art and science of interference hunting

8/11/2019 Art and Science of Interference Hunting

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The Art and Science ofInterference Hunting

Paul Denisowski, Applications Engineer



Art and Science of Interference Hunting

Paul Denisowski, Rohde & Schwarz

Topics

• Overview

• All about interference

• Interference hunting tools

• Identifying and analyzing signals• Directivity and propagation

• Importance of antennas

• Fundamentals of direction finding

• Q&A / discussion





Modulated sources

• Modulated sources are deviceswhich are intended to generate

RF signals.

• Problems occur when these

devices are faulty or are

operated incorrectly.

• Even signals from well-behaving

transmitters may produceinterference due to harmonics,

intermodulation, overload, etc.

• Because modulated sources are

meant to carry information,extracting this information can

help us identify the signal source.





Interference from modulated noise sources

• Since modulated noise sources usually have a bandwidth of < severalMHz or kHz, they appear as relatively narrow signals.

• These signals create interference either by being superimposed on

another signal or by generating harmonics, intermodulation, etc.

• Sometimes these signals will exceed their normal spectral limits due tomalfunctions (e.g. a broken Tx filter) or improper operation

(overamplification, frequency instability, etc.).

A narrowband intermittent

source interfering with TV

channel 48





Harmonics

• A harmonic of a signal is acopy of that signal

appearing at a whole

number multiple of the

original (fundamental)frequency.

• For example, a transmitter

at 155 MHz can produce

harmonics at 310 MHz, 465

MHz, etc.

• Always check to see if a

copy of the interfering signalis a harmonic of another

signal.The 4th harmonic of a signal at 195.25 MHz

appears at 781 MHz (LTE Band 13)





Intermodulation

• Intermodulation resultsfrom two or moresignals appearing in anonlinear circuit.

• Sum and differencefrequencies arecreated from themixing of

fundamentals andharmonics.

• Because intermodinvolves the mixing ofmultiple signals, it willonly occur when allcomponent signals arepresent.

Signals at 440 MHz (f1) and 445 MHz (f2)

produce intermod products at 435 MHz (2f1 –

f2) and 450 MHz (2f2 – f1) MHz





External rectification (“rusty bolt effect”)

• The junction between two pieces of metal can create a rectifier (diode),

especially when corrosion is present.

• This effect can generate spurious signals that are then radiated by

metallic elements in the joint.• Towers and guy lines are a

good starting point, since

they can rust, have long

metal elements, and areclose to powerful

transmitters.

• Utility poles/wires, metal

fences, and gutters arealso prime suspects.





Repeaters / BDAs

• Cellular repeaters or bidirectionalamplifiers (BDAs) can be used to

extend cellular coverage in

buildings or in fringe areas.

• May also be installed on boats.

• The main interference issues are

the retransmission of unwanted

signals at the input of the BDA aswell as malfunctioning BDAs.

• Difficult to troubleshoot but a very

common source of interference in

the cellular bands.

http://cell-phone-accessories-wholesale.com/images/products/detail/BDA1800603A.jpg





Umodulated sources

• Unmodulated sources are devices which unintentionally generateRF signals.

• Common sources are electric motors, faulty transformers, vehicle

ignition systems, electrical fences, fluorescent lighting, etc.

• Easy to recognize, often shows up as jumps in the noise floor or awide, random spectral pattern.

Noise in the aircraft

band (generated by

a nearby electrical

motor)





Deliberate interference

• Deliberate interference may be narrow-band (e.g. talking on a public safety

frequency) or broad-band (jamming).

• Pirate or unlicensed (“freeband”) operations

can also cause issues to licensed users.

• Sources may be mobile, possibly to avoid

detection / radiolocation.

• Although most businesses and individualsare very cooperative in resolving

interference, deliberate interferers will

usually deny or conceal their activities.Mobile GPS jamming

device





Recognizing jammers

• Jammers are typicallyeasy to identify and

locate : strong, broad,

always-on signal.

• Tend to increase the

noise floor even outside

of their nominal operating

range.





Interference Hunting Tools

• Two primary tools : spectrum analyzer and

the monitoring receiver .

• Two major differences between spectrum

analyzers and monitoring receivers :•Internal architecture

•Heterodyne principle

•FFT (Fast Fourier Analysis)

•Operational features

•Spectrum analysis functions

•Task-oriented features

•Direction finding

•Offline, remote, and coordinated

operation





Heterodyne principle

• The basis for most spectrum analyzers.• Input signal converted to an intermediate frequency (IF) using a

mixer and a local oscillator (LO).

• Signal is swept past a fixed-tuned filter to determine resolution

bandwidth.

• Signal is then logarithmically amplified and passed to the display.





What does a spectrum analyzer do?Make high accuracy measurements

of known signals.• User must specify a wide variety of

parameters and settings.

• Measure and display modulation

characteristics (e.g. EVM, ACLR).• Verify conformance to a standard

(e.g. SEM).

• Heterodyne (swept) principle : very

precise measurements, butinformation may be missed/lostduring a sweep.

• Usually connected to a cable, not to

an antenna.• Primary use is in a lab / production

environment.





What does a monitoring receiver do?

Makes rapid measurements ofunknown signals.

• Designed for speed and high POI.

• Signals can be demodulated and

monitored.

• Can make measurements at

discrete frequencies (scanning).

• Received signals almost alwaysimpaired/distorted, so features like

attenuation, AGC, preselection, etc.

needed.

• Can often be integrated into adirection finding system.





Understanding propagation

• A good knowledge of radio propagation is vital in localizinginterference.

• The distance at which a signal can be received is a function of both

power and frequency.

• Signals also have different characteristics based on frequency.

These include multipath, directivity and penetration.

• Spectrum refarming means that services traditionally found in one

portion of the spectrum may now be found in a different portion withdifferent propagation characteristics (700 MHz LTE, digital TV).





Directivity and penetration

• Generally speaking, higherfrequencies (GHz range) tend to beline-of-sight and more easily reflected.Penetration of signals into structures

can be poor depending on buildingcomposition.

• Lower frequencies (VHF/UHF) mayrefract or “bend” around structuresand these frequencies penetrate wellinto buildings.

• HF signals can propagate for greatdistances depending on ionsphericconditions. Ground-wave propagation

is also possible.• Naturally, penetration also is very

dependant on transmitter power.





Multipath• Multipath means receiving a

signal from differentdirections simultaneously.

• The severity of multipath is

also a function of the

frequencies involved.

• Caused by reflections, most

commonly in an urban

environment.• Multipath can make direction

finding difficult.

• Careful selection of

monitoring / DF sites canreduce the impact of

multipath.





Antennas

• Radio transmitters require antennas.

• Antenna design/size is related to frequency and function.

• Antenna direction also useful in identification.

• Even if an antenna is not in active use, it can act as a reradiator.

• Check the antenna site for signage and other clues as to owner/purpose.

• Antennas may be hidden or disguised.

Jammer disguised as

a pack of cigarettes

Cell phone repeater antenna

http://1.bp.blogspot.com/_89Wkw-wbK0s/S8rybF8obNI/AAAAAAAAN3I/ayRfm7IgZW0/s1600/DSC_0911.JPG





Identifying signals• Some of the more common ways of identifying signals are :

•Pattern of interference

• Audio demodulation

•Spectrum characteristics

•Signal analysis and digital demodulation•Online resources

•Direction finding

Signal splatter (overmodulation) Carrier drif t





Looking for patterns

• Important questions to ask in analyzinginterference :

•When does the interference occur?

•Is the interference constant orintermittent?

•Does the interference coincide with any

other events?

• If possible, attempt to see if interferencecan be eliminated by disconnecting or

powering down transmitters or other

electrical devices.

• For some types of signals, propagationwill change depending on time of day or

season.Outdoor sources may change

based on weather conditions





Audio demodulation

• Audio demodulation means listening to thesignal

• Modulation generally is AM or FM, but

there are variations of these (narrow band

FM, single sideband, etc.)

• Recording signals for later analysis /

documentation is useful

• What are we listening for?•Station ID / call signs

•Language and content (what are they

saying?)

•Morse code IDs

• Even digital signals can sometimes

identified this way





Spectral analysis

• The most fundamental display in interference hunting is a spectral

display (amplitude vs. frequency).

• A max hold function is also important in detecting short duration

signals or looking for an elevated noise floor.

Max hold (red)shows maximum

signal amplitude

for frequency and

also indicates

signals that arenot currently

present





Waterfall analysis

Duration of

jamming signal

exactly 9 seconds(T1 – T2)

Signal width of 870MHz signal is 1.4

MHz

• A waterfall display shows frequency, time, and level informationand is extremely useful in analyzing signals.





Signal analysis / digital decoding

• The ratio of digital to analog transmitters is

increasing at an exponential rate

• Signal analysis involves modulation type,

bandwidth, baud rate, etc.• Much easier to locate a digital signal if we

know what kind it is.





Online Resources

• The Universal Licensing System of theFederal Communications Commission (FCC)

provides access to a database of all licensed

transmitters.

• Searchable by location, frequency, call sign,etc.

• URL : http://wireless.fcc.gov

http://www.fcc.gov/





Example of ULS Results

Search results contain licenseinformation, contact address

and phone, transmitterlocation, power output, etc.





Direction Finding (radiolocation)

• If a signal cannot be identified

via demodulation or signal

analysis, locating its source is

often the only way to resolve

interference

• DF requires a receiver and a

directional antenna. Mapping

and triangulation software arealso extremely helpful

• Understand the strengths and

weaknesses of various DF

techniques and equipmentTriangulation of a transmitter based on

multiple DF bearings





DF equipment

• DF can be performed using fixed, mobile(vehicle) or portable (handheld) units, or a

combination of these.

• Fixed / mobile units are most useful during

initial hunting and when covering largegeographical areas

• Portable units are used best used for the

last hundred meters, as well as in cases

where vehicle access is not practical





DF antennas

• Directional antennas are a criticalcomponent in DF.

• Handheld antennas should have

good directionality and wide

bandwidth.

• Integrated compass/GPS allows for

greater accuracy when determining

location and bearings.• More advanced DF methodologies

such as Watson-Watt or Correlative

Interferometry usually require more

elaborate antennas, but providemuch higher accuracy.

Typical gain pattern of a

handheld DF antenna –

note the directivity

Vehicle mounted

interferometer

R di b i





Recording bearings

• For results to be useful, we need to record both position and bearing(azimuth) information.

• Traditionally, this information was calculated / recorded manually (e.g.

using a magnetic compass and a map/ruler) but this is prone to human

error.

• Modern equipment can use GPS and electronic compass data for an

automated determination.

An example of a handheld DF

antenna with an integrated electronic

compass and GPS receiver

T i l ti





Triangulation

• The main reason forrecording bearings is to

combine them in order to

locate a transmitter.

This is known astriangulation.

• Bearings are plotted

(manually or

automatically) on a map,and the transmitter is

near the intersection of

the lines.

Automatic triangulation calculation, including

calculated uncertainty radius

DFb t ti





DF best practices

• Rotate handheld antennas slowly.Tripod mounts can also help obtain

stable / reproducible readings.

• Binoculars or a telephoto lens are

extremely useful for finding and

identifying antennas.

• A good DF location is one away

from obstructions. Higher isusually better.

• Rooftops and top floors of parking

garages are good urban DF sites.

• Be aware of the influence ofreflection / multipath.

Q&A/Di i




P l D i ki R hd & S h

Q&A / Discussion

art and science of interference hunting

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