Maximum tolerance for a loop trace-antenna's capacitor
Here a 55mm square loop gives the highest gain using 0.8mm FR4

Reducing area 50% correlates with ~3dB loss each time from this.

The author's calculation of v.f.= 0.82c is an indication of FR4 dielectric effects on wavelength and C values required.

Contrary to what the author says, the shunt capacitance controls a parallel resonant pole and not the series zero.

Here is my simulation.


You can play with the values and aim at peak to get a reading. Note how phase changes abruptly at series resonance.

THe 1 Ohm ground shunt measures current and the Out before the cap measure transfer function while moving out to the 1 Ohm measures Zin.  Q here is about 320 or 42.73dB gain limited by Series ESR and sense R ratio as well as X(f) for a crude approximation... set to 434 MHz.

Here showing the effects of a shunt C with a notch in radiated power by parallel resonance which comes closer to series f with larger C values.