Some RFID (radio frequency identification) transmitters operate near 10 MHz and their antennas often look like that shown in Figure 1. In that figure, the RFID transmitting antenna is composed of three turns of printed trace around the perimeter of the PCB. The electronics on the PCB have been blurred as they are not germane to this discussion, except to note that the driving waveform of the RFID transmitter often has a high harmonic content. This harmonic content can be the source of problems if one of the harmonics fall on a self-resonance of the RFID antenna.

In Figure 1, there is a 2.5 cm/1 inch magnetic loop constructed out of US 16 AWG stiff brass wire covered with heat shrink tubing. This loop is being held so that a good portion of it couples into the RFID antenna. By measuring the reflected signal of the loop over a frequency range, driven from the tracking generator of the spectrum analyzer, we can determine the self-resonant frequencies of the RFID antenna. This method is described in my Technical Tidbit: June 2006, Measuring Structural Resonances.

Figure 2 shows the resulting plot from the configuration of Figure 1 from 100 MHz to 500 MHz, well above the ~10 MHz intended frequency of the RFID circuit. The plot is similar that of the reflection coefficient of the wire loop. The smooth waves are a small impedance mismatch somewhere in the system. But the two sharp dips represents energy absorbed from the loop, and therefore not reflected, by the RFID antenna. Note the vertical scale of one db/div. Such an expanded scale is needed as the coupling from the loop into the RFID antenna is not strong so we are looking for small changes in the reflected signal from the loop.


In the plot of Figure 2, one can see that there are two self-resonant frequencies, ~230 MHz and ~470 MHz. In this case, the resonance at ~230 MHz lined up with one of the harmonics of the RFID signal and caused a radiated emissions problem at that frequency. Murphy's Law in action.
For most physical structures, like RFID antennas, components, PCBs, etc., there are self-resonant frequencies that are much higher in frequency than one might think and often much higher than the normal frequency of operation. These self-resonant frequencies can cause problems, such as in this example.