenna type Received Power and developing a theory of general application. Rumsey Square patch 71.1 dBV proposed that an antenna that could be defined only by angular Folded dipole 74.1 dBV geometric dimensions (if a scale factor applied to the Spiral square 66.3 dBV geometric shape of its conductive structure is transformed into a structure identical to the original) would have its For the rectangular patch antenna presented in Fig. 1, electromagnetic behavior independent of frequency. simulations have been carried out using Ansoft Designer software in order to evaluate the resonant frequency and the input impedance at the patch border. The obtained resonant frequency value, by the conducted simulations, is approximately 842 MHz, which corresponds to channel 67. The antenna input impedance is about 251. As the coaxial cable has different impedance value, it was necessary to build a λ/4 transformer to convert into 251 in 50. From the results it can be observed that the matching network works properly with a VSWR equal to one (0dB) at the working frequency, as shown in Fig. 12.

Fig. 10. Microstrip spiral antenna.

Fig. 10 shows a simple square spiral antenna printed on a substrate with a thickness h, dielectric constant
r and length L. The spiral arm is composed of multiple filaments, whose lengths are a0, a0, 2a0, 2a0, 3a0, 3a0, (M-1)a0, (M-1)a0, Ma0 and Sa0 [8]. The value 4Ma0 is regarded as the outermost peripheral length of the spiral arm. The antenna is fed by a coaxial cable at the point ‘0’. Fig. 12. Microstrip square patch antenna: VSWR.

As can be seen in Fig. 13, the simulated antenna is actually resonant to 842 MHz, with an input impedance which is real and equal to 50.

Fig. 16. Microstrip spiral antenna: VSWR.

Fig. 13. Microstrip square patch antenna input impedance. One powerful advantage of a folded dipole antenna is that it has a wide bandwidth, near to one octave bandwidth [2, 9]. This is the reason it was often used as a TV antenna for multi channel use as a driving element of Yagi antennas. Fig. 14 illustrates the VSWR values as a function of the frequency in the 830-850 MHz range. As observed, the VSWR is always lower that 0.6 dB what led to the centre of a Smith Fig. 17. Microstrip spiral antenna input impedance. chart normalized to 300 . Using a 4:1 voltage BALUN the typical input impedance value of 75  is expected. V. CONCLUSIONS This paper shows that microstrip antennas can be used in TDT. They showed good performance at frequencies near to 842 MHz (VSWR near to unit and a very high Return Loss) and are quite stable. When compared to conventional antennas, these antennas are smaller, lighter and more adaptable to any surface. For the set of rest locations, the signal level received by these antennas was in the order of 70dBV. This is 25 dB above the minimum level, demonstrating its great potential for reception of TDT signals. Of the three antennas implemented, Fig. 14. Microstrip folded dipole antenna: VSWR. as expected, the folded dipole is the one that is more stable and As observed in Fig. 15, the folded dipole antenna is actually that is capable of receiving higher signal level. resonant at 842 MHz, being the input impedance real and almost constant near to 300 for the considered bandwidth.

VI. REFERENCES [1] ETR 101 190 (V 1.3.1), “Implementation guidelines for DVB terrestrial services”. Section 9, ETSI Technical Report, Oct 2008. [2] Balanis, C.A., Advanced Engineering Electromagnetics, John Wiley & Sons, New York, 1989. [3] D. M. Pozar, Microwave Engineering, 2nd edition, John Wiley & Sons, Inc., 1997. [4] C. A. Balanis (3 ed),” Antenna Theory: Analysis and Design”, John Wiley & Sons, New York, 1997. [5] I. Bahkl and P. Bhartia, “Microstrip Antennas” Artech House Inc, 1982. [6] Hammerstad, E.O., “Equations for Microstrip Circuit Design,” Proc. Fifth European Microwave Conf., pp. 268-272, September 1975. Fig. 15. Microstrip folded dipole antenna input impedance. [7] Rumsey, V. “A solution to the equiangular spiral antenna problem.” IEEE Trans. on Antennas and Propagation, vol. 7, issue 5, pp. 117-117. Both the VSWR graph and the Smith chart obtained by the [8] Hisamatsu Nakano, Jun Eto,Yosuke Okabe e Junji Yamauchi “Tiled- Ansoft simulator showed that at the resonance frequency, the and Axial-Beam Formation by a Single-Arm Rectangular Spiral spiral antenna has a stable behavior being completely adapted, Antenna With Compact Dielectric Substrate and Conducting”, IEEE Transactions on Antennas and Propagation, Jan. 2002. because the curve is near the center of the chart, and the [9] James, J.R. and Hall, P.S., Handbook of Microstrip Antennas, Vols 1 VSWR near to 0dB as seen in Fig 16. and 2, Peter Peregrinus, London, UK, 1989.