A VSWR bridge and a magnetic near field probe (NFP) are very valuable tools to find resonances in RF and EMI/EMC applications as tuning, shielding, etc.
A Voltage Standing Wave Ratio (VSWR) bridge (or Return Loss (RL) bridge) is a passive device usually used by RF engineers to discern between the forward and reflected waves in a transmission line or RF circuit.
The bridge provides a sample of electric field and another of magnetic field in the line or circuit and they are summed resulting in addition in the forward orientation and subtraction in the reflected orientation.
As shown in Figure 1, the VSWR bridge has three ports and it is designed for use with 50 ohm systems): RF input port (RFin), RF output port (RFout) to be connected to the device under test (DUT) and coupled port (reflected signal from the DUT).
By connecting the tracking generator output (TG) of your spectrum analyzer to the input (RFin) of the VSWR bridge and connecting the bridge output (RFout) to the spectrum analyzer input (RF IN) we can measure the degree of mismatch between a transmission line or circuit and its load (e.g antenna).
If the DUT input impedance is near 50 ohms, no signal will be reflected to spectrum analyzer RF IN. As the mismatch is higher, the signal level in the analyzer increases.
Now, we will take a typical magnetic near field probe (NFP) as used in EMI/EMC debugging techniques replacing the DUT by the probe as shown in Figure 2.
The NFP looks like a short circuit (it is a loop), with input impedance very far from the nominal 50 ohms value (please, check that condition in your probe).
Then, a great part of signal from the tracking generator is reflected back from the NFP and redirected to the spectrum analyzer input through the Coupled port of the VSWR bridge.
If you hold the NFP near a resonance in components, circuits, cables, PC boards, shields, etc., while sweeping with the spectrum analyzer frequency in a range of interest, some part of the incident energy will be
absorbed at that frequency and a dip will appear in the screen (part of the tracking generator output is being absorbed by the resonant system).
Usually the system is calibrated so the REF LEVEL of the analyzer (top line in screen) is obtained when the NFP is positioned far from the system under test.
Two lab experiments have been prepared in my lab to show the technique. In Figure 3, the NFP is placed on top of a RFID (13.56MHz) card.
As explained before, a dip near the center of the grid appears. The frequency is 14MHz and that is the resonant point of the tuned internal circuit in the card. The nominal frequency is 13.56MHz (ISM frequency) and the system must be tuned to that frequency when the reader is located on top of the card (not when the card is far from reader).
Another example has been prepared in Figure 4 with a 19” rack in a radio transmitter.
The NFP is positioned on top of the rack, moving slowly around the perimeter, and a dip is found when the probe is near the front slot of cover (see picture). The dip frequency is around 655MHz (the full length of the slot is 45cm approximately but the contact is not uniform along it). The technique is very useful to find problems in the design of the enclosure before the electronics are ready. The dip must disappear removing the slot especially if the frequency is near to some internal frequency (e.g. internal RF oscillator or harmonics from clock). Reasons for these kind of slots are non metallic paints, anodized surfaces, etc.
My final advice: learn how to use your near field probes to find resonances and you will discover a great technique for EMI/EMC troubleshooting.
