RADIO FREQUENCY INTERFERENCE (RFI)

INVESTIGATION AND RESOLUTION GUIDE

CONTENTS

01 Purpose and scope 01

02 The interference location process 02

03 Interference location techniques for domestic interference 03

04 Interference location techniques for commercial interference 09

05 Interference at radio station sites 11

06 Suppression methods 14

07 Safety and compliance 16

08 Contacts/Bibliography 17

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 01
PURPOSE AND SCOPE

THIS RADIO FREQUENCY INTERFERENCE (RFI) AND RESOLUTION GUIDE IS

DESIGNED FOR USERS WITH A MODERATE TO HIGH LEVEL OF ELECTRICAL,
ELECTRONIC OR TELECOMMUNICATIONS KNOWLEDGE, SKILL AND EXPERIENCE.

Radio Spectrum Management (RSM) has BACKGROUND

produced this self-help guide to locating and
identifying RFI sources. The information outlines Regulatory measures are in place to prevent
some common causes of interference and unacceptable levels of RFI pollution to the
suggests a methodology to resolve them. The New Zealand radio spectrum resource.
use of the term interference in this publication These measures are provided for under
is in accordance with normal industry practice, the Radiocommunications Act 1989 and
but does not imply that any particular situation or Radiocommunications Regulations 2001,
example is necessarily interference as defined in and managed by RSM.
the Radiocommunications Act 1989.
RSM’s responsibilities include:
Changing technology means that the sources of
licensing of radio transmitters and
interference are also continuously changing, and
this will often mean adopting new technology managing compliance frameworks for
and techniques. Nevertheless, well-established electrical products and radio transmitters.
methodology that is systematically employed
These frameworks are supported by random
will normally serve practitioners well.
audits and inspections to ensure the integrity
Although best practice in installation and of New Zealand’s self-regulatory system
maintenance is essential to preventing and protecting the communication services essential
minimising RFI, this Guide does not address to the New Zealand economy. Compliance
such aspects. These requirements are generally requirements for electrical “noise”, and the
covered in manufacturer instructions and in engineering of radio licences assume that
standards available from Standards New Zealand. radiocommunication services cannot be
provided with a “radio-silent” background,
and that the services should work satisfactorily
in the presence of reasonable levels of
unwanted signals.

PA G E 01
 02
THE INTERFERENCE LOCATION PROCESS

1. Obtain detailed information from affected 4. Determine if poor installation practices are
parties as to the nature of the interference causing or contributing to the interference.
such as:
eg for land mobile sites, is the configuration
what time does it occur? in accordance with industry standards such
as AS 3516.2-1998 – Part 2: Guidelines for
what frequencies are affected?
fixed, mobile and broadcasting services
when did it start? operating at frequencies above 30 MHz.

which geographical area is affected?

5. Conduct measurements of the incoming
what does it sound like or look like? interference to determine:
does it happen in certain weather conditions? levels

We recommend you never prejudge a case as modulation characteristics

to the source of the interference.
direction.

2. Determine how the affected service is

6. Review your original information and
being impaired.
follow up on relevant information
Determine the way the affected service as required.
operates (system configuration) as this will
Consult others in the industry as appropriate.
assist in understanding how the interference
If necessary, request further information from
is affecting the wanted service and hence the
the affected parties so you can be certain
likely nature of the interference.
to understand the affected system and the
eg for systems such as linked voting systems interference mechanism before proceeding.
there may be several possible points of entry
for the interference.

3. Check the receiving installation and

where possible test or change the affected
equipment to ensure the equipment or
receiver settings is not the source of the
interference.

PA G E 0 2
 03
INTERFERENCE LOCATION TECHNIQUES
FOR DOMESTIC INTERFERENCE

THIS SECTION RELATES TO INTERFERENCE TO TV AND BROADCAST RECEPTION AND

OTHER NON-COMMERCIAL SERVICES LIKE AMATEUR, DX LISTENING, CITIZEN BAND
AND PUBLIC RADIO SERVICE.

1. Check the equipment and installation. largely occur from wiring that emerges from the
ground at street fuse boxes and at houses.
Ensure the affected receiving installation is
The signal level (see below) works best in this
configured correctly, and working effectively.
situation.
This, together with sufficient wanted signal is
critical to ensure that it is as immune as possible Watch for multiple interference noises and
to external interference. frequently check that you are following the
correct one.
Faulty systems eg incorrect wiring,
overloaded and/or mismatched amplifiers,
faulty distribution equipment, etc may create LOCATING WIDEBAND INTERFERENCE
their own interference.
Techniques for locating the source of wideband
Installers tend to use amplifiers that have
interference are:
excessive gain, thinking that this will provide
better system performance. High gain 1. Measure interfering signal levels.
amplifiers are considerably more susceptible
Normally the stronger the RFI level, the closer
to interference. Use only the gain needed for
you are to the interference source.
the task – the lower the gain, the lower the
system susceptibility. Lower frequencies eg AM broadcast (521 to
1612 kHz) wavelengths are long and standing
2. Trace the signal along the wiring. waves will occur. They will likely occur a great
distance away from the source, meaning the
The diagram on the following page shows
level may be strongest away from the source.
how interference travels to a typical
domestic residence. It is easy to confuse direction when tracing
signals in the low frequency (300 kHz) to high
While unwanted signals will radiate directly from
frequency (30 MHz) range. This is particularly
the RFI source they will also travel considerable
so when the conducted component meets
distances along wiring. It is thus possible to
other powerlines or junction points. The
trace the noise along the lines. It is likely that
measured level at the junction may exceed
radiation from these lines will also occur at
levels closer to the source.
multiple points, hence a number of radiated
signals will likely require attention. When following conducted noise along an
uninterrupted power line, each standing wave
Wiring attached to RFI sources frequently acts
peak will be a little lower than the previous.
as an aerial. The nature of this wiring provides for
This is particularly noticeable at VHF and UHF
a multiplicity of radiated and conducted patterns
frequencies. When close to the source, the
of RF energy which will vary widely from one
standing wave pattern changes into a steady
frequency to another.
level of signal.
Urban situations with underground power will
also behave in this way although radiation will

PA G E 0 3
 Propagation of interference signals

Interference Source

Legend

Conducted noise Secondary radiated noise

Secondary conducted noise

Induced noise
Radiated noise

PA G E 0 4
2. Measure highest interference frequency. Cordless phones, baby monitors, amateur radio,
citizen band or wireless networks. Interference
The closer you get to the source, the higher in
may occur because these are in close proximity
frequency the interference will be detectable.
or on the wrong frequency. A recording of
Due to shorter wavelengths at higher
the interference may be useful in determining
frequencies, there is a tendency for more
the source. Beware of receiving installation
radiated signal closer to the source.
deficiencies which may cause the appearance
of interference even though the transmitter
3. Direction of signal.
operation may comply with all requirements.
Reflections from metallic surfaces will provide
multiple signals and the reflections may be
stronger. The screening/shielding effect of
Additional tips to help you locate wideband
buildings and topography can help in determining
interference:
the direction to an interference source.
Suitable equipment will be needed for
4. Band fill (on spectrum analyser). this task, with a quality continuous tuning
A fuller band will likely mean you are closer to wideband receiver covering the range
the interference source. 550 kHz to 3 GHz with a signal strength
meter and attenuator. Efficient wideband
antennas will also be required.
EQUIPMENT AND APPLIANCES THAT
COMMONLY CAUSE INTERFERENCE When tracing TV interference, a TV receiver
INCLUDE: is often useful if the interfering signal is
contained within and thus masked by the
All electrically-powered equipment produces TV signal.
electromagnetic fields (radio waves). For most
equipment this effect is unwanted (unintended Understanding the propagation of signals at
radiation) and is limited by design to comply low frequencies (AM band) as opposed to
with mandatory standards. For other equipment high frequencies (TV bands) is useful.
(transmitters) the radiation is intentional and The “T” intersection of power lines and
necessary for communication or material treatment. telephone lines act like half- or quarter-
Some examples of interfering equipment are wavelength stubs (or multiples of, or
listed below: parts of) and thus the noise will behave
accordingly. These intersections are often
Switch mode power supplies, TVs, stereos, the location of strong standing waves and
DVD players, satellite receivers, videos, radiation, and may be a long way from the
computers, 12 volt lighting, energy saver bulbs interference source.
and other lighting equipment.
An understanding of power reticulation
Variable speed drives, water pumps, is useful.
air conditioners, lifts.
Often it is not possible to differentiate the
Motorised equipment, drills, food processors, noise source among two or more houses.
model trains, vacuum cleaners, water blasters. In this instance requesting help from the
Microprocessor controlled equipment, residents and turning off the mains will help
computers, sewing machines, alarms, TVs, identify the correct location.
DVD players, heat pumps. Turning off the power to a house may reduce
Unstable masthead or distribution amplifiers, the interference to your receiving equipment
incorrectly installed distribution hardware, and due to the proximity of your equipment to
faulty receiving equipment. the house wiring. This may give you the
incorrect impression that the source is
located in that house. Always check back
with the complainant at this point.

PA G E 0 5
 Alarm systems and other backed up equipment Consider the following during the location
(battery and uninterruptible power supplies) process:
will keep associated equipment active, even
Equipment for measuring interference
with the house mains off. Check for the
level, frequency and direction is effective in
presence of such equipment before assuming
locating power line interference. In some
the premises do not hold the interference
instances the arc producing interference
source. An alarm technician may be needed
can be seen (particularly at night) or heard,
to check security systems.
but should not be confused with corona
discharge.

At times the exact source hardware

POWER LINE INTERFERENCE
may be determined with the use of an
Power line interference usually comes from ultrasonic detector, but only where an arc
broadband noise. is present which is exhibiting ultrasonic
noise. Binoculars are helpful in looking
It may be traced using the same techniques
for faulty equipment as is a wooden maul
as tracing interference from an appliance. The
for vibrating the poles to determine the
interference is conducted along power lines and
presence of loose hardware.
is also radiated in a similar manner. It is usually
noticed only where power is reticulated via It may be necessary to employ the
overhead power lines and receiving aerials assistance of power authority staff
are relatively close to the lines. to test hardware to determine the
exact source.
It is a complex task to trace power line interference
and a number of techniques will likely be required
but the final analysis may still not lead to the
source. Patience and concentration over a period of AUDIO RECTIFICATION
time is often required.
Audio rectification usually occurs in narrow band
Power line interference tends to affect a wider transmissions.
area and is often moisture and wind dependent.
Strong RF signals are picked up by audio amplifying
Band one TV signals are more susceptible to
or computer equipment and are rectified by one
this type of interference. The nature of the
or more of the many diodes and transistors in this
interference is largely determined by the voltage
equipment. This produces DC voltages that corrupt
of the power lines. The majority of interference
operation. At times these diodes and transistors will
occurs with 11 KV, 22 KV, and 33 KV equipment.
demodulate the RF signals and this may be heard
The location of interference on 22 KV and 33 KV over speakers attached to the system.
equipment may be difficult where multiple
Please consult texts in the bibliography on page 17
sources are likely. They also tend to have a high
for techniques to correct these cases.
level of ambient noise where wooden structures
are used, due to loose hardware (caused
by shrinkage) and tracking between metal
hardware. Corona discharge may also add to the
ambient noise.

At all times ensure that you, and any measuring

equipment you are using, maintain a 4 metre
safety distance from any overhead power lines.

PA G E 0 6
POWER LINE INTERFERENCE

Radiation patterns

A
A Induction area

B Power line
C
C Source

D Radiation area

B Typical single source, single line, power line

radiation and conduction patterns.

Standing waves
A Voltage level C Low frequency

B Distance from source D High frequency

Standing waves (a series of noise peaks and

troughs) appear along the line. Note the noise
between peaks lessens as they approach the
C D
source. Likewise, the distance between a peak and
a trough lessens as the source is neared.

Variations in noise levels along the line may be

due to secondary line radiation, down leads,
ground wires, and nearby metallic objects.

Usually, the higher the frequency, the less the

A distance between peaks and troughs eg the
B standing waves become shorter.

Noise voltage levels

A Voltage level D Noise source

F
B Distance E Secondary lines

C Main line F Standing waves

Affect on standing waves of secondary power

lines joining the main line.

D E
C B

PA G E 0 7
 Some of the sources of interference found on a power pole

A
C E

B
G
F

Legend

A. Retaining nut and bolt head on cross arm G. Tracking between non-associated hardware eg possibly too close

B. Cross arm brace bolt head and over lap joint H. Earth wire lightly touching pole stay wire (stay wire not shown)

C. Insulator mounting post, loose insulator binder (tie) wire, and I. Dissimilar metals in contact causing corrosion
loose insulator
J. Wire over line
D. Loose shackles on bell type insulators (disc strain type shown)
K. Long tail on line connection
E. Loose hardware eg bolt head to associated washer

F. Loose clamps on insulator tops

PA G E 0 8
 04
INTERFERENCE LOCATION TECHNIQUES
FOR COMMERCIAL INTERFERENCE

THIS SECTION RELATES TO COMMERCIAL INTERFERENCE FROM UNINTENDED

RADIATORS, OR FROM INTENDED RADIATORS.

UNINTENDED RADIATORS Typical interfering equipment:

Motor controlled equipment, motor commutator

Some electrical equipment operates at high voltage
brushes, associated variable speed drives
or with high loads giving the potential to cause
(thyristor noise).
severe interference.
Microprocessor controlled equipment,
Preventing interference from unintended computers and other devices, air-conditioning,
radiators. conveyor control.
To prevent RFI from occurring, all electrical RF equipment, radio frequency welders, sealing
equipment must meet an appropriate standard or equipment, medical equipment.
specification as described in the relevant notices on
Switch mode power supplies, lighting, battery
www.rsm.govt.nz.
chargers.
Products must not be marketed or used in
New Zealand unless the equipment meets the
INTENTIONAL RADIATORS
designated specification:
(TRANSMITTERS)
Suppliers must hold a Supplier Code Number
and mark their equipment with compliance Interference from transmitters is primarily managed
labelling. through the licensing system by separation in
Equipment must be installed in accordance with frequency, location or time. This management
the required (usually manufacturer’s) installation process can fail where equipment becomes
specifications. faulty (spurious radiation) or where separation is
insufficient (co-channel).
Locating interference from unintended
radiators. Identifying spurious radiation or co-channel
signals.
Techniques for tracing the interfering noise are
similar to that for domestic interference but there The determination will be made from several
is often a higher radiated component and levels of observed factors.
interference tend to be higher. Spurious radiation interference usually occurs
Location in the urban environment presents where a fault condition exists while co-channel
additional challenges, particularly in respect of usually occurs when the equipment is installed on
reflected signals. Reflections and multiple standing the wrong frequency. A suitable directional aerial
waves may require detailed analysis before (correct band) and a good receiver is normally used
meaningful results are obtained. If the interference to direction-find these sources.
is especially difficult to locate, it may be helpful for
the tracing to be conducted concurrently on several
different frequencies.

PA G E 0 9
 Suggested steps in the location process: Co-channel signals.

Use RSM’s online SMART spectrum search These are same-channel transmissions which
utility (area search in particular) to help locate may occur when transmitters are incorrectly
or eliminate suspect transmitters. programmed, mobiles are operating outside of their
normal operating area or where they are located in
Use open reflection-free sites to conduct the
geographically high positions.
direction finding process where possible.
Watch for reflections and discard these readings. Check SMART online for cancelled licences that
Watch for receiver overload in areas of high may be still in use on the affected channel. A check
radio usage. with the previous licensee may identify this source.

Weak signals may initially require a visit to the This type of RFI may also occur during periods of
affected site and the use of sensitive but robust stable weather where a high pressure weather
receiving equipment. Filters may be needed system may be located across the country resulting
to prevent receiver overloading. in long range transmission of VHF and to a lesser
extent UHF signals. This is called anomalous
A good spectrum analyser may help to determine
propagation. It will usually last for the duration of
the nature of the signal. A small resolution
the high pressure system (may be several days)
bandwidth and narrow span is required for
and normally occurs in mid- to late-summer. Often,
weak signals.
trans-Tasman signals may be heard at the same
Listening to the modulation may indicate the time where interference is often received from high
nature of the offending transmitter where power FM broadcasting and TV stations.
you may be able to match it to the primary
Co-channel interference should not be confused
transmission.
with the normal shared channel arrangements
Narrow the area by triangulation and focus in on prevailing in simplex services. Where channel
the triangle. In urban areas, portable equipment sharing is in use, selective calling or tone squelch
will be required and reflections will likely be a systems may be used to provide user privacy.
major problem. Improper operation of those systems may be
confused by users as being co-channel interference.
Use buildings as “RF shelters” if they will help to
determine the direction of the signal.
Adjacent channel signals.
The direction and level of signals are the best
All receivers have limited ability to reject adjacent
tools to locate RFI, but there will be many false
channel transmissions when in close proximity to
readings to discard.
a transmitter on an adjacent channel. The ability of
Helicopters with fixed aerials are often the most receivers to reject adjacent channel transmissions
efficient way to track transmissions, especially in varies among equipment makes and models.
remote areas.
If several channels are used in the same band
Spectrum analysis may allow signal repetition simultaneously, like at port company premises
rates and modulation characteristics to be while unloading a ship, careful planning of
determined which can help narrow the type of frequency usage will help reduce the occurrence
offending transmitters. of interference.

PA G E 10
 05
INTERFERENCE AT RADIO STATION SITES

DETERMINING AND LOCATING THE SOURCE OF UNWANTED INCOMING RADIO

SIGNALS AT A RADIO STATION SITE IS LARGELY A PROCESS OF ELIMINATION. REMOTE
SITES MAY WELL PROVE A CHALLENGE WHEN THE SIGNALS ARE INFREQUENT AND
INTERMITTENT IN NATURE.

Key steps in the process are: 8. If on-site interference is determined, locate

the transmitters causing the problem. An
DETERMINE IF THE INTERFERENCE IS intermodulation calculation may help. Otherwise,
CREATED ON-SITE OR REMOTE FROM note which transmitters are on during the
THE SITE presence of the interference. Narrow the search
by observing the problem over a period of time
1. Use our online SMART spectrum search utility
sufficient to give confidence in the results.
to create an area search both wide ranging (eg
80 kms), and local (eg 1.5 kms). This information 9. Check the proximity of the transmit aerials to
is useful for comparison purposes, for Approved the receive aerials. Aerial isolation is critical even
Radio Engineers (AREs) and Approved Radio between widely spaced bands (eg UHF to VHF
Certifiers (ARCs). land mobile bands).

2. Determine if the signals are coming from the

radio station site or from off-site. This may LOCATING THE SOURCE OF THE
require connecting the radio site aerial to a INTERMODULATION PRODUCT
separate receiver or, if the signals are strong
This source will likely be a transmitter not fitted
enough, setting up a temporary receiving
with a circulator and filter, or a corroded joint
system on-site with directional capabilities.
enabling rectification and mixing of two or more
3. Try another receiver or other equipment as transmissions to occur. Generally the contributing
necessary to eliminate equipment faults. transmitters will be two or more on-site
transmissions though a strong off-site transmission
4. A sensitive spectrum analyser is a very useful
has been known to contribute.
tool for analysis of the parameters of the
incoming signal. Watch for possible overloading Site operators have been known to replace entire
of your receiving equipment. masts with a type that does not use bolted joints as
the most efficient way to solve ongoing problems.
5. Observe the nature of the interfering modulation
and its amplitude/time/frequency characteristics. Useful advice

6. If garbled or mixed modulation is heard it may 1. All transmitters must be fitted with filters and
be an intermodulation product created on-site. circulators to prevent intermodulation being
This is more likely at multi-transmitter sites. formed in the final stages of transmitters and
If no audible noise is heard the problem may retransmitted. This is a primary source
well be receiver de-sensitising (see receiver of interference.
de-sensitising on page 12).
2. It is possible to direction-find the source of the
7. Note any other transmitters that may be active faulty joint in many circumstances. The use of
when the interference comes on or goes off. sensitive receivers at multi-transmitter sites
requires the use of filters.

PA G E 11
 3. Look for any bolts or screws that may have receive signals. The affected receivers may suffer
become corroded. Lattice towers where joints a reduced sensitivity by 10 to 30 dB. It is not
are secured by bolts are often a major source of possible to audibly hear this noise and the receiver
rectification. Similarly, gridpack type antennas outwardly appears to be fine (receiver “hiss” is
that are held together with screws often cause apparent and appears normal). It is possible to
intermodulation. determine receiver de-sensitising by testing the
signal to noise ratio through the use of a noise
4. A connector or cable may have suffered water
bridge and signal generator.
entry causing internal corrosion.

NEAR AND FAR FIELDS

RECEIVER DE-SENSITISING
Radio wave propagation close to antennas should
FM noise is a major contributor to receiver
be understood. Signals behave differently close
de-sensitising. It often occurs at sites where there
to radiating antennas, where inductive principles
are high-power FM broadcast transmitter aerials
predominate rather than the electromagnetic and
close to sensitive land mobile receive aerials.
electrostatic principles of radiation as you move
The white noise created in high power linear away from the antenna. This may affect the
amplifiers raises the noise floor often over a wide measurement techniques needed to determine
range of spectrum and affects sensitive narrow signal strength and direction for interference
band FM receivers by reducing their ability to location.

Dipole near field diagram

A Current curve shown with I max at top of the curve

The way an RF field develops as it moves away from its transmission source is depicted in this drawing.

PA G E 1 2
ENGINEERING PRACTICES AT Notch filter
MULTI-USER SITES

Frequencies are engineered at sites on the basis of

sound radio frequency engineering practices being
employed. Where these practices are observed the
chance of on-site interference is minimised.

RSM recommends that multi-user sites comply with

specification AS 3516.2-1998 and that all contracts let
Notch filter plot
for site construction contain this specification as part
of the requirements. RBW 3 MHz Marker 1 (T1)
TG -20 dBm VBW 10 MHz 57.20 dBµV
Ref 91 dBµV *Att 10 dB SWT 100 ms 136.196163135 MHz
90
CROSS MODULATION
85

Cross modulation is a form of intermodulation where 80

the mixing occurs in the front end of a receiver.
75
Re-radiation does not normally occur from the
receiver. It is commonly noticed where mobile 70

equipment is not protected with circulators and/ 65

or filters.
60
At base stations, filters and circulators are normally
55
fitted and this prevents this type of interference.
50
This is likely to be the primary problem when
investigating interference sources. So a wise first 45

step is to try to eliminate the effect with filters.

Center 125.7961631 MHz 20 MHz/division Span 200 MHz
Remember to use only the minimum required gain in
any broadband amplifiers, as overload and mixing will
Small indoor and outdoor commercial filters are available
likely occur in the presence of a strong signal. to prevent interference to TV and FM receivers.

Consult your Master Antenna TV equipment supplier.

Typical UHF passband plot

CF7W-300/456
10
ch.36 (Fc1) 5 ch.62 (Fc2)
0

1: 1 2

-20

-30

-40

-50
3 4
-60
ch.33 ch.66 (Fs2)
-70

Start 228.000 MHz Stop 528.000 MHz

PA G E 1 3
 06
SUPPRESSION METHODS

ALL ELECTRICAL EQUIPMENT PRODUCES ELECTROMAGNETIC NOISE. ACCORDINGLY,

AWARENESS OF THE NEED FOR DESIGN AND INSTALLATION PROCEDURES THAT MINIMISE
THE NOISE IS IMPORTANT. WHERE THESE PROCEDURES ARE INADEQUATE TO ENSURE
EQUIPMENT COMPATIBILITY AND COMPLIANCE, SUPPRESSION BECOMES NECESSARY.

INTENDED RADIATORS Low frequency interference will require higher

value inductors and capacitors. Note that these
For intended radiators, good engineering practices components will likely be ineffective at high
will prevent and eliminate sources of interference. frequencies due to capacitive and inductive
Even radio frequency ‘intentional radiators’ like ISM reactance which is as a result of their size.
(industrial, scientific and medical) equipment are
required to be installed correctly and used in the Some products are difficult to suppress, eg

appropriate spectrum. thyristor-controlled devices and microprocessor-

controlled devices, as it is often difficult to
get close to the sources of the interference.
UNINTENDED RADIATORS Suppression may also affect the operation of
The following process and considerations should the device so check with the supplier before
be noted when suppressing these devices. proceeding. Caution! Electrical safety and
operational performance may be adversely
Before fitting filters determine that the installation affected by suppression components. Ensure
has not developed a fault condition, and that it has that the addition of suppression components
been installed correctly. Instability often occurs does not cause the device to become
where correct practices are not followed, eg RF electrically or operationally unsafe.
distribution systems act like random transmitting
aerials where correct matching is not employed Special attention to earthing and screening
consistently. when considering suppression of devices
capable of being contained in a metal housing
Check with the supplier/manufacturer as they is recommended. See the diagram on page 15
may have custom suppression components in relation to the current paths that have to be
available or have the fix already. considered. All these paths must be blocked and
In the majority of RFI product cases a filter will require a combination of both inductors and
made from RF-rated inductive and capacitive capacitors. The use of screened cabling may also
components may be employed. Select the be an essential part of suppression for equipment
components of the correct value and performance using mains frequency conversion (inverters and
and fit them in the correct place, eg for brush feed variable speed drives etc).
commutator motors the components should be RF sources often have a low impedance thus any
attached directly to the brushes with short wires. filtering must be designed with this characteristic in
Fitting filters away from the exact source of the mind. A capacitor alone will not usually be sufficient.
noise is usually ineffective due to radiation from
the wiring between the interference source and
the filter.

PA G E 1 4
 Even the most unlikely devices may be a major
AC mains suppressor
source of interference. For example, RSM
dealt with an interfering 121.5 MHz signal,
blocking the 121.5 Emergency Locator beacon
receivers aboard satellites some 850 to
1000 kms above the earth.

The source was a cumulative one, traced to

tens of thousands of satellite receivers used
in New Zealand which have a 121.5 MHz
oscillator on the mother board.
This Schaffner suppression socket is effective at suppressing
conducted noise from switch mode power supplies such as
in satellite receivers and similar devices. When fitted in a
complete metal housing that the supply is contained in it also
removes the radiated component.

Suppression of a commutator motor

B
A High frequency chokes

B Field windings

C Low frequency chokes

D D Suppression capacitors

Suppression of a brush motor. Note the

TVI chokes will be the most effective
suppression device when fitted right on the
A C brushes. They will also considerably reduce
commutator sparking.

Combined screening and suppression principles

B C A Interference source

B Electromagnetic screen

C Radiation is intercepted by screen

D RF filter or suppressor to prevent

currents from propagating along
supply leads
A
E RF currents returned to source
via capacitors

Method works for almost all devices capable

of having a screen placed around them
and filtering placed on all leads entering
the enclosure.

D E

PA G E 1 5
 07
SAFETY AND COMPLIANCE

NEW ZEALAND HAS PRESCRIBED REGULATORY REQUIREMENTS INTENDED

TO PROTECT PEOPLE AND PROPERTY (THE ELECTRICITY ACT 1992), THE RADIO
COMMUNICATIONS ENVIRONMENT (THE RADIOCOMMUNICATIONS ACT 1989) AND
OUR ENERGY RESOURCES (THE ENERGY EFFICIENCY AND CONSERVATION ACT 2000).
THE FOLLOWING ADVICE RELATES TO OTHER PERSONAL SAFETY ISSUES WHICH MAY
ARISE DURING INTERFERENCE INVESTIGATION.

The fitting of suppression components or working at such sites have taken part in a suitable
modification of equipment to reduce interference training course and wear RF radiation badges.
may cause non-compliance with those requirements.
The climbing of structures is not normally permitted
Always check that proposed changes will not cause
unless it has been prearranged with the site operator
non-compliance before commencing suppression
and the task is conducted by a qualified rigger.
or modification. Check with the manufacturer or
supplier if uncertain. Access to sites often requires appropriate training
in site procedures and safety requirements, and
When working at radio station sites you will be
the use of access tracks requires four wheel drive
exposed to radiation. Radiation causes heating of
skills and experience. Site owner permission
the organs of the human body and thus the level and
should always be arranged first, to ensure that any
duration of exposure should be minimised.
necessary safety information is obtained.
Determine the magnitude of this radiation before
entering the site and seek advice on areas within
the site where access is not permitted due to high
levels of RF energy. It is recommended that persons

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CONTACTS/BIBLIOGRAPHY

PHONE 0508 RSM INFO (0508 776 463) SANJAI RAJ GROUP MANAGER
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BIBLIOGRAPHY

1. http://strategis.ic.gc.ca/epic/internet/insmt-gst.nsf/ 10. Page-Jones, R, (1998), The RSGB Guide to EMC, Radio Society
en/h_sf06086e.html, Industry Canada, Guides on locating of Great Britain.
communications and domestic interference sources.
11. Moell, J, Curlee, T, (1987), Transmitter Hunting, Radio Direction
2. http://web.acma.gov.au/radcomm/publications/better_tv_ Finding Simplified.
radio/index.htm, Better Radio and Television Reception, detailed
12. Hare, E., Schetgen R, (1991), Radio Frequency Interference,
down-loadable booklet on locating domestic interference, etc.
How to Find and Fix It, Newington, CT, The American Radio
3. http://www.kyes.com/antenna/interference/tvibook. Relay League Inc.
html, Interference Handbook, USA, Federal Communications
13. Hutchinson, C. L., (1989), Radio Frequency Interference,
Commission.
How to Identify and Cure it, Newton, CT, The American Radio
4. http://www.rsm.govt.nz/cms/product-compliance/suppliers/ Relay League Inc.
standards-and-compliance-requirements, Standards,
14. Nelson, W. R., (1981), Interference handbook, How to locate and
Gazette Notices.
cure RFI: Radio Frequency Interference, Wilton, Conn, Radio
5. http://www.rsm.med.govt.nz, The Register of Radio Frequencies Publications Inc.
(SMART).
15. NZ Technical Correspondence Institute, (1981), Electrical Theory
6. http://spectrumonline.med.govt.nz/, public extract from the and Practice, Wellington, Government Printer.
Register of Radio Frequencies.
16. Rowe, F. D., (1961), How to Locate and Eliminate radio and TV
7. Standards Association of Australia, (1998). Siting of Interference, New York, John F Rider Publisher Inc.
radiocommunications facilities, AS3516.2-1998 Part 2: Guidelines
17. Priestly, B., (1972), Television Interference Manual, London, Radio
for fixed, mobile and broadcasting services operating at
Society of Great Britain.Whitaker, J., National Association of
frequencies above 30MHz, Standards Australia, New South
Engineering Handbook, 9th Edition.
Wales, Homebush.
18. Whitaker, J., National Association of Broadcasters Engineering
8. Loftness, M., (2003), AC Power Interference Handbook, USA,
Handbook, 9th Edition.
The American Radio Relay League (ARRL), Percival Technology.

9. The ARRL RFI Book - 2nd Edition (2007), Practical Cures for Radio
Frequency Interference, The American Radio Relay League Inc.

RSM offers this information as a Guide only and is not responsible or liable for any misadventure that occurs due to its use.

The paper used in the production of this document comes from a sustainable source and the bleaching process is environmentally friendly.
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ISBN: 978-0-478-31073-3 Print www.rsm.govt.nz
978-0-478-31075-7 Online
Printed June 2008