3M ScanEM-C User manual

3M™ ScanEM-C
ElectroMagnetic
Near-Field Probe Kit
Model CTK015
User’s Guide

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3M™ ScanEM-C Electro-Magnetic Near-
Field Probe Kit, Model CTK015
Read, understand, and follow all safety information contained in these user
instructions prior to use of the 3M ScanEM-C Electro-Magnetic Near-
Field Probe Kit, Model CTK015. Retain these user instructions for future
reference.
Explanation of Signal Word Consequences
WARNING: Indicates a potentially hazardous situation, which, if not
avoided, could result in death or serious injury and/or
property damage.
CAUTION: Indicates a potentially hazardous situation, which, if not
avoided, may result in minor or moderate injury and/or
property damage.
NOTICE: Indicates a potentially hazardous situation, which, if not
avoided, may result in property damage.
WARNING:
To reduce the risks associated with impact related injury, which if not avoided,
could result in death or serious injury:
• When working with the automated equipment, always observe safety precautions
as recommended by equipment manufacturer and your company’s practices

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CAUTION:
To reduce the risks associated with environmental contamination related injury,
which, if not avoided may result in minor or moderate injury:
• Dispose of the probes in accordance with local, state, and federal regulations
NOTICE:
To reduce the risks associated with property damage:
• Periodically check that the probe is functioning properly
Industry Canada
This Class A digital apparatus complies with Canadian ICES-003.
FCC
This device complies with Part 15 of the FCC Rules. Operation is subject to the following
two conditions: (1) this device may not cause harmful interference, and (2) this device must
accept any interference received, including interference that may cause undesired operation.
Note: This equipment has been tested and found to comply with the limits for a Class A digital
device, pursuant to part 15 of the FCC Rules. These limits are designed to provide reasonable
protection against harmful interference when the equipment is operated in a commercial
environment. This equipment generates, uses, and can radiate radio frequency energy and,
if not installed and used in accordance with the instruction manual, may cause harmful
interference to radio communications. Operation of this equipment in a residential area is
likely to cause harmful interference in which case the user will be required to correct the
interference at his own expense.

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Table of Contents
Contents of the kit ..........................................................................................3
Product description ........................................................................................4
3M™ ScanEM-C Electromagnetic Near-Field Probe Kit
controls and connector ..................................................................................6
Using ScanEM-C Probes ................................................................................7
Basics of operation ....................................................................................................7
Stand-alone operation ...............................................................................................8
Quick set-up: ............................................................................................................8
Setting the level dial...................................................................................................9
Sound on/off ............................................................................................................10
EM level LED bar ......................................................................................................11
Operation as a probe................................................................................................11
Basics ......................................................................................................................11
Amplified near-field probes for a spectrum analyzer ................................................12
Non-contact oscilloscope probes .............................................................................13
Broadband field strength probe for a multimeter .....................................................15
ScanEM-C as a probe ..............................................................................................16
Analyzing the results ....................................................................................18
Troubleshooting ...........................................................................................20
Common places to look for excessive emissions .....................................................20
During the design phase ..........................................................................................21
Checking the prototype ............................................................................................21
Before the EMC test .................................................................................................21
Should things go wrong ...........................................................................................22
Seeking help ............................................................................................................22
EMC101 ........................................................................................................ 23
Why EMC? ...............................................................................................................23
Near-field primer .....................................................................................................23
The validity of the near-field measurements ............................................................24
Taking care of your ScanEM-C probes ........................................................26
Changing batteries ...................................................................................................26
What if: ....................................................................................................................26
Specifications ...............................................................................................30
Quick Guide ..................................................................................................31

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The 3M™ ScanEM-C Electromagnetic Near-Field Probe Kit will help
improve your EM compliance process and will be a valuable addition to your
bench and field tool collection.
ÎThe ScanEM-C Kit is designed for board-level EMC pre-compliance and
diagnostics. For EMC product-level diagnostics, use the 3M™ ScanEM-QC
Electro-Magnetic Near-Field Probe Kit, model CTK019.
ScanEM-C Kit Contents
The ScanEM-C Kit, model CTK015 consists of:
One ScanEM-EC probe (electric field), model CTM030
One ScanEM-HC probe (magnetic field), model CTM032
One Six foot (~1.8 m) SMB-to-BNC cable, model AWD001
AAA batteries (Two per each probe—already installed)
Plastic storage box, model CTA015
User’s Guide
Optional ScanEM-C Kit accesories:
Accessories Pack, model CTA101
Includes:
• An additional six foot SMB-to-BNC
cable, model AWD001
• One BNC-to-N adapter, model JBA001
• One BNC-to-Banana Plug Adapter,
model JBA002

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Product Description
The 3M™ ScanEM-C Electromagnetic Near-Field Probes detect the presence
of an electromagnetic field and indicate its relative strength. The probes are
most effective in the near field (close to the radiating object). The ScanEM-
EC Probe detects the electric component of the electromagnetic field and the
ScanEM-HC Probe reacts to the magnetic component. Knowing which is
more prominent is helpful when it comes to engineering a solution.
What is EMC?
ÎIf you are new to the world of electromagnetic compliance, please refer to a
brief primer on page 23 of this User’s Guide.
The ScanEM-C Kit offers a universal solution to electromagnetic
compliance problems. By themselves, ScanEM-C Probes are completely
self-contained EM field detectors. When connected to test equipment,
ScanEM-C Probes become broadband amplified near-field probes.
In a stand-alone mode, ScanEM-C Probes indicate the presence and strength
of an electromagnetic field by two means: audio indication and an LED level
bar. The more LEDs that are on and the higher the pitch of the sound, the
stronger the electromagnetic field is at the point of measurement. In stand-
alone mode, ScanEM-C Probe doesn’t provide absolute levels. It serves only
to single out the areas with relatively high emissions.
As a probe, the ScanEM-C Probe provides both AC and DC outputs.
ScanEM-C Probes serve as: broadband amplified probes for a spectrum
analyzer; as non-contact high-bandwidth voltage (ScanEM-EC Probe) and
current (ScanEM-HC) probes for an oscilloscope; and as broadband field
strength probes for a multimeter and other instruments.

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The 3M™ ScanEM-C Electromagnetic Near-Field Probes can:
Pinpoint the location of a component, circuit area or trace with high
emission levels
Predict radiated EMC test results
Pre-test your product prior to an official compliance test in order to
identify potential problems early
Quickly troubleshoot a product that has failed a compliance test
Find EM leakage on enclosures, cables and connectors
Find sources of EM interference
Show the presence of a high-frequency signal on a trace or wire
without contact
Help to troubleshoot circuits and wiring
Work as amplified near-field probes when connected to a
spectrum analyzer
Work as non-contact high-bandwidth voltage and current
oscilloscope probes
Work as field-strength probes when used with a multimeter

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3M™ ScanEM-C Electromagnetic Near-
Field Probes Controls and Connector
Antenna Level Dial Sound Switch LED Level Bar Output
This is the
ScanEM-C’s
antenna.
Adjusts the
reference level
of the EM field.
Turns sound
indicator On
and Off.
Indicates
relative EM field
strength.
Provides AC and
DC output.
Stand-alone operation only
Point it at the
area or the
object you
want to check.
The closer the
antenna is to
the object, the
more accurate
the test result.
The level dial
is used to set
the “normal
background
level.” It also
works as a
squelch control.
The stronger
the emission
level, the higher
the pitch of the
sound.
Five LEDs
indicate relative
field strength.
The LEDs can
be used with or
without sound.
SMB male
connector.
Connects
ScanEM-C
Probe to a
spectrum
analyzer,
oscilloscope,
frequency
counter,
multimeter,
and other
instruments.
Operate Button
Keep this button
depressed when
you are using your
ScanEM Probe.
ScanEM-C is
“ON” as long
as this button is
held down.
Two AAA alkaline batteries are already installed in
each ScanEM-C Probe. See page 26 for instructions
on how to replace the batteries.

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Using the 3M™ ScanEM-C
Electromagnetic Near-Field Probe Kit
Basics of Operation
Both ScanEM-C Probes come with the batteries installed and are ready to
work right out of the box.
The best way to hold the ScanEM-C Probe is in the palm of your right hand,
just as you would hold a pen, with your thumb on the red Operate button
and with your index finger above the Level dial and Sound On switch (see
illustration on page 6). The red Operate button turns the power on. The
ScanEM-C Probe will be “on” as long as the button is held down.
ScanEM-C Probes are non-contact probes. Do not touch circuit elements
with the ScanEM-C Probe’s antenna. The test results may be meaningless,
and normal operation of the measured device may be influenced by parasitic
capacitance introduced by the ScanEM-C Probe in direct contact. The
ScanEM-C Probe itself may be damaged as well if it is directly exposed to
high voltage.
For the best results, position the ScanEM-C Probe perpendicular to the
tested area. This way, stray electromagnetic emissions coming from the
outside of the measured area may not affect the ScanEM-C Probe’s antenna
as much, and the ScanEM Probe can pinpoint the source of the troubling
emissions with greater accuracy.
The ScanEM-C Probe needs a few seconds to “warm-up”
ÎBefore scanning or taking measurements, allow a few seconds for the
ScanEM-C Probe to stabilize after first turning it on.

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3M™ ScanEM-EC Probe
The small round red tip at the end of the ScanEM-EC Probe is it’s antenna.
Point it at the suspected area and keep it close to the measured spot without
making contact.
3M™ ScanEM-HC Probe
There is a pickup coil located in the tip of the ScanEM-HC Probe. In order
to locate a magnetic field emanating from a wire or cable, the coil has to
be parallel to that wire. Hold the ScanEM-HC Probe in such a way that the
seam on the plastic case is parallel to the tested wire, cable or trace. You
can easily differentiate which trace emits the most radiation by just rotating
the ScanEM-HC Probe around its longest axis (see illustration below). The
ScanEM-HC Probe will show the strongest readings when its coil is parallel
to the offending wire.
Stand-Alone Operation
In stand-alone mode, the ScanEM-C Probe is completely self-sufficient and
requires nothing else to operate.
Quick Set-Up
Hold the ScanEM-C Probe in the palm of your
right hand as if you were holding a pen.
Set the Sound switch in the desired position
(if in doubt, set it to On).
Press the red Operate button with your
thumb and hold it down.
Point the ScanEM-C Probe’s antenna away
from the tested product.
Adjusting the position of the
3M™ ScanEM-HC Probe

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With your index finger adjust the Level dial so that only the green LED
is On.
Position the 3M™ ScanEM-C Probe perpendicular to the tested area
and bring its antenna as close as possible without actually touching the
tested product.
Move the ScanEM-C Probe around the tested product or area and
observe changes in sound and LED output.
The closer the ScanEM-C Probe’s antenna is to the tested area, the greater
its resolution. A scan performed an inch or two away from the product can
direct you to the general area where high emissions are generated, but the
precise location of the source can be found only when the antenna is in close
proximity to the product.
Setting the Level Dial
The Level dial sets the reference level for field strength measurements. The
sensitivity of the ScanEM-C Probe stays the same regardless of the position
of the Level dial (Figure A). If you let go of the Operate button and then
press it again, you will not need to readjust the level.
Set the Level dial so that when the antenna is pointed away from the tested
device, only the green LED is “On” (the pitch of the sound will be at the low
end of its range). If the tested product
is “noisy” as far as the EM level is
concerned, the LED bar may be totally
lit most of the time and the pitch may
be constantly high. In this case, point
the ScanEM-C Probe at an area of
expected “average” emission on the
tested product, and adjust the Level
dial to the new background level. Figure A: 3M™ ScanEM-C Probe Reaction

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You can use the Level dial to put the 3M™ ScanEM-C Probe in squelch
mode. Follow the procedure outlined in the previous paragraph, only set the
reference level just below the point where the green LED turns on. This way
your ScanEM-C Probe will be quiet and the lights will be off until you pass it
over an area where the EM level is higher than your reference setting. When
testing a product for compliance, for example, you may want to scan some
other product that has passed the emission test before, and set the background
level of the ScanEM-C Probe to the average emission level of that product.
Then scan the product you are trying to get agency approval for in search of
spots with higher emission levels.
Setting the Level dial to zero completely disables the ScanEM-C Probe’s own
indication and is used for its operation as a probe only. Use Level settings of
one and above in stand-alone operation.
Level Dial Numbers
ÎThe numbers from zero to nine on the Level dial do not refer to any specific
field strength, nor do they signify a preset spacing in field strength between any
two numbers. These numbers are put there for convenience so the user can
refer to them when recording relative field strength. You can use these numbers
to “calibrate” your ScanEM-C Probes. Once the correlation between the field
strength at the tip of the ScanEM-C Probe, the number on the dial, and the status
of the LED bar is established, it can be used for simple Go/No Go tests.
Sound On/Off
The human ear is very sensitive to pitch variation and thus can detect the
slightest change in field strength when the speaker is on.
With the Sound turned on, you can detect much finer variations in EM field
strength than by using lights as the only indicator. You may prefer to have the
sound turned off to cut down on noise, but this limits your resolution to just
five steps (five LEDs).

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EM Level LED Bar
The EM level bar is for relative estimates only and shouldn’t be used for
precise measurements.
The 3M™ ScanEM-C Electromagnetic Near-Field Probe’s LEDs are spaced
on a somewhat linear scale. The figure to the right shows typical LED scale.
The scale is always relative to the green LED. On this scale, green LED is
always a reference point or “0
mV/m,” regardless of where the
Level dial is set. One should
be careful not to mistake the
situation when the LED bar is
overloaded with the second red
LED just turned On.
Figure B is provided for
reference only. Specification
is subject to change without
notice.
Operation as a Probe
1400
1200
1000
800
600
400
200
0
Green Yel 1 Yel 2 Red 1 Red 2
LEDs
m V/m
(relative)
CAUTION!
UNLIKE SOME PASSIVE PROBES, THE SCANEM-C PROBE IS NOT A REVERSIBLE DEVICE—DO NOT
SUPPLY A SIGNAL INTO ITS OUTPUT! IT CAN PERMANENTLY DAMAGE YOUR SCANEM-C PROBE.
Basics
The ScanEM-C Probe provides both AC and DC outputs to virtually any
piece of test equipment: spectrum analyzer, oscilloscope, frequency counter
or multimeter. Output is provided via an SMB connector and the supplied
cable. For some types of test equipment you may need to use additional
Figure B: Typical 3M™ ScanEM-C Probe LED Response

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adapters (typically, BNC-to-N or BNC-to-banana plugs). Such adapters are
widely available. In case of difficulties in obtaining them, you can order
them from 3M.
The DC voltage that the 3M™ ScanEM-C Electromagnetic Near-Field Probe
provides has a high output resistance—60 kOhms. While it doesn’t present
a problem when using the ScanEM-C Probe with a multimeter with high
input impedance, connecting the ScanEM-C Probe to a 50 Ohms input of
a spectrum analyzer limits the DC signal to a maximum of 2.5 mV without
affecting the AC performance.
Always Set the Level Dial to Zero When Using ScanEM-C as a Probe
ÎWhen the Level dial is set to zero, it disables the ScanEM-C Probe’s own
indication—the LEDs and the speaker, —preventing any influence on the
output signal.
Do not load the ScanEM-C Probe’s output with less than 50 Ohms load
(such as connecting it in parallel to the inputs of a spectrum analyzer and 50
Ohms input of an oscilloscope). This will not damage the ScanEM-C Probe,
but its performance will be reduced.
Though the ScanEM-C Probe has a wide dynamic range, a very strong field
may overload it. This may result in harmonic distortions of the output signal.
The overload condition can be clearly seen on the screen of a spectrum
analyzer or an oscilloscope. Should this occur, simply move the ScanEM-C
Probe farther away from the emission source until distortions go away.
Amplified Near-Field Probes for a Spectrum Analyzer
Connect the ScanEM-C Probe to the INPUT of the spectrum analyzer with
the supplied cable. Some spectrum analyzers require the use of a BNC-to-N
adapter.
The performance of the the ScanEM-C Probes is superior to those of

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passive probes. The 3M™ ScanEM-C
Electromagnetic Near Field Probes
have broad and flat frequency
responses within a specified range
(see page 13 for details). The
ScanEM-C Probe’s built-in amplifiers
provide ~20 dB of gain, virtually
eliminating the need for an external
amplifier.
Figure C shows a typical frequency
response of the ScanEM-EC Probe
vs. an equivalent passive probe. As
seen, passive probes typically fail at
lower frequencies (below 200 MHz),
where most of the fundamental
frequencies and many of the strongest
harmonics of the tested equipment
are located. Figure D shows
comparative frequency responses of
the 3M™ ScanEM-HC Probe and its
passive counterpart at lower frequencies.
Avoid overloading the probes with a very strong field—it may induce
distortions in the ScanEM-C Probe amplifiers. The ScanEM-EC Probe, for
example, begins to overload at field strengths of approximately 4 V/m.
Non-Contact Oscilloscope Probes
When connected to an oscilloscope, the ScanEM-C Probe becomes an
extremely broadband non-contact probe, which is a great diagnostics and
troubleshooting tool. The typical challenge for oscilloscope measurements
-5
-15
-25
-35
-45
-55
-65
-75
-85
-5
-15
-25
-35
-45
-55
-65
-75
-85
Passive Probe
Passive Probe
O MHz 1000 MHz
VBW 10 kHz RBW 10 k Attn 0 dB
O MHz 1000 MHz
VBW 10 kHz RBW 10 k Attn 0 dB
ScanEM EC
ScanEM EC
Figure C: Typical Frequency Response of 3M™
ScanEm-EC Probe vs. Passive Probe
Figure D: Typical 3M™ ScanEM-HC Probe
Frequency Response vs. Equivalent Passive Probe
(Lower Frequency Range)

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is that the probe loads the circuit and affects its behavior. The typical
solution for this problem is to make 100x1 or even 1,000x1 probes that
have extremely high input impedance and low capacitance. That may
work in some applications; but for many, the signal attenuated 100 or even
1,000 times is too low to analyze with the scope. The 3M™ ScanEM-C
Electromagnetic Near-Field Probe makes no physical contact, therefore it
Figure E: Typical Response of 3M™ Scan-
EM-EC Probe as a Voltage Probe
Figure F: Typical Response of 3M™ Scan-
EM-HC Probe as a Current Probe
offers no loading whatsoever on the circuit under test.
Another problem inherent with scope probes is their bandwidth. Probes
have 60, 100 or 250 MHz bandwidth. Cost for higher-frequency probes
is prohibitively high. With today’s high-speed circuits, low-bandwidth
tools may easily miss nanosecond-long transients, leaving engineers to
wonder why their circuits behave the way they do. The bandwidth of the
3M™ ScanEM-EC Electromagnetic Near-Field Probe is at least 2 GHz,
outperforming most any oscilloscope probe on the market. In addition, the
ScanEM-C Probe amplifies the signal instead of attenuating it, allowing the
user to view even the weakest signal on the screen.
The ScanEM-EC Probe measures an electrical field that is a representation
of the voltage in the circuit. The ScanEM-HC Probe measures a magnetic
field that is a representation of the current in the traces. You will be able to
monitor high-frequency currents on a signal or power trace without cutting

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the trace or spending hundreds or thousands of dollars on a special current
probe that only works at much lower frequencies.
The 3M™ ScanEM-C Electromagnetic Near-Field Probes, as any field probe
would, show the signal slightly differently than the one actually residing on
the trace. This is due to an antenna factor of the trace. Experimenting with the
ScanEM-C Probe and the oscilloscope probe will show you the difference.
Also, the ScanEM-C Probes pick the sum of the emissions from the adjacent
traces and wires, so some discretion is needed when assessing the signal on
the screen.
Broadband Field Strength Probe
for a Multimeter
The ScanEM-C Probe provides DC
voltage that is a function of field
strength. Connect the ScanEM-C
Probe to the multimeter set to Volts
DC (typical scale of 2 V). You will
need a BNC to banana plug adapter.
Set the Level dial to zero (left-most
position). In the absence of the
electromagnetic field, the multimeter should show 0V. Should the multimeter
display any other voltage reading, use it as the correction factor to any further
field measurements. You can characterize the ScanEM-C Probe’s output using
a GTEM cell, and then convert the ScanEM-C Probe’s output into absolute
values of the field strength.
When using the ScanEM-C Probe with a multimeter, avoid touching the tip of
the ScanEM-C Probe with your hand—it may induce a feedback between the
typically non-shielded circuitry in a multimeter and the ScanEM-C antenna
that is amplified simply by your touching it.
A typical transfer function of the ScanEM-EC Probe is shown in Figure G to
10000
1000
100
10
1
Field Strength, mV/m
0 500 1000 1500 2000 2500 3000 3500
DC Output mV
Figure G: 3M™ ScanEM-DC Probe Response
(Typical)

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the right. This chart is provided for reference only. Specification is subject to
change without notice.
Know What You are Measuring
ÎA word of caution: Avoid misleading numerical readings of the field strength
by understanding what you are actually measuring. The 3M™ ScanEM-C
Electromagnetic Near-Field Probe measures the field strength at its tip. In the
near-field the field strength fluctuates dramatically depending on the distance
from the source. Slight repositioning of the tip of the ScanEM-C Probe may
result in different readings.
What You Can Do With the ScanEM-C Probe
Measuring Shielding Efficiency
Measure the field strength before and after the shield is applied to know the
actual effectiveness of the shielding. Measuring the field strength on both
sides of the shield also helps to evaluate shielding.
The ScanEM-C Probes and a multimeter can be used to check the quality of
applied shielding and gaskets in production. A quality control inspector can
be supplied with the maximum allowed DC voltage at critical points.
Locating Defective Components
Often one doesn’t need a spectrum analyzer in order to locate
malfunctioning ICs and other components. The strength of the emitted field
next to the component can be a good indicator of its status.
EMC Pre-Compliance and Troubleshooting
Connect the ScanEM-C Probe to the input of the spectrum analyzer. Set the
spectrum analyzer to the desired settings (if in doubt, use default settings
on power-up). Press and hold the red Operate button. Pass the tip of the
ScanEM-C Probe over the area in question. Should the emission level be too
high, you may need to change the attenuation of the spectrum analyzer’s input.

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Monitoring the Signals
You can monitor the signal on the traces or wires without contact with the
3M™ ScanEM-C Electromagnetic Near-Field Probe. Connect the output of the
ScanEM-C Probe to an oscilloscope. Set the scope input coupling to AC. Bring
the tip of the ScanEM-C Probe to the trace or wire you need to test and observe
the waveform of the signal on the screen. Keep in mind that the waveform
of the radiated signal is different from that of the signal on the trace itself.
You won’t be able to measure distortion of the signal, nor will you be able to
precisely assess undershoot/overshoot of pulses. However you should be able
to reliably detect the signal, estimate its waveform, measure the frequency of
the signal, for example.—all without influencing the signal. This is especially
important for tuned circuits, crystals or high-impedance circuits—anywhere
where direct contact with the circuit disrupts its normal operation.
Locating Bad Connections
The ScanEM-C Probe can help in locating bad connections and broken
traces by showing sharply different radiation levels and patterns before and
after the connection in question.
Identifying Defective ICs and Other Components
Quite often it is difficult to identify a defective chip on a board by just
measuring the signals on its pins since the chip receives signals from the
other components. You can determine whether the IC is functional by
comparing its radiation pattern with a good IC on a similar working board.
Identifying Wires and Traces Carrying RF Signals
By monitoring radiation patterns of individual wires or traces in a live circuit,
you should be able to (1) follow the specific trace on a complex circuit board
or the wire in a cable assembly and (2) check whether that trace or wire is
connected properly. the ScanEM-C Probe can save you hours of guesswork.

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Analyzing the Results
If the 3M™ ScanEM-C Electromagnetic Near-Field Probe indicates the
presence of an electromagnetic field, it may not necessarily mean that your
product has emission problems. It is normal for an electronic device to
produce some electromagnetic noise. As you pass the ScanEM-C Probe over
the tested product, you may notice that the level of the electromagnetic field
changes. You may find that several areas in the tested product have higher
than average emission levels. These areas require special attention. Should
the product fail an agency emissions test, those areas are suspect. It is a good
idea to take whatever steps necessary to reduce the excessive EM field level
at these areas to as close to the background level as possible prior to the
agency test.
Often the ScanEM-C Probe may indicate excessive EM field strength
in places where high frequency signals are not normally expected—i.e.
AC power lines. This happens because high-frequency signals produced
elsewhere “ride” on these lines and generate an electromagnetic field.
Many low-frequency signals contain a sufficient amount of high-frequency
harmonics for the ScanEM-C Probe to detect. If any of these non-sinewave
signals are generated by your equipment, it may be necessary to reduce the
emission level in order to satisfy EMI regulations. For a far-field agency
test, one or two spots on the product with high emission levels detected in
the near-field may be less problematic than a broad area with a somewhat-
higher-than-average EM level.
High-level radiation registered with a near-field probe is not necessarily
an indication of a problem in the far-field. Conversely, a near-field probe
may not show the field strength of the area that is causing the compliance
failure. In order to understand why, one needs only to realize that in the
near field, the probe picks up the signal only at a particular spot, while in