All about amateur radio EMI, interference and filtering to minimize. This relates to Section 29 of the NZART Radio Syllabus and may be used to teach this section of the exam.

1. 29 – Interference & Filtering29 – Interference & Filtering
• When you interfere with Broadcast Stations
• When you interfere with Television Stations
• When you interfere with Audio devices (Stereos etc)
• When you interfere with other Amateurs
• When you interfere with telephone devices
• When you interfere with garage doors etc etc

2. When your Signals InterfereWhen your Signals Interfere
BCI and TVI:BCI and TVI:
Interference is inevitable from time to time. You should face the problem whenInterference is inevitable from time to time. You should face the problem when
it arises and not worry about it beforehandit arises and not worry about it beforehand
Make sure You are “Clean” FirstMake sure You are “Clean” First
Make sure all the apparatus in your own home do not cause any interferenceMake sure all the apparatus in your own home do not cause any interference
with any of your devices in your home. You should be “Holier than thou”with any of your devices in your home. You should be “Holier than thou”
BCIBCI
The BC Receiver should have a RF Amp stage and be connected to anThe BC Receiver should have a RF Amp stage and be connected to an
outside Antenna. It is interference only when you interfere with an actualoutside Antenna. It is interference only when you interfere with an actual
station.station.
Audio Devices (Stereos – Radiograms – Record Players etc)Audio Devices (Stereos – Radiograms – Record Players etc)
These devices are not designed to be radio receivers. Courteously discuss theThese devices are not designed to be radio receivers. Courteously discuss the
matter with the owner and advice him to contact the supplier of the device ormatter with the owner and advice him to contact the supplier of the device or
his agent to have the deficiency cured. Courteously is the operative word.his agent to have the deficiency cured. Courteously is the operative word.

3. What kind of EMI have you got?What kind of EMI have you got?
 Troubleshooting an EMI problem is a three-step process,Troubleshooting an EMI problem is a three-step process,
and all three steps are equally important:and all three steps are equally important:
 Identify the problemIdentify the problem
 Diagnose the problemDiagnose the problem
 Cure the problem.Cure the problem.
There is the “Social” aspect of any Interference problemThere is the “Social” aspect of any Interference problem
You don’t want to be “The Neighbour from Hell!”You don’t want to be “The Neighbour from Hell!”

4. Either your neighbour or theEither your neighbour or the
RSM RI will contact you first.RSM RI will contact you first.
Don’t Panic! Check your operating activities against your logbookDon’t Panic! Check your operating activities against your logbook
It is a good idea to keep a logbook!It is a good idea to keep a logbook!
 Is it you? Have you changed antennas?Is it you? Have you changed antennas?
 Have you built a new Amplifier or changed modes?Have you built a new Amplifier or changed modes?
 What did your neighbour actually see or hear?What did your neighbour actually see or hear?
 Show “Concern” but do not admit responsibility.Show “Concern” but do not admit responsibility.
 Wait for tests to be conducted. Get all the details, time, date andWait for tests to be conducted. Get all the details, time, date and
 nature of the interference. Do not Worry!nature of the interference. Do not Worry!
Do not hesitate to contact the RSM by phone and it willDo not hesitate to contact the RSM by phone and it will
help to contact a local radio club member for advicehelp to contact a local radio club member for advice

5. Causes of InterferenceCauses of Interference
The Technical cause of interference may be:The Technical cause of interference may be:
A) At the Transmitter installation or,A) At the Transmitter installation or,
B) At the Receiver installation or,B) At the Receiver installation or,
C) Somewhere else or,C) Somewhere else or,
D) Combinations of all of those aboveD) Combinations of all of those above

6. Interference TestingInterference Testing
A) Have another Ham operate your equipment while youA) Have another Ham operate your equipment while you
communicate with him by a VHF handheld. Observe what happens tocommunicate with him by a VHF handheld. Observe what happens to
colour, sound, channels etc and note it in your notebook. Have acolour, sound, channels etc and note it in your notebook. Have a
cursory look at the installation of his antenna and feedline and notecursory look at the installation of his antenna and feedline and note
those details down. Be very non-committal with your communicationthose details down. Be very non-committal with your communication
to the neighbour.to the neighbour.
B) Substitute another TV set know to be TVI free in your location.B) Substitute another TV set know to be TVI free in your location.
C) Confine your tests to operational ones. Do not remove the back ofC) Confine your tests to operational ones. Do not remove the back of
his set.his set.
D) Keep your tests short. Do not offer an opinion. Withdraw andD) Keep your tests short. Do not offer an opinion. Withdraw and
consult textbooks and others for advice and decide on a course ofconsult textbooks and others for advice and decide on a course of
action.action.

7. You May Have HarmonicsYou May Have Harmonics
How do we get rid of Harmonics?How do we get rid of Harmonics?

8. Curing Harmonic InterferenceCuring Harmonic Interference
((Harmonic and Fundamental Overload Problems)Harmonic and Fundamental Overload Problems)
Your transmitter operates from 1.8Mhz to 30 Mhz in various bands.Your transmitter operates from 1.8Mhz to 30 Mhz in various bands.
The 2nd harmonics exist from 3.6Mhz to 60Mhz and 3The 2nd harmonics exist from 3.6Mhz to 60Mhz and 3rdrd
harmonics operate right up to 90Mhz. 4harmonics operate right up to 90Mhz. 4thth
, 5, 5thth
and 6and 6thth
harmonics areharmonics are
all over the television receiver frequencies. You need a device onall over the television receiver frequencies. You need a device on
your transmitter which “passes” Low frequencies (1.8 – 30 Mhz)your transmitter which “passes” Low frequencies (1.8 – 30 Mhz)
and is extremely “lossy” above 30 Mhz.and is extremely “lossy” above 30 Mhz.
This is aThis is a LOW PASS FILTERLOW PASS FILTER

9. Connecting a Low Pass FilterConnecting a Low Pass Filter
Your transmitter operates from 1.8MhzYour transmitter operates from 1.8Mhz
to 30 Mhz in various bands. Theto 30 Mhz in various bands. The
LPF (Low Pass Filter) “passes”LPF (Low Pass Filter) “passes”
your low frequency transmissionsyour low frequency transmissions
(1.8 – 30 Mhz) and is extremely(1.8 – 30 Mhz) and is extremely
“lossy” above 30 Mhz. In effect, the“lossy” above 30 Mhz. In effect, the
filter rejects anythjing above 30 Mhzfilter rejects anythjing above 30 Mhz

10. The TV Receiver operates from 50Mhz to 800 Mhz in variousThe TV Receiver operates from 50Mhz to 800 Mhz in various
bands. The trick is to reject the 2bands. The trick is to reject the 2ndnd
, 3, 3rdrd
, 4th, 5th and 6th, 4th, 5th and 6th
harmonics from the offending radio transmitter. You need aharmonics from the offending radio transmitter. You need a
device on TV receiver which “passes” High frequencies (50device on TV receiver which “passes” High frequencies (50
to 800 Mhz) and rejects low frequencies below 30 Mhz.to 800 Mhz) and rejects low frequencies below 30 Mhz.
The HIGH PASS TV FilterThe HIGH PASS TV Filter

11. The TV Receiver operates fromThe TV Receiver operates from
50Mhz to 800 Mhz in various50Mhz to 800 Mhz in various
bands. The High Pass Filters willbands. The High Pass Filters will
“pass” high frequencies (50 to 800“pass” high frequencies (50 to 800
Mhz) while rejecting lowMhz) while rejecting low
frequencies below 30 Mhz.frequencies below 30 Mhz.
Connecting a High Pass TV FilterConnecting a High Pass TV Filter

12. The Line filter is designed to operate from VLF toThe Line filter is designed to operate from VLF to
VHF Mhz and stop RF generated by the transmitterVHF Mhz and stop RF generated by the transmitter
going back into the mains power. RF is stopped bygoing back into the mains power. RF is stopped by
the low pass filter and shorted to ground.the low pass filter and shorted to ground.
Line FiltersLine Filters

13. What else can cause interference?What else can cause interference?
• Electric Blanket switches (They tend to regularly click on and off)
• Refrigerator thermostats (They tend to regularly click on and off)
• Washer and Dryer electronics (Tends to change pitch with various wash cycles)
• TV Sets (Generates a Line Oscillator signal around 15 Khz with harmonics)
• Some Garage Door electronics (Continuous signals on 1 or more frequencies)
• Poor Commutation in an electric motor (On continuously with motor on)
• Plasma TVs
Some Broadband noise generators to beware of!Some Broadband noise generators to beware of!

14. Types of FiltersTypes of Filters
Band Pass Filters reject frequencies outside
their bandwidth design parameters but pass
the band of frequencies it is designed for.
Band Stop Filters reject frequencies inside
their bandwidth design parameters but pass
all frequencies outside it’s bandwidth.
Band Notch Filters reject frequencies inside
their bandwidth design parameters but pass
all frequencies outside it’s bandwidth. The
Notch Filter has an extremely narrow
bandwidth and is usually designed for one
specific frequency

15. Active FiltersActive Filters
Filters can be active or passive. Passive filters are comprised of inductors and
capacitors and are used for the suppression of unwanted signals and interference.
Active filters use amplifying devices such as
transistors or integrated circuits with feedback applied
to achieve the required filter characteristics.
The “Operational amplifier” is an active device with
features making it particularly suitable for filter
applications up to a few megahertz. This diagram
shows a typical example.
Active Filters can have a very high gain but with
negative feedback applied, are usually operated to
produce a circuit with unity gain. The input impedance
to such a circuit can be very high.
These circuits are compact, and able to have variable
Q, centre, and cut-off frequencies. The circuit gain and
performance can be adjusted by changes to the
feedback network.
Op Amp Active Filter

16. Key Click FiltersKey Click Filters
Older transmitters often generated “Key Clicks” These were most annoying to
other amateur radio ops. RC filters were used to tame the key clicks.
In a CW transmission, the envelope of the keyed RF
output waveform may be as shown in this upper
diagram - a square-wave. When analysed this will be
found to be composed of a large number of sinewaves.
These sidebands may extend over an wide part of the
adjacent band and be annoying to listeners - a form of
click or thud each time your key is operated.
To prevent this happening, the high-frequency
components of the keying waveform must be
attenuated. In practice this means preventing any
sudden changes in the amplitude of the RF signal. With
suitable shaping, it is possible to produce an envelope
waveform as shown in the lower diagram.
Key Click Filter

17. We Are Done !We Are Done !