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https://designers-guide.org/forum/YaBB.pl Design >> RF Design >> rx antenna impedance matching https://designers-guide.org/forum/YaBB.pl?num=1065493598 Message started by monzie on Oct 6^{th}, 2003, 7:26pm |
Title: rx antenna impedance matching Post by monzie on Oct 6^{th}, 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. |
Title: Re: rx antenna impedance matching Post by pancho_hideboo on Oct 24^{th}, 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 Z can 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 itself Here 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 input You 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 ? ??? ??? ??? |
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