Ⅰ. What is the meaning of near and far?

The first point to keep in mind is that any electromagnetic wave has impedance, which is known as wave impedance. The wave impedance Zw is equal to the ratio of the wave's electric (V/m) and magnetic (A/m) fields, hence Zw=E/H. Note that V/m divided by A/m equals ohms.

The impedance of the wave is a Zw wave of 250 ohms if the measured signal is 10V/m in an electric field and 0.04A/m in a magnetic field.

High impedance waves have a high E value and a low H value, while low impedance waves have a low E value and a high H value  (note the equivalent point when using the circuit theory method: circuits with high voltage and low current are high impedance circuits, A low voltage and the high current circuit is a low impedance circuit).

Another thing to consider is that electric and magnetic fields diminish as you get further away from the source. As a result, upon traveling away from the circuit in Figure 1, both the electric (E) and magnetic (H) amplitudes decrease.

E and H are greater closer to the signal's origin. When E and H are far away from the circuit, they decay by 1/x, 1/x2, or 1/x3, where x is the distance from the source.

It's worth noting that only the radiated signal can go a long distance away from the circuit (when the circuit has antennas that are needed or not). Other signals will surround the circuit's components and will not be radiated.

Ⅱ. What is the ratio of E to H (wave impedance)? When far or near the signal  source

The wave impedance Zw=E/H is plotted in Figure 2 assuming the circuit is at point X=0. When you measure the ratio between E and H when you are far away from the circuit (ie d >), you will get Zo = 377, which is the wave impedance in the ar field.

So, if you know the amplitude of the electric field in the distant field, you can easily determine the amplitude of the magnetic field. Because both E and H decrease by 1/x in the far-field, the ratio remains constant as distance increases.

You won't be able to tell if the signal's source is a low impedance circuit (i.e. a loop) or a high impedance circuit in this scenario (i.e. an open circuit). In the far-field, you receive a plane wave, which we pick up with an antenna.

Consider that you've gotten a little closer to the finish line. The E/H ratio is around 377 ohms, but as the distance approaches /6 (d/=0.16), the E/H ratio will grow or fall from the Zo value.

The wave impedance will assume values close to the impedance of the circuit from which the signal is formed. For example, if the signal is generated by a low impedance circuit as a loop (high current - low voltage), Zw will be lowered from 377 ohms as you go closer to the circuit. If the signal is produced as an open circuit (high voltage - low current) by a high impedance circuit, Zw will rise from 377 ohms as you go closer to the circuit.

This is because we utilize near-field magnetic or electrical probes instead of antennas in the near-field.

Let's say a circuit at 250MHz has a problem with radiation. /6 is 20cm for this frequency. You'll be in the distant field if you measure/receive a  signal at 5 meters. You'll be in the near field if you measure/receive the signal at 5cm 

It's vital to remember that far-field and near field values are related; yet, if you measure signals in the near field, you may not be able to detect them in the far-field since they can't be radiated (they are stored in your measured area, but without some kind of antenna sending the signal into the far-field). 

Ⅲ. What is the difference between near-field testing and far-field testing?

Near Field: The three-wavelength zone centered on the field source. The strength of the electric field and the strength of the magnetic field have no defined proportional relationship in the near field.

E 377H, to be precise. Under typical conditions, the electric field is substantially greater than the magnetic field for field sources with high voltage and little current (such as transmitting antennas, feeders, and so on). The magnetic field must be substantially larger than the electric field for field sources with low voltage and high current (such as the mold of some induction heating equipment).

The near field's electromagnetic field strength is substantially higher than the distant field's. The focus of electromagnetic protection should, in this case, be in the near field. The near-field electromagnetic field strength varies fast with distance, and the inhomogeneity non this space is significant.

The far-field field is a three-wavelength radius field that exists outside of the field source. All electromagnetic energy is radiated and propagated in the distant field in the form of electromagnetic waves, and the attenuation of this field's radiation strength is significantly slower than that of the induction field.

The electric field strength and the magnetic field strength have the following relationship in the far-field: The electric and magnetic fields are perpendicular to each other in the international system of units, E=377H, and both are perpendicular to the propagation path of the electromagnetic wave. The electromagnetic field strength of the far-field field is quite minimal, as it is a weak field.

Ⅳ. What kind of probe is needed for near-field measurement?

The three probes illustrated in Figure 1 are near-field probes of varying sizes used to detect various H-field or magnetic field intensities, and I suppose many electrical engineers must have used them.

These probes are only utilized for the most extreme situations, such as debugging and troubleshooting prior to compliance. Come observe how the probe is used to measure near-

Connect the EMC  probe to the oscilloscope, then set the coil in your PCB  and move the coil location while watching the waveform on the oscilloscope to trace the issue component or trace on the PCB.

The magnetic field direction is detected by the probe in Figure 5. When we alter the direction of the magnetic ring, the waveform on the oscilloscope will change as well, because the magnetic flux going through the ring changes as well.

Ⅴ. When will the near field be used?

We must all understand that the standard tests we send to the testing agency are all done in the far field, and that if the far field test fails, the approach indicated in Figure 5 will be utilized to track the components on the PCB  that exceed the standard. Of course, you can scan your own board ahead of time to see if there are any issues, such as excessive radiation.

Let's talk about near and far fields and how they relate to electromagnetic radiation now. The field might go up on the z axis like this, and the H field is 90 degrees, so they actually travel in different directions, as shown in Figure 6, which shows conventional electrical. Of course, we can see the wavelength as well, here with Figure 6. This cute little animation is only to demonstrate how it works, as it immediately moves on to what we really need to observe, which is wave impedance.

 a probe that is not sensitive to electric fields; you may move it in any direction on the PCB  and the waveform on the oscilloscope will not change because there is no magnetic field coupling; it is just pure electric field coupling, it is just pure distance.