Match antenna with very low resistance to 50 Ohm; feasible?

[flanello]

Hello,

I need to match an antenna with a very low resistance (~0.5Ohm) with discrete components using a low pass matching network at 900MHz.
I know how to calculate the matching network, but I'm not sure if this can be done in practice.
Does someone have an advice for such a matching?

 

[FvM]

Did you calculate or actually measure the impedance? In both cases, the result might have a considerable error term.

It's possible in principle, you should consider at least a two stage matching network. As you can easily calculate, inductor ESR will seriously affect matching network efficiency.

 

[Electro nS]

it is not feasable , that why infinitesmall dipoles are theoritical and cannot be used in practice , in addtion the Prad for such antenna is very low.
you can try using the smith chart and see the value of the components (L and C) obtained and that would determine if it can be done or if the values are unrealistic .
in addition the tolerances will affect the results slightly.

cheers

 

[FvM]

"Not feasible" misses the point. Radiation resistance of 0.5 ohm isn't that unrealistic for electrically small antennas used in real life, although efficiency will be indeed poor. The most annoying property of this kind of antennas isn't the small radiation resistance rather than the high Q and respective small bandwith.

0.5 ohms at 900 MHz sounds unusual, though. Presuming a (l/λ)² dependency of Rr, we arrive at cm antenna sizes. Is this your design?

 

[biff44]

Maybe your brain is caught in a rut? Why try to match it to 50 ohms. Why not instead drive it with a high frequency current source?

 

[flanello]

The antenna impedance is obtained by a simulation of the antenna structure.
It's a dipole antenna near a metallic wall.

 

[FvM]

It might be helpful to implement an impedance transformer as part of the antenna structure.

I have presumed so far, that 0.5 Ohm is the real part of the antenna impedance, caused by a very small size relative to wavelength. Is thus true?

 

[flanello]

Yes, the 0.5 Ohm is the real part, and it is caused by a very small size relative to wavelength.
Thanks for the advice with the impedance transformer.

 

[FvM]

If the size is <<λ, distributed impedance transformer might be not feasible. In this case, lumped inductors and capacitors are probably the only realistic option, or a micro size transformer.