High Frequency Electronics High Frequency Design SMAL - PDF document

High Frequency Electronics High Frequency Design SMALL ANTENNAS classic dipole and loop as examples.For more information on electricallysmall patch antennas,readers arereaders areThe Short DipoleFigure 1(a) shows a short dipoleantenna.At 100 MHz,a 1758 ohms,as determinedas determinedThis low resistance and high capaci-tive reactance illustrates that a largeimpedance transformation will berequired to match this antenna to awave resonant dipole.In this case,angular (Figure 1(b)).This currentradiation pattern of Figure 1(c),in aplane containing the antenna wire.efficiency.The maximum gain of 1.77wave dipole’s 2.14 dBi gain.However,story.As will be shown later,thematching system is the primary con-trically small antennas.Figure 2(a) shows a small circularloop,with a diameter of /10.TheTheRr= 31,171 (A/O2)2where Rris radiation resistance,the,with (wavelength) in the sameunits./10 diameter loop,,the radiationresistance is found to be 1.92 ohms.include the resistive loss of the con-ductor (with skin effect),plus theinductance of the loop,which willhave a result in range of 3.0 +ohms.The radiation pattern and gain/10 short dipole.reveal much about its behavior.large reactive component.Of concernis the loss within the matching cir-cuitry.Even with relatively high have significant resistance that con-tributes to system loss.For example,Figure 3(a) shows anideal,lossless matching network toimpedance.Mathematically,this pro-vides a proper match,albeit narrow-However,ideal inductors do notexist.A practical between 50 and 200,depending onthe surrounding environment.For aof 100,each inductor will have a,or 879/100 =8.79 ohms.Since there are two induc-tors,the total additional resistance in17.58 ohms.Ignoring the smaller lossfrom the capacitor,the finite 20log[1.96/(17.58+1.96)] = 21 dB.Figure 3(b)shows a modifiedmatching network that accommo-dates the additional loss.The differ-empirically-derived matching net-work (e.g.determined by trial-and-for losses.The matching process is similarfor the small loop,except that thematching involves a large value of .Since capacitors havemuch higher than inductors,itwould seem that small loop matchingwould have lower losses than anequivalent dipole match.This is gen- = 1.96 –j1758 Max.gain Figure 1 · Short dipole example:(a) dimensions and impedance;(b) current distribution, and (c) D = (r = r= 1.92 Figure 2 · A small loop also has alow radiation resistance. erally true,but note that the loopexample occupies an area much larg-er than the example dipole.A loopsmaller and have a lower value of which will increase matching net-work losses.Mitigating the Loss ProblemMany technical papers andstructure of small antennas in waysand/or implement lower loss match-ing technqiues.Loading—the addi-ments,both lumped and integral tothe antenna,is the most commongroup.Top hats,folded elements,3-dimensional structures,dielectriclength to a small antenna,raising theradiation resistance.included in the link budget calcula-tem gain,transmit power or simplyrange.This may work for some appli-cations,but all these consequencesmance,decreasing performance andshortening battery life.A usefuland matching network can be easyand cheap,but requires the designerto be aware that such an improve-Hopefully,this tutorial raises theawareness of loss and efficiencyissues with small antennas.The nextfuture product designs.References1.D.Miron,,Newnes 2006.2.R.Garg,P.Bhartia,Inder Bahland A.Ittipiboon,,Artech House3.As implemented in EZNEC+ byRoy Lewallen,www.eznec.com.4.J.Kraus,,McGraw-Hill 1950,Ch.6,section 6-8. 1.407 µH Q= 100 Q= 100 50 Figure 3 · Short dipole matching: (a) ideal lossless components, and (b)values required for practical inductors with a