Interference and Filtering

Question File Number 29

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Filters can be active or passive. Passive filters, comprised of inductors and capacitors, are used for the suppression of unwanted signals and interference. These are treated below. 


Active filters use amplifying devices such as transistors or integrated circuits with feedback applied to achieve the required filter characteristics. 

The operational amplifier is one such active device with features making it particularly  suitable for filter applications up to a few megahertz. This diagram shows a typical example. 

These 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. 


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. 

One means for doing this is a key-click filter as shown in this diagram. When the key contacts close, the inductance of the iron-cored choke prevents the key current from rising too suddenly. When the contacts are broken, the capacitor keeps the keyed current going for a short period. The resistor prevents the discharge current from being excessive. 

Note that the body of the key is at earth potential at all times - for safety reasons.



Radio transmissions can cause interference to other Radio Services and to nearby electronic equipment. Some Radio Frequency Interference (RFI) can render some equipment completely useless. 

The term Electromagnetic Compatibility (EMC), is the preferred title and reflects the need for all devices to co-exist together in the same electromagnetic environment. 

The responsibility for avoidance of, and the suppression of, interference to other Radio Services, is a Radio Regulatory matter is  considered in the section on   Regulations


This Interference and Filtering section will consider the causes of and solutions to common RFI problems - problems that arise when your transmitted signal gets into your own and other television receivers and other appliances. 

It is important, for domestic and for neighbourhood harmony, to be able to correct manufacturing deficiencies in consumer electronics. 

Filter Passbands

Filters form the basis of many RFI circuits. A filter is a frequency-selective circuit which passes signals of certain frequencies while attenuating others. Filters are able to select desired frequencies from undesired frequencies so they are fundamental to suppressing interference.


Typical measures of a filter are its cut-off frequency and its Q.

The cut-off frequency is defined as the frequency at which the signal will be reduced to half the power of the maximum signal passed. The Q (or quality) of a filter is a measure of how sharp the filter is. High-Q filters are those with a relatively narrow bandwidth, while low-Q filters have a relatively wide bandwidth. A filter's bandwidth is the frequency separation between cut-off frequencies.

This diagram shows the four common filter types. They are easy to recognise.

Low Pass filters exhibit the typical characteristic shown in 1.

High Pass is shown in 2

Band Pass is shown in 3

Band Stop in 4

These diagrams are for demonstration only. Practical filters exhibit considerable differences and more pronounced characteristics.


Broadcast And Television Interference

TV interference is of two types -

TV receivers which radiate spurious emissions and cause interference to the signals you are trying to receive on the amateur bands, and

interference which your transmissions cause to TV reception on adjacent television receivers.

It is the second variety that is the more important and the more difficult.

The text following is based on the NZART document: A Code of Practice for Radio and Television Interference Cases dated 1981, published in Break-In October 1981. 

A copy of the original document can be obtained from NZART Headquarters, P.O. Box 40-525, Upper Hutt or at  NZART Headquarters. Please give this reference to the month and year of this Break-In issue and your postal address. 


A Code of Practice for Radio and Television Interference Cases

1. Introduction

This is a guide for radio amateurs whose operations come to the attention of neighbours through disturbance to reception of sound broadcast and television transmissions (BCI and TVI). This disturbance is a continuing risk in amateur radio, and all radio amateurs can expect to cause or to be accused of causing BCI or TVI at some time. The interference is not damaging and the accusation does not bring any disgrace.

Interference between one radio service and another is inevitable from time to time, because all services share the one radio frequency spectrum.

You must face the problem only when it arises, and you should not worry about it beforehand. You should not fear a TVI or BCI report in any way or restrict your activities or hours of operation because a report may arise.

The best advice is this: ensure that the apparatus in your own home is free from interference caused by your amateur radio activities--and be active on the air. In all cases of interference, a cure is possible. Problems can be cured only as they arise. In reading this guide, which treats TVI in greater detail, bear in mind that in BCI cases you must take a similar approach.

The exact procedure to follow in interference cases cannot be laid down. Each case differs. Neighbours have been known to complain of interference after a radio amateur has erected a new aerial but before it has been used for transmitting. In other cases, neighbours have tolerated overhearing transmissions because they like to feel informed. Few hard and-fast rules can be offered.

2. BCI

Interference to broadcast-band receivers is often reported. The broadcast receiver cannot be considered to be of adequate design unless it has a radio-frequency amplifier stage and is connected to an outside aerial. An internal aerial or an aerial in the same room as the receiver is not to be accepted as satisfactory.

The amateur's transmissions may be able to be received at various points on the tuning dial, but the generally-accepted rule is that the case is one of interference only when reception of the local broadcast stations is disturbed.

3. Interference To Audio Devices

By some reports, an amateur's transmissions are heard from record players, stereo grams, and similar audio devices that are not designed for the reception of radio transmissions.

On receiving a report of interference to such an audio device, courteously discuss the matter with the owner, and advise him to contact the supplier or his supplier's agent to arrange for it to receive attention and to have the deficiency cured. These devices are not designed to be radio receivers.

4. TVI

The important point to remember about TVI is that it can be cured. Bear this point in mind at all times. TVI must be challenged head-on and a cure found for each separate case. Unfortunately, there may be no easy way or shortcut.

5 Preliminaries

When you start transmitting from a new neighbourhood or with a new rig, first ensure that your own television set is absolutely free from TVI. Then operate without any self-imposed restrictions of any sort. That is, operate when you want to, for as long as you wish, on any authorised band, with any power up to your legal limit, and with no disturbance to your own television receiver.

A radio amateur's first operating concern should be to ensure that the television receiver in his own home is disturbance-free. It should not display any interference.when operation is taking place on the frequented amateur bands.

Your television receiver is very close to the transmitter and its aerial. Having your TV receiver clean is important for several reasons, the first being that it promotes domestic or family harmony. Your receiver will be the subject of the first tests the MED RSM Official may want to make--and revealing a clean display on your own television set will incline him in your favour. Revealing a clean set can also help you to deal with neighbours who do not believe that the fault lies in their own installation. If your own TV set is not TVI free, therefore you should make it so.


6. The Wait

Do not ask the neighbours for TVI reports. Let the neighbours first report the matter either to yourself or to the MED RSM. Wait for the TVI reports (if any) to come to you-- they may never come.

7. Reports

TVI reports can come from several directions and in several ways.

The neighbour may contact you or a member of your family. An MED RSM Official may contact you. The report may be very complete, may be garbled or incomplete, may be casual, or may be second or third hand. Be sure you recognise a TVI report as such, and note it well.

8. Action Upon Receiving A Report

a. Do not delay. Attend to the matter promptly.

b. Check what you learn against your own operating activities and against your log. Have you changed bands, changed aerials, or built a new amplifier? Does the report coincide with changes to your installation or operating habits?

c. Check that any interference is in fact due to you. Be sure that it is not from a neighbour's new electric drill, arc welder, or other appliance, or from some other source.

d. Check with family members who view your own television set. Was any interference observed at the time claimed?

e. Show concern, but do not admit any responsibility for the interference at this stage. Wait until tests have been conducted.

f. Determine whether the MED RSM staff have or have not been notified.

g. Get full details of the interference, the time, the channel, and the nature of the interference on picture and on the sound. Has it just started, or is the problem of long standing?

h. Details of the model or type of television receiver, feeder, and aerial are also useful.

i. Start a notebook with date, time, and details of the report. Because even cases with big problems have small beginnings, start an accurate record early. You cannot be sure of the final outcome.

j. Above all don't worry.


9. When Should You Contact The MED RSM?

This depends on the nature of the TVI report reaching you, the degree of co-operation shown by the neighbours, and how well you know them. If the neighbour directs threats or abuse at you, or is not known to you, or claims that the fault is wholly yours, do not hesitate to notify the MED RSM by telephone.

You would be wise to be prepared to give a short history of any previous TVI problems you have experienced in this same location. Have you cured similar problems? This is where the notebook becomes useful.

If you do not show any TVI on your own set, continue to operate until the matter can be investigated.

If your neighbour is co-operative and is prepared to let you or a friend examine the set; then offer to do some tests to try to reproduce the conditions that gave rise to the interference. You may be able to cure the problem without involving the MED RSM staff at all.

Please be aware that the MED RSM may charge someone for their services. Make enquiries first to determine any costs involved and where their account is likely to be directed. This may depend on where the source of the interference is finally found.

10. The Cause

The technical mechanism or whatever generates the interference or disturbance must be established early to determine:

a. The cure necessary, and,

b. Who is responsible for affecting a cure, and,

c. Who is to pay any expenses involved.

Because tests must be carried out to determine this mechanism, the following are necessary:

a. Access to the television set for tests,

b. Operation of the transmitting equipment, and,

c. Someone with TVI tracing experience to decide which tests should be done, to carry out the tests, and to interpret the results.

This means that the radio amateur and the neighbour must be present for the period of the tests. That is, co-operation is necessary.

The tests may or may not be conducted by the MED RSM. They could be conducted by some other competent person provided the co-operation of the neighbour is assured.

Note that one or more mechanisms may be creating the interference, and so more than one cure may be necessary at any television installation. At any one transmitter site, the disturbance in adjacent television receivers may be generated by quite different mechanisms.


11. The Problems

The two problems that arise with TVI are:

a. Technical, and,

b. Social.

Few people will comment on which is the more difficult! The technical cause may be:

a. At the transmitter installation, or,

b. At the receiver installation, or,

c. Somewhere else, or,

d. Combinations of these.

12. The Tests

The first tests should be elementary:

a. Check the TV installation. Is the aerial in good order? Is it installed in accordance with accepted practice? Is the ribbon / coax in good order? Is a balun fitted? Is the aerial adequate for the TV field strength at the site? Is the aerial suitable for the TV channels received at the site? Check the suitability of the aerial mount. Check the joints between feeder and aerial elements. Do not assume that because a television aerial has been commercially installed that it will have been correctly installed. The requirements of a TV aerial to reject interference are more stringent than those for satisfactory reception when interference is absent. An aerial which gives satisfactory reception when installed may prove inadequate later when a source of interference comes into being.

b. Have another operator work the transmitter on the frequency from which interference is suspected. Note any disturbance to picture, colour, or sound. Make adjustments to accessible controls - fine tuning, contrast, and colour. Check all television channels. A VHF link to the transmitter operator is useful for co-ordination

c. Substitute another television receiver (perhaps a different model) and repeat the tests. Use a television set known to be TVI-free in a similar location.

d. Do not remove the back from the television set. Confine tests to operational tests, intended only to identify the nature of the disturbance, but try a high-pass filter (if available) in the television aerial lead if a quick diagnosis decides that this might help, if the neighbour agrees.

e. Obtain details of the set's make and model. Is it under guarantee? How old is it? Who supplied it? Is it under a service contract? Who maintains it? Is it a rental set?

f. Has an official from the MED RSM viewed the set? Does the MED RSM know of the problem ?

g. Keep the test short, make no promises, and do not give an opinion at the site. Withdraw, consult textbooks and other persons for advice, and then decide on a course of action.


13. The Rusty-Bolt Effect

High-pass filters (at the television receiver aerial terminals) and low-pass filters (at the transmitter) do not always cure TVI problems. Substitution of other TV sets can generally show if the cause is a faulty transmitter or faulty television receiver, but if substitution shows the interference effect to continue, then the cause becomes more difficult to establish. The rusty-bolt effect is one of the hardest of all these TVI causes to locate.

If a known clean transmitter is causing interference to a known good television receiver, then an external cause can be suspected. Perhaps the transmitter signal is being picked up by a local conductor such as a clothes-line or fence-wire. A rusty or corroded joint in this conductor may be acting as a diode. Harmonics of the transmitter signal could be produced by this spurious diode detector and re-radiated. These harmonics can be received by the television receiver and cause interference to the picture or sound.

Such interference may vary with the weather. It may be intermittent and be affected by wind as well as rain.

Typical offenders are metal-tile roofs, metal gutters and down pipes. A heavy blow with a hammer may sometimes correct an offending joint. Applying water from a hose can sometimes change or remove the interfering source and help to identify the culprit.

Either bonding or insulating the offending joint may solve the problem. More than one joint may be causing trouble. Bonding is generally impossible with metal tiles. Shifting the television aerial away from the offending harmonic source or sources is a more practical cure. A bonded wire mesh over the offending joint may be considered. It is unlikely that a complete metal roof will have to be bonded to effect a cure.

Bonding suspect joints can sometimes produce problems. With bonded conductors, a better signal pick-up may result, larger radio frequency currents may flow, and the problem may shift to another joint that was hitherto not suspect. Insulating the suspect joints may sometimes be more effective. A change to nylon guy-wires may sometimes eliminate problem joints.

The accepted rule is that if the offending joints are on the amateur's property, the problem is his. If the offending joints are on the property of the television set's owner the problem is his, Unfortunately, few set-owners understand this problem and so the radio amateur should offer technical assistance and advice. Re-siting the television set aerial or the transmitting aerial is often the only practicable cure.

14. Guarantees And Service Contracts

If the television set has been shown to be faulty and is under a guarantee or a service contract, then give the firm concerned early advice of the problem. This is best done after the MED RSM has been advised and the problem discussed. Advise the firm concerned that the MED RSM is aware of the problem. These actions are really the concern of the television set's owner, but the radio amateur may offer to assist.

15. Rental Sets

Rental sets should be treated in the same way as a set with a service contract. A rental set has the advantage that a change to another model may be possible, which could cure an otherwise difficult problem.

16. Getting Involved With Other People's Gear

As a radio amateur, you should be aware of the undesirability of agreeing to fix a neighbour's equipment. The equipment may be under guarantee, may be covered by a service contract, or may be rented. It may not belong to the person who is using it, who may not always be honest and forthright about ownership. Where to draw the line depends very much on how well you know the neighbour, and other factors, such as the age of the set, and the nature of the problem itself.

Your unauthorised tampering may invalidate guarantees and service contracts. Future problems with the equipment - in no way related to the interference problem - will without doubt be blamed on the radio amateur. No radio amateur wants to be concerned with the maintenance of his neighbour's equipment for evermore. The possibility of double-blame must be avoided (first the interference, and then of damaging the set).

Safety and Regulations are good reasons for keeping out of a neighbour's set. Many modern television receivers may operate with the chassis alive - at about half mains voltage. This also means that short lengths of coaxial cable inside the set (to the aerial isolation unit) could appear to have the outer at earth potential, but in fact this outer could be at a hazardous potential. Under the various Electrical Acts and Electrical Regulations, a radio amateur is not qualified to service mains-operated television receivers.


17. What Level Of Interference Is Tolerable?

Slight disturbances on a television test pattern which are barely noticed by a trained eye will not be seen on a television picture.

Disturbances of the same level as the noise on the picture, and less than interference from motor vehicle ignition, electrical appliance noise or aircraft flutter, are acceptable.

Tearing of the picture, herringbones, or switching between colour and black and white are unacceptable.

The last trace of TVI may be slight changes at the areas of intense red in a picture. This is acceptable for unless attention is drawn to it, it will be unnoticed.

Noises from an adjacent transmitter should not be heard during pauses in the television sound.

It is wise not to draw the attention of the owner to minor disturbances. Instead, check if he is satisfied with the quality of reception. The neighbour should be unable to tell when you are transmitting.

18. Contact With The Equipment Manufacturer

Where substitution of another set or other tests have shown that the fault is within a particular television receiver or other piece of commercial equipment, consider approaching the manufacturer of the equipment. When or how this should be done depends on the attitude of the local agent for the equipment, and the status of the guarantee or service contract. Ideally, the local supplier of the equipment should handle communications with the manufacturer in cases where a manufacturer's modification or the expense of a local modification arises. Again the radio amateur may offer to assist the owner.

If you approach a manufacturer, be certain to include details of model type and serial numbers, age of set, installation arrangements, tests conducted and their results, and any other details that will help in an analysis of the problem, diagnosis of its cause, and the development of a cure.

19. The Radio Amateur's Responsibility For The Cure

The radio amateur should accept responsibility for being the cause of TVI, only if carefully conducted tests have established:

a. That his transmitting installation is faulty, or,

b. That, in the substitution of another transmitter of comparable characteristics, the problem disappears, or,

c. That, in more than one adjacent television set, previously TVI-free, the same interference symptoms suddenly appear at the same time, and coincide with transmissions from the amateur's transmitter, or,

d. That a parasitic rectifying joint on the radio amateur's own premises is generating interfering signal components.

20. The Television Set Owner's Responsibility For The Cure

The owner or user of the television set must accept responsibility for curing the interference if carefully conducted tests show:

a. That no interference is exhibited on the radio amateur's own television receiver on the radio amateur's own premises, or,

b. That a high-pass filter or other trap device on the television aerial eliminates the interference, or,

c. That any parasitic rectifying junction is shown to be located on the property of the television set's owner or user, or,

d. That another television receiver substituted at the television installation fails to display the same interference, or,

e. That other attention at the television installation will cure the interference; for example, repairs to the aerial or feeder, or a shift of the television aerial to another position.

22. The Viewers' Choice

If a television viewer chooses to view television programmes on a defective set, or a set with a defective installation, he should not expect a radio amateur to cease transmissions to remove the disturbances to his viewing.


25. Terminology

Be careful with the use of words. An amateur transmitter does not interfere with or cause interference to television reception until properly conducted tests have clearly established that the fault is in the transmitting equipment or the transmitting installation.

A properly adjusted transmitter, radiating a clean signal, does not radiate interference or cause interference. Disturbances to television reception should not be described as interference if the television set has deficiencies in its design or installation that cause it to respond to signals from a clean transmitter.

A faulty television receiver or installation that responds to the amateur transmitter's clean signal does not exhibit interference - although this is the term often given to it (TVI). Reception is being disturbed is a better description.

If the transmitter is faulty and radiates energy that enters the television set at the television channel frequencies, then this is clearly a case of interference. The amateur transmitter is then radiating an interfering signal.

If the fault is at the television receiver, and the transmitter is blameless, then the transmitter cannot be said to be causing interference.

26. The Approach

Be tactful when explaining to a neighbour that his television receiver or installation is faulty. An explanation such as follows is satisfactory and typical:

You have a very good set. It displays each channel very well, with good crisp pictures and pleasant sound. Unfortunately, it also responds to signals not meant for it, and this means it is defective. Other sets in the area are known to be unaffected in this way ... By means of some tests, we can determine if the fault is inside the set, or if it can be cured by changes to your aerial, or if your installation needs a filter or trap added to the aerial lead. The punch line is it also responds to signals not meant for it, and this means it is defective. This should be carefully explained.

Contact with the neighbours may be by a visit, telephone, or a formal letter. The procedure to adopt depends how approachable they are, how well you know them, and where the TVI report came from, and how it was conveyed to you. There is a need to explain to the layman what Radio Frequency Interference (RFI) is, and what radio amateurs do.

27. No Guarantees Possible For TVI Cures

The possibility of a TVI report is ever present. Once a cure has been effected to a TVI case, there is no known way of ensuring that the same set will not again become subject to TVI at some later time, perhaps by other cause. Damage and corrosion takes its toll of aerials and earthing systems.

Sets age and become faulty. The radio frequency spectrum is a shared resource, and until we have new knowledge or techniques, all radio amateurs must learn to live with the possibility of a TVI case arising at any time and be trained in how to handle it when it does arise.

A radio amateur should not, and can not, give a neighbour a guarantee that a TVI cure just made will remain effective for any period.

28. Fitting Devices To A Neighbour's Set

It may be found that a high-pass filter, traps, stubs or other device fitted at the aerial terminals of a neighbour's TV set will cure disturbances to his viewing. It is important to leave a label or tag securely attached to the set, which gives reason for the installation of the device - otherwise the device may be removed by someone in the absence of an interfering signal because it has no effect.

29. Extra Assistance

Every NZART Branch should designate a member of its Committee as Interference Officer, his duties being to receive requests for assistance on BCI/TVI matters from members. He should have power to enlist other technically qualified members of the Branch into a team to help any member who needs tests, diagnosis, negotiations, advice, and other support until the case is closed. Amateurs should be seen to be united - this is important.

An independent expert third party may be acceptable to a neighbour in difficult cases.

Branches should be aware that the NZART Council is in a position to help with problem BCI/TVI cases, particularly where added technical assistance is required, or where an amateur is under pressure from a local dignitary or influential person. NZART Council has the route through the NZART Administration Liaison Officer available for official negotiations on behalf of a member if the Council deems them necessary.

Difficult technical or social TVI/BCI interference problems should be notified to NZART promptly.

30. Conclusion

Remember that all BCI and TVI cases are capable of being technically cured. All you need is patience to test, diagnose the problem, and work out a cure. Many good textbook and magazine articles have been published and are available. The problem is not yours alone. Other radio amateurs are available to help you, many having experience with the problem. The MED RSM are there to help. TVI and BCI are accepted as a part of radio life and technical progress.

The neighbour also has a part to play. Cooperation and patience are necessary. Don't allow yourself to worry, and don't allow your neighbour to think that you should stop your operations.


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Compiled Sun Nov 28 2010 at 8:41:44pm

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