Folded antenna + balun : matching question
I have a question about matching a printed folded dipole.
I am working at 2.45GHz
My antenna is connected to a 50 ohms /50 ohms balun, then connected to a 2-ports VNA, for s11 evaluation.
At the frequency of interest, the impedance is Z; is this the impedance of the antenna seen from the VNA?
Because I need to tune this antenna, to the correct frequency: I can use some discrete components, but I also need to match the antenna to 50 ohms.
Is it mission: impossible ?
Regards
The antenna, if it is a pure folded dipole do it have a rather high impedance, 200-300 Ohm.
It can be impedance transformed with a 1/4 wave transmission line, or by using a 1/2-wave balun.
In the later case is not any extra baluns needed.
Impedance matching and conversion from balanced to unbalanced can also be done with discrete components. It does cost less space but is also a bit more complicated to design.
This balun converts from balanced 200 Ohm to unbalanced 50 Ohm:
A normal folded dipole in free space has an impedance of around 73*4 = 292 Ohms, which with a 4:1 balan will give an SWR of about 73/50=1.5:1. BUT you can improve things if you make one side of larger diameter than the other. IIRC, the side which is not fed needs to be thinner. I suspect you can do the same on a PCB dipole. If you can make it 200 Ohms, then use a 4:1 balun, you have a better match to 50 Ohms.
since it is a narrow band, it is probably possible.