Identifying and Locating Radio Frequency Interference (RFI)
Introduction
With the plethora of wireless
devices, increasing broadcast, communications, and other RF sources all
competing for radio spectrum, the chances of radio frequency interference (RFI)
will only increase. This article explains how to identify, characterize, and
locate typical interfering sources.
CATEGORIES OF INTERFERENCE
There are two
broad categories of interference; narrow band and
broadband (Figure 1).
Narrow Band – this would
include continuous wave (CW) or modulated CW signals. Examples
might include clock harmonics from digital devices, co-channel
transmissions, adjacent-channel transmissions,
intermodulation products, etc. On a spectrum analyzer, this
would appear to be narrow vertical lines or slightly wider modulated vertical
bands associated with specific frequencies.
Broadband – this would
primarily include switch-mode power supply harmonics, arcing in
overhead power lines (power line noise), wireless
digitally-modulated systems (such as Wi-Fi or
Bluetooth), or digital television. On a spectrum analyzer, this
would appear to be broad ranges of signals or an increase in the noise floor.
Power line noise or switch-mode power supplies are the most common sources.
Figure 1. An example spectral plot from 9 kHz to 200 MHz of
narrow band harmonics (vertical spikes) riding on top of broadband interference
(broad area of increased noise floor). The yellow trace is the baseline system
noise.
TYPES OF INTERFERENCE
Some of the most common types of
interference are described below.
Co-Channel Interference – more than one
transmitter (or digital harmonic) using, or falling into, the
same receive channel.
Adjacent-Channel Interference – a transmitter operating on
an adjacent frequency whose energy spills over
into the desired receive channel.
Intermodulation-Based Interference – occurs when energy from two, or
more, transmitters mix together to produce spurious frequencies that land in
the desired receive channel. Third-order mixing products are the most
common and usually, this occurs from
nearby transmitters. An example of potential intermodulation might occur in
a strong signal area for FM broadcast.
Fundamental Receiver
Overload – this is normally caused by a strong, nearby,
transmitter simply overloading the receiver front-end
or other circuitry, causing interference or
even suppression of the normal received signal. A
common example is VHF paging transmitters interfering with receivers.
Power Line Noise (PLN) – This is a relatively common
broadband interference problem that is typically caused by arcing on electric
power lines and associated utility hardware. It sounds like a harsh raspy buzz
in an AM receiver. The interference can extend from very low frequencies below
the AM broadcast band, and depending on proximity to the source, into the HF
spectrum. If close enough to the source, it can extend up through the UHF
spectrum.
Switch-Mode Power
Supplies – Switch-mode power supplies are very common and are used
for a variety of consumer or commercial products
and are a common source of broadband interference.
Lighting devices, such as the newer LED-based lights or commercial agricultural
“grow” lights, are another strong source of interference.
Other Transmitters – There are several transmitter types
that commonly cause RFI:
·
Two-Way or Land Mobile Radio – Strong interfering FM signals may result in “capture
effect”, or over-riding of the desired received signal.
·
Paging Transmitters – Paging transmitters are generally very powerful FM or
digitally modulated transmissions that can overload
receiver Digital paging will sound very raspy, like a power saw or
buzzing, and may interfere with a
wide range of receive frequencies. Fortunately, most of the
VHF paging transmitters moved to the 929/931 MHz frequency pairs, so this is
not the issue it once was.
·
Broadcast Transmitters – Broadcast transmitter interference will have modulation
characteristics similar to their broadcasts – AM, FM, video
carriers, or digital signals.
Cable Television – Signal leakage from cable
television systems will generally occur on their prescribed channel
assignments. Many of these channels overlap existing over-the-air radio
communications channels. If the leaking signal is a digital channel, interference
will be similar to wideband noise (a digital cable channel is almost 6 MHz
wide).
Wireless Network Interference – Interference to wireless networks
(Wi-Fi, Bluetooth, etc.) is increasingly common, and with the proliferation of
mobile, household (IoT), and medical devices incorporating
Wi-Fi and other wireless modes, this issue is likely to get
worse. More details on wireless interference will be found in the companion
article, Wireless Network Interference and Optimization.
LOCATING RFI
SIMPLE DIRECTION FINDING (DFING)
DF Techniques – There are two primary methods for
DFing. (1) “Pan ‘N Scan” where you “pan” a
directional antenna and “scan” for the interfering signal, recording the
direction on a map, while keeping note of intersecting lines. (2)
“Hot and Cold” where an omni-directional antenna is used while
watching the signal strength. In this method, the rule of thumb is for every 6
dB change you’ve either doubled or halved the distance to the interfering
source. For example, if the signal strength was -30 dBm at one mile from the
source, traveling to within a half-mile should read about -24 dBm on the
spectrum analyzer.
DF Systems – Radio direction-finding (RDFing)
equipment can be installed into a vehicle or used portable. For vehicular use,
there are several automated Doppler direction-finding systems available. Some
examples include:
·
Antenna Authority (mobile, fixed and portable) www.antennaauthorityinc.com
·
Doppler Systems (mobile and fixed) www.dopsys.com
·
Rohde & Schwarz (mobile, fixed, and portable) http://www.rohde-schwarz.com
Step Attenuator – You’ll also find a step attenuator
quite valuable during theprocess of DFing. This allows control over the signal
strength indication (and receiver overload) as you approach the interference
source. The best models come in steps of 10 dB and have a range of at least 80
dB, or more. Step attenuators may be purchased through electronics
distributors, such as DigiKey, etc. Commercial sources would include Narda
Microwave, Fairview Microwave, Arrow, and others.
LOCATING POWER LINE INTERFERENCE
For Low Frequency
Interference – particularly power line noise
(PLN) – the interference path can include
radiation due to conducted emissions along
power lines. Therefore, when using the “Hot and Cold” method you’ll need
to be mindful that the radiated noise will generally follow the route of the
power lines, peaking and dipping along the route. The maximum peak usually
indicates the actual noise source. As a complication, there may be several
noise sources – some possibly long distances away.
Antennas – For simply listening to power line
noise, the built-in “loopstick” antenna on an AM
broadcast band radio or telescoping antenna on a shortwave radio may work
well. However, for tracking down power line noise to the source pole, and
typically for DFing other interfering sources, you’ll want to use higher
frequencies. A simple directional Yagi, such as the Arrow II 146-4BP (Figure
17) with three piece boom (www.arrowantennas.com) can be assembled quickly
and attached to a short length of pipe and works well to receive this type of
broadband RFI.
Use of VHF Receivers – Whenever possible, you’ll
generally want to use VHF or higher frequencies for DFing. The shorter
wavelengths not only help in pinpointing the source, they also make smaller
handheld antennas more practical.
Signature Analyzers – These are time-domain
interference-locating instruments that produce a distinct “signature”
of an interfering signal. This would
include instruments produced by Radar Engineers (Figure 2). They
are the best solution for tracking down power line noise and consumer
devices that produce repetitive noise bursts with known
periodicity.
Figure 2. A signature analyzer from Radar Engineers that tunes
from 500 kHz to 1 GHz and which displays an electronic “signature” of a
specific interference source. Receivers such as
this are used by professional
investigators to track down power line noise (photo courtesy, Radar
Engineers).
LOCATING NARROW BAND INTERFERENCE
For most narrow
band interference sources, such as co-channel, adjacent channel,
and intermodulation interference, the recommended tool is the spectrum
analyzer, as this allows you to focus on particular frequency channels or bands
and see the big picture of what’s occurring. Once the interfering signal is
identified, the analyzer can then be used to DF the signal.
USING SPECTRUM ANALYZERS
Spectrum analyzers display frequency
versus amplitude of RF signals. They can be helpful in determining the type and
frequencies of interfering signals, especially for narrow band interference.
There are two types of analyzers; swept-tuned and real time.
Swept-tuned analyzers are based on a superheterodyne
principle using a tunable local oscillator and can display a desired bandwidth
from start to stop frequencies. They are useful for displaying constant, or
near constant, signals, but have trouble capturing brief intermittent signals,
due to the lengthy sweep time.
A real-time analyzer samples a
portion of the spectrum using digital signal processing techniques to analyze
the captured spectrum. They are able to capture brief intermittent signals and
are ideal for identifying and locating signals that may not
even show up on swept analyzers. Most real-time
bandwidths are limited to 27 to 500 MHz,
maximum. The Signal Hound BB60C and Tektronix RSA306 are both relatively
inexpensive real-time spectrum analyzers that are USB-powered and use a PC for
control and display.
One important point to keep in mind
regarding the use of spectrum analyzers is that because they have
an un-tuned front end, they are
particularly susceptible to high-powered nearby transmitters off frequency
from where you may be looking. This can create internal intermodulation
products (spurious responses) or erroneous amplitude
measurements that are very misleading. When using spectrum analyzers in an “RF
rich” environment, it’s important to use bandpass filters or tuned cavities
(duplexers, for example) at the frequency of interest.
Spectrum analyzers are also useful to
characterize commercial broadcast, wireless, and
land mobile communications systems. For wireless or intermittent
interference, real-time analyzers work best. If used for
tracking PLN, it’s best to place the analyzer in “zero-span”
mode to observe the amplitude variation. Placing the analyzer in “Line Sync”
may also be helpful.
COMMERCIAL INTERFERENCE HUNTING
SYSTEMS
There are several manufacturers of
interference hunting or direction-finding systems. I’d like to describe four of
these, Aaronia, Narda, Rhode & Schwarz, and Tektronix. As
mentioned previously, for intermittent interference (particularly
for commercial communications
installations) or digitally-modulated signals, a real-time spectrum analyzer is
the best tool and has the ability to capture brief, intermittent, signals; some
as short as a few microseconds. Examples might include the Aaronia Spectran V5
series. Tektronix RSA-series, or Narda IDA2.
Aaronia – Aaronia not only has the lightest
portable system for Dfing, but the biggest and heaviest-looking. Their Spectran
V5 Handheld is the smallest real time analyzer. Mapping is not an option on
this model, but the larger Spectran V5 XFR PRO is a ruggedized laptop that can
use open-source maps and has triangulation features. Aaronia also has a variety
of affordable directional antennas and a combination
GPS/compass may be mounted on some models.
Figure 3. The Aaronia Spectran V5 handheld real-time analyzer is
the smallest self-contained unit and tunes from 9 kHz to 6 GHz. Other models
have upper frequencies of 12 and 18 GHz.
Aaronia is also unique in that
they’ve developed a drone detection system comprised of a 3D tracking antenna,
the model IsoLOG 3D with options from 9 kHz to 40 GHz in 360 degrees. This
matches up with their Spectran Command Center with triple LCD screens. See the
references for more information on that system.
Figure 4. The Aaronia Spectran V5 XFR PRO in the field portable
configuration.
Figure 5. The Narda IDA2 spectrum analyzer and interference
hunting system. The frequency range is 9 kHz to 6 GHz. Photo, courtesy Narda
STS.
Narda Safety Test Solutions – Narda has a similar interference
analyzer, the Model IDA2 with a real-time bandwidth of 32 MHz and frequency
range of 9 kHz to 6 GHz. There are a variety of directional antennas available
with built-in GPS and compass. This system also relies
on open-source mapping tools, such as Open
Street Maps (http://www.openstreetmaps.org). It is battery-operated
for easy portable use.
Figure 6. The mapping software with bearing lines drawn showing
triangulation of an interference source. Photo, courtesy Narda STS.
Rohde & Schwarz – Rohde &
Schwarz has a portable system (Figure 7) that can quickly identify most
interference sources and can also use imported mapping feature and GPS/compass
in the antenna to triangulate the interfering source. Several fixed, mobile, or
portable antennas are available for different frequency bands. This system also
relies on open-source mapping tools, such as Open Street Maps
(http://www.openstreetmaps.org). It is battery-operated for easy portable use.
Figure 7. The Rohde & Schwarz R&S®PR100 custom spectrum
analyzer with mapping and triangulation and R&S®HE300 antenna. The R&S®
FSH analyzer may also be used. Photo courtesy, Rohde & Schwarz.
Tektronix – Tektronix also has a means of
Dfing and mapping with their real time DSA-series
spectrum analyzers. The USB-controlled RSA507A is noteworthy due to it’s
built-in battery and portable capability. It also offers 40 MHz real-time
bandwidth. By connecting it to a tablet PC, such as the Panasonic Toughpad
model FG-Z1 and
with the Alaris DR-A0047 antenna, you
have a self-contained portable DF hunting tool (Figure
9). This system also relies on open-source mapping tools, such
as Open Street Maps (http://www.openstreetmaps.org).
Figure 8. The mapping application for the R&S® FSH analyzer.
Photo courtesy, Rohde & Schwarz
Figure 9. The Tektronix spectrum analyzer with
mapping/triangulation and Alaris DR-A0047 antenna. Photo, courtesy Tektronix.
Figure 10. When the SignalVu-PC
software with mapping option is connected
to one of their RSA-series real time
spectrum analyzers and Alaris directional
antenna, the compass direction is automatically shown, along with the spectral
display of the signal in question. Photo, courtesy Tektronix.
Tektronix provides
their SignalVu-PC with Mapping option to
help identify and capture interfering signals.
The mapping option allows bearing lines to be marked on the map to triangulate
the source of interference.
Figure 11. Flipping over to
the mapping option of SignalVu-PC, allows
you to record bearing lines to the interfering source, with the
triangulation showing the approximate location of the source. Photo, courtesy
Tektronix.
Summary
With today’s increasing use of
wireless devices, broadcast, communications, military and other RF
sources all competing for radio spectrum, the chances of radio frequency
interference (RFI) will only increase. With the proper tools,
broadcast and communications engineers are able to quickly
identify and eliminate sources of interference as they are detected. The latest
real-time spectrum analyzers make the job even more efficient.