Electronic – Patient Connection Radiated Emissions problem

The reason for this is that the most common cause of radiative emissions is due to the power or signal lines acting as antennas. This is because most circuits are small compared to the wavelength of the frequencies that you care about for emissions. For instance, c/(100 MHz) is 3 meters. A trace on a 15 cm PCB will be an inefficient antenna at 100 MHz. However, if you let any noise at 100 MHz leak onto your 5' power cord, it can act as a very effective antenna.

Obviously this reasoning doesn't necessarily apply if you are worried about radiative emissions at 1 GHz or if your PCB is a meter long. It is also possible to use impedance matching networks made from inductors and capacitors that allow a short antenna to radiate effectively, but this tends to not happen by accident.

If you have radiated emissions from the power cord at 30 MHz, you probably also have a conducted emissions problem. I would address both now, not wait until you flunk the conducted emissions test later.

The simple answer is a off the shelf line filter. These can be quite effective, and come with specs as to how much they attenuate common mode and differential mode signals at various frequencies. 30 MHz is "high" in this context, so relatively easy to filter out.

Off the shelf line filters are simple and robust, but can also be large and expensive relative to the components inside. If you don't feel comfortable with filtering emissions and dealing with line power, just get a line filter.

If you want to do this yourself, I'd start with a balun and a capacitor to ground on each of the two line side terminals of the balun. That will mostly knock down common mode signals, but will also attenuate differential signal somewhat at this frequency. Keep the requirements for the capacitors in mind. You want capacitors specifically rated for power line use.