Actual Wheeler cap measurements

I wanted to determine the antenna efficiency of the printed antennas that I designed. I constructed a Wheeler cap for this purpose and carried out the respective measurements.

The Wheeler cap dimensions are said to be not critical; in my setup, at 2.4GHz, the radian sphere should be ~2cm in radius. So what I did is ensure that the box is never closer than 2cm to the antenna under test.

In my measurements I distinctly see that wire antennas have higher efficiency than printed antennas and that the electrically smaller printed antennas have lower efficiency than the larger ones. So this is all according to expectations.

However, I am struggling with the following: the literature dictates that the S11 measurements with and without Wheeler cap must be "all real" measurements. But when I take a reference point on the Smith chart of the antenna impedance on the real axis and then place the Wheeler cap on, there's always a non-negligible imaginary part of the impedance at this reference frequency point (and this is true for any antenna). So it seems to me that "all real" measurements cannot be taken with the Wheeler cap method (at least not at one frequency point for both with and without Wheeler cap). Should I just take the modules of S11, or use a different Wheeler cap size, or ...?

Any help or user experience with actual Wheeler cap measurements is highly appreciated!

Hi

can you tell me what was the shape of the box you use for your wheeler cap measurements?


regards

The box is roughly 8x8x8 cm3.
I think this should be OK for 2.4GHz, given the fact that my antennas under test are about 3x3cm2 planar models.

hi there

well it seems the size of your metallic box is fine.

also i think you need to measure the input impedance of your antenna without the cap at the resonant frequency. so it must be real

now when you put the wheeler cap , u again need to measure input impedance at the previously mentioned frequency. now it will complex so u need the find out the real part of that impedance and measure your efficiency

i hope my information is helpful to you

regards