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rx antenna impedance matching (Read 5127 times)
monzie
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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.
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pancho_hideboo
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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 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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« Last Edit: Oct 25th, 2003, 4:13pm by pancho_hideboo »  
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