pancho_hideboo

> 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 30050j. > 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)^2j*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 30050j?
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 30050j. 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 ? ??? ??? ???
