  Forum Pages: 1 rx antenna impedance matching (Read 5135 times)
 monzie New Member Offline Posts: 1 rx antenna impedance matching Oct 06th, 2003, 7:26pm   I am new to rf.I am looking at a LNA IC for a model airplane receiver at 72MHZ.The s11 parameter (ie. input impedance) at 100MHZ for this IC is 300-50j.My antenna is a single wire driven at one end, 26 cm long (1/16 of a wavelength).  A formula I am using gives me a radiation resistance of 3 ohms.  Should I use a matching network to match 3+0j to 300-50j?Is the radiation resistance of the antenna in the ballpark?Thanks for any help. Back to top IP Logged
 pancho_hideboo Senior Fellow      Offline Posts: 1424 Real Homeless Re: rx antenna impedance matching Reply #1 - Oct 24th, 2003, 6:17pm   > I am looking at a LNA IC for a model airplane receiver at 72MHZ. > The s11 parameter (ie. input impedance) at 100MHZ for this IC is 300-50j.> My antenna is a single wire driven at one end,> 26 cm long (1/16 of a wavelength).freq=72MHz --> wavelength/16=lamda/16=26cm> A formula I am using gives me a radiation resistance of 3 ohms. � I'm not sure of your antenna of "a single wire driven at one end".But if you mean very short monopole antenna, its input impedance Zcan be approximated like following. � � �Z=10*(k*length)^2-j*30*Omega/(k*length) � �where k=(2*pi)/lamda, Omega=4.61*log10{(2*length)/radius} So Real[Z]=1.5ohm for length=lamda/16.If you mean dipole antenna with total_length=2*(lamda/16), Real[Z]=3.0ohm.Anyway Imag[Z] is not zero at freq=72MHz unless antenna have any loading coil.Generally an input impedance of antenna Z is expressed like following. � �Z=R+j*X, R=Rr+Rl, X=Xr+Xa � � � Rr ; Radiation resistance of antenna � � � Rl ; Resistance due to loss of antenna � � � Xr ; Reactance due to reactive power in medium around antenna � � � Xa ; Reactance due to antenna structure itselfHere I set very rough assumption instead of you. � Rl=0.0 ; Lossless antenna � Xr=0.0 ; Perfect vacuum around antenna � Xa=0.0 ; Antenna is resonant at 72MHz with loading coil or something. �So antenna input impedance is Z=Rr=3ohm. �> Should I use a matching network to match 3+0j to 300-50j? There are four options for matching.(1) Power matching(=Conjugate matching)(2) No reflection for voltage wave(3) Noise matching(4) Maximum condition for voltage at LNA inputYou mean (2) ? If so you should match 3+0j to 300-50j.If you mean power matching, you should match 3+0j to 300+50j.There seems to be no feeder line between antenna and LNA. �And since your application is low frequency,so I think (4) is preferable than other three.> Is the radiation resistance of the antenna in the ballpark? I can't understand your question.You mean radiation pattern ?If so, horizontal plane or vertical plane ? ??? ??? ??? Back to top « Last Edit: Oct 25th, 2003, 4:13pm by pancho_hideboo » IP Logged Pages: 1