WiFi / GNU Radio / USRP

In the last quarter, I helped some students who worked on a feasibility study that included near field measurements.

To be honest, I have absolutely no idea what’s going on in the near field, but according to Wikipedia some fancy stuff happens.

The students used my WiFi transceiver and experimented with very small distances.

Using a center frequency of 5.62GHz and a VERT 2450 antenna with a height of about 17cm, the reactive near field spans about 18.8cm.

$$ \mbox{reactive} = 0.62 \cdot \sqrt{\dfrac{D^3}{\lambda}} $$

c = 3e8
f = 5.62e9
wavelength = c / f

D = .17
reactive = .62 * (D**3/wavelength)**.5

Out[7]: 0.18809298643667358

In a first experiment they moved the antenna around while watching the constellation plot. It just went crazy when the antennas were in the reactive near field. At first, I thought that we see clipping effects as the receiver overdrives, but we double-checked and asserted that the power level was still fine.

The students also did some quick measurement that showed reasonable results.

That might not be rocket science, but I found it very cool to play with stuff from textbooks in a real experiment in our lab.