Normally, one would not expect that a resonance is caused by the core tuning the cable because of the resistive part of its common mode impedance. One way this can happen is illustrated in Figure 1 below. We will assume that the equipment enclosures are made of metal and are large enough to be a significant part of a wavelength at the frequencies of interest. For instance, a personal computer enclosure would meet this criteria at 150 MHz. The cable length that connects the two pieces of equipment is one half wavelength at F 2 and therefore is one quarter wavelength at F 1 .
Some background is needed here.
A half wavelength wire can translate a low impedance at one end to a low impedance at the other end whereas a quarter wavelength wire translates a high impedance at one end to a low impedance at the other. A quarter wavelength wire makes a good antenna where the chassis of the equipment to which the wire is fastened becomes the other half of the antenna. This is because the quarter wavelength wire, with its end in free space, presents a low impedance at the equipment end. This allows high frequency currents to flow into the wire and radiate. It only takes on the order of 20 microamps to be a potential radiated emissions problem.