How can i measure the impedence of antenna?

I want to measure the antenna impedence by NA,but it change with the surrounding,how can i get the correct result of antenna impedence?And anyone share me your experiment or document?
Thank you,

This is a common problem. You need to be out doors and have the antenna high enough to be many wavelengths away from all objects including the surface of the earth.

Another source of error to avoid is the transmission line being part of the antenna. You need to have some form of isolation like a transformer or quarter wave sleeve at the antenna.

There is another way. It depends on wavelenght and VNA capability.

Namely: Time Domain (TD).

If your VNA is equipped with TD option, it may act similary as a "radar" , so relatively far reflections may be cut away into TD and then converted once again into Frequecy Domain

u need an anichoic chamber to do the job , this champer full of absorbing materials on the walls , so the antenna will be like in a free space , and connect the VNA to the antena , and measure S11 , then u can get the impedance

khouly

Dear Sergio
Do you mean that with TDR one can distinguish between the external and internal reflections in a VSWR measurement?

Yes, You can.
Genarally what the user want, is to cancel the reflections generated by the room.

Of course the user should knows the electrical lenght of the antenna (from ref.plane to the radiation end).

As example, the predicted lenght is easy to know for an horn antenna, but not easy for log-periodic.

So, how is possible to know the boundry between antenna and free space?

By joining a rough estimation of the antenna lenght and the observation of the TDR trace on the VNA; a sharp reflection almost everytime occour at that boundry.