Sdd11 for dipole antenna measurement with VNA (without a balun)
can I use the Sdd11 parameter for measuring the input impedance of a dipole antenna with a VNA without a balun?
I would connect two ports with the two halfs of the dipole. Is this correct?
What is Sdd11? You will still use a single port for the antenna. Easy way to measure the the impedance is to measure S11 in Smith Chart.
You can measure differential S-parameters only with 4-port VNA, you can not measure with 2-port VNA.
Hello,
Regarding the differential S parameter, consult the manual of your VNA. What is the frequency and bandwidth of interest?
If it is relatively narrow band, you can feed your dipole just with a 50 Ohm line and apply two quarter wave stubs at the coaxial screen. Distance between stubs and antenna is 0.25*lambda. Orient the stubs perpendicular to the dipole and the coaxial cable.
You can make the stub of 1 piece of wire of about 0.54lambda. you can solder this directly to the braid.
The stubs provide a low impedance to the shield of the coaxial cable, via the quarter wave formula this transforms to a high common mode impedance at the dipole.
Depending on frequency, there are many other solutions to make the balun function.
Sorry for the absence.
@BigBoss: Why do I have to use a 4 port-VNA? I use only two ports for this task.
@WimRFP: The frequency is 1.5GHz and the bandwidth is 100MHz.
Do you have a picture of your suggestion? Because I'm not sure if I have it correctly understood.
---------- Post added at 14:28 ---------- Previous post was at 14:16 ----------
@sv1437: S11dd is the differential S11-parameter of the 4-Port Mixed-Mode S-Parameters:
4-Port Mixed-Mode S-Parameters
Can I measure a dipole with this parameter in order to avoid a balun?
Hello,
The two 0.27lambda wires or strips (connected to the braid) provide a low impedance for the common mode current. This converts to a high common mode impedance at the feedpoint of the dipole (quarter wave transformer).
With your two port VNA, you can optimize the position and length of the strip for optimum performance. Note that the strip is perpendicular to the E-field plane of the dipole to avoid interference.
Use small diameter coaxial cable for the run between the strip and the dipole. Your 100 MHz at 1.5 GHz is relatively narrow band and therefore no problem for this type of 1:1 balun