DESIGN FEATURE Microstrip Lines Reviewing The Basics Of Microstrip An understanding of the fundamentals of Lines microstrip transmission lines can guide high- frequency designers in the proper application of this venerable circuit technology. Leo G. Maloratsky RINTED transmission lines are widely used, and for good reason. Principal Engineer They are broadband in frequency. They provide circuits that are Rockwell Collins, 2100 West Hibiscus Pcompact and light in weight. They are generally economical to pro- Blvd., Melbourne, FL 32901; (407) duce since they are readily adaptable to hybrid and monolithic inte- 953-1729, e-mail: lgmalora@ grated-circuit (IC) fabrication technologies at RF and microwave fre- mbnotes.collins.rockwell.com. quencies. To better appreciate printed transmission lines, and microstrip in particular, some of the basic principles of microstrip lines will be reviewed here. A number of different transmis- with respect to the others. In Fig. 1, sion lines are generally used for it should be noted that the substrate microwave ICs (MICs) as shown in materials are denoted by the dotted Fig. 1. Each type has its advantages areas and the conductors are indicat- ed by the bold lines. The microstrip line is a transmis- Basic lines Modifications sion-line geometry with a single con- W W t a ductor trace on one side of a dielectric H hhhW W substrate and a single ground plane line a h Suspended Inverted on the opposite side. Since it is an Microstrip Microstrip line Shielded microstrip line microstrip line microstrip line open structure, microstrip line has a t W W1 major fabrication advantage over b b t stripline. It also features ease of a W Stripline 2 Stripline Double-conductor stripline interconnections and adjustments. In a microstrip line, the wave- W W W t b h t b length, L, is given by: d aaa Λ=λε 0.5 stripline Shielded suspended / (eff ) (1) Suspended Shielded high-Q Shielded suspended stripline suspended stripline double-substrate stripline where: t W ttS W e h h h eff = the effective dielectric con- aaa stant, which depends on the dielec- Slotline Slotline Antipodal slotline Bilateral finline tric constant of the substrate materi- S a S W H W b al and the physical dimensions of the b h microstrip line, and Coplanar Symmetrical l waveguide = the free-space wavelength. coplanar line Shielded coplanar waveguide In a microstrip line, the electro- hhhh magnetic (EM) fields exist partly in WWWbbb W b the air above the dielectric substrate aaaa and partly within the substrate itself. Finline Antipodal finline Antipodal overlapping Finline Bilateral slotline finline Intuitively, the effective dielectric constant of the line is expected to be 1. These are commonly used types of printed transmission lines for MICs. greater than the dielectric constant MICROWAVES & RF ■ MARCH 2000 79 DESIGN FEATURE Microstrip Lines of air (1) and less open to the air and, in reality, it is than that of the desirable to have circuits that are 4.0 e = 20 dielectric sub- covered to protect them from the 1 e strate. Various environment as well as to prevent = 15 curves for effec- radiation and EM interference e = 12 tive dielectric con- (EMI). Also, the microstrip configu- 3.0 e = 10 stant are shown in rations that have been so far dis- 0.5 e ) = 8 Fig. 2 as a func- cussed are transversally infinite in eff e = 6 e tion of physical extent, which deviates from reality. ( e = 5 dimensions and Covering the basic microstrip config- 2.0 e = 4 e = 3 relative dielectric uration with metal top plates on the e = 2 constant. top and on the sides leads to a more Referring again realistic circuit configuration, a e 1.0 = 1.5 to Fig. 1, it should shielded microstrip line with a hous- 0.1 0.2 0.4 0.6 0.8 1.0 2.0 4.0 be apparent that a ing (Fig. 1). W/h basic (unshielded) The main purposes of the housing 2. The values of effective dielectric constant are shown for microstrip line is or package are to provide mechanical different substrate relative dielectric constants as a not really a practi- strength, EM shielding, germetiza- function of W/h. cal structure. It is tion, and heat sinking in the case of high-power applications. Packaging A comparison of various transmission-line types must protect the circuitry from mois- ture, humidity, dust, salt spray, and Transmission Impedance Chip other environmental contaminants. line Q factor Radiaton Dispersion range mounting In order to protect the circuit, certain methods of sealing can be used: con- Microstrip ductive epoxy, solder, gasket materi- (dielectric) 250 Low Low 20 to 120 Difficult for als, and metallization tape. (GaAs, Si) 100 to 150 High shunt, easy An MIC mounted into a housing for series may be looked on as a dielectrically loaded cavity resonator (Fig. 3, left) Stripline 400 Low None 35 to 250 Poor with the following inner dimensions: a is the width, l is the length, and H is Suspended stripline 500 Low None 40 to 150 Fair the height of the enclosure. These Slotline 100 Medium High 60 to 200 Easy for dimensions should be selected in a shunt, diffi- way so that the waveguide modes are cult for below cutoff. series The parasitic modes appear in this resonator if: Coplanar 150 Medium Low 20 to 250 Easy for =−ε − waveguide series and H {h[1 (1 / )]R}1(R 1) (2) shunt where: =+λ 22 2 Finline 500 None Low 10 to 400 Fair R (0 // 2) [(Ml ) (N / a) ] (2a) 5.0 H H 4.0 h a 3.0 a = 24 mm 2:2 1:2 l = 30 mm 2.0 h = 0.5 mm Height (H)—mm h l 2:1 M = 1, N = 1 1.0 a 0.5 15 25 35 45 55 Wavelength (l)—mm 3. Housing dimensions are selected for microstrip circuits (left) to minimize losses. The effects of unfavorable housing height versus wavelength and different parasitic modes is shown (right). MICROWAVES & RF ■ MARCH 2000 80 DESIGN FEATURE Microstrip Lines and M and N = positive integers. 2.6 W/h = 2 From eq. 2, it is possible to obtain e = 9.6 W/h = 1 the condition of absence of parasitic 2.5 W/h = 0.6 W/h = 0.5 modes: W/h = 0.4 2.4 W/h = 0.3 R – 1 < 0 ; R < 1 W/h = 0.1, 0.2 2.3 or λ222<+ 0 4/[(M /l ) (N / a) ] (3) 1.3 W/h = 2 e = 2.0 or Effective dielectric constant W/h = 1 λ <+2/)[(M /l )22 (N / a) ]05. (4) W/h = 0.6 0 W/h = 0.3, 0.4, 0.5 1.2 W/h = 0.2 Equation 4 is known as the condi- W/h = 0.1 tion for wave propagation in a 123456789 (H – h)/h waveguide with dimensions l 2 a. In the case of this article, it can also be 4. The effective dielectric constant is shown as a function of the relative considered the condition for the dielectric constant and physical dimensions for a shielded microstrip line. absence of parasitic modes in a waveguide of cross-section a 2 H or assumed that the side walls are suffi- ious geometries and substrates of dif- l 2 H. If eq. 4 is not satisfied, para- ciently spaced so that they only see ferent relative dielectric constants sitic modes can arise, and the height weak fringing fields and, therefore, while Fig. 6 illustrates the relation- H must be chosen to suppress these have a negligible effect on the effec- ships between characteristic im- modes. Figure 3 (right) illustrates tive dielectric constant. The top pedance and the physical dimensions the resulting graphs of unfavorable cover tends to lower the effective of shielded microstrip lines for two l H versus 0 for housing dimensions dielectric constant (which is consis- examples: substrates with low (2) and of a = 24 mm, l = 30 mm, and dielec- tent with intuition). The top wall high (9.6) relative dielectric con- tric substrate with a dielectric con- enables electric fields in the air above stants.2 The top cover tends to reduce stant of 9.8 and THK of 0.5 mm. the strip conductor thereby giving the impedance. When the ratio of the The top and side covers essentially the air more influence in determining distance from the top cover to the redistribute the field of the more the- the propagation characteristics. dielectric substrate and the substrate oretical microstrip and understand- The characteristic impedance of a thickness [(H – h)/h] is greater than ably have an influence on the effec- microstrip line may be approximate- 10, the enclosure effects can be con- tive dielectric constant. ly calculated by assuming that the sidered negligible. The characteristic Figure 4 shows the relationship EM field in the line has a quasi trans- impedance range of a microstrip line is between the effective dielectric con- verse-EM (TEM) nature. The char- 20 to 120 V. The upper limit is set by stant and the physical dimensions of acteristic impedance of a microstrip production tolerances while the lower the shielded microstrip line for dif- line can be calculated using the limit is set by the appearance of high- ferent values of the relative dielec- Wheeler equations.3,4 er-order modes. tric constant of the substrate materi- Figure 5 shows the characteristic There are three types of losses al.2 In these curves, it has been impedance of microstrip lines for var- that occur in microstrip lines: con- 140 e = 5 V V 110 e = 7 100 e = 10 80 e = 16 e = 1 50 e = 2 Characteristic impedance— Characteristic impedance— e = 4 20 10 0.1 0.2 0.3 0.4 0.5 0.6 1.0 0.1 1.0 10.0 W/h W/h (a) (b) 5.