The New Zealand Association of Radio Transmitters Incorporated
Approved by NZART Council June 1982 (Revised 2002)
Interference to radio transmissions can be classified in several ways. The following is used for this statement.
A. Interference by other amateur stations to amateur transmissions.
B. Interference by amateur stations to other services.
C. Interference by transmissions of other radio services to the reception of amateur transmissions.
D. Interference by electrical / electronic apparatus other than radio communication transmitters to reception of amateur transmissions.
A. Interference to Amateur Transmissions by Other Amateur Stations
The Amateurs' Code (see annual NZART Call Book) sets the standard for operations by amateur stations.
Operator training by experience ensures a disciplined and controlled system of operating to ensure that each station gets its turn in a regulated way. High operating standards are expected in the Amateur Service.
Crowded bands lead to interference. Operating skill to avoid interference to others, and patience to wait until the frequency is clear, requires effort from all concerned.
Deliberate interference is sometimes reported. It may be unwitting and due to propagation conditions but if it is obviously malicious the action to take can vary with the circumstances. Orginators of such interference are usually seeking an audience so the best action is to ignore them completely. No reaction deprives the offender of satisfaction. Take note of the circumstances and date, time and frequency and the form of interference. Persistent offences should be considered by Branch Executives and suitable action decided. Some form of direction-finding (DF) capability should be available in each Branch. This is important to pin-point signals by triangulation or other techniques. A direct approach to the suspected originator of the interference should be made by Branch Executives or a group of amateurs. The originator may or may not be licensed, and if licensed may or may not hold a grade of licence for the band concerned. Tact and firmness with authority are needed in every case. If this approach fails, the local office of the Ministry of Economic Development (MED) should be informed and the NZART Administration Liaison Officer asked to assist.
A good receiver helps to minimise the effects of interference. Poor sensitivity (cannot hear the other stations so assumes a clear frequency), poor selectivity (hears stations on adjacent frequencies) are examples where the specification of equipment is important
The use of excessive power is a prime cause of interference particularly to some solid-state receiving equipment whose front-end performance may be inadequate. The minimum power needed to sustain a contact should be used in the interests of other operators and to minimise the impact of interference.
B. Interference by Amateur Stations to Other Services
This includes interference to audio devices, broadcast and television receivers and other electrical and electronic equipment. Reference should be made to the Code of Practice in Broadcast and Television Interference Cases available in the New Zealand Amateur Radio Study Guide In difficult cases contact the NZART EMC Officer for assistance.
Each case of interference by amateur stations to other services must be dealt with on its merit, based on the procedures in the BCI/TVI Code of Practice.
C. Interference by Stations of Other Radio Services to the Reception of Amateur Transmissions
Many amateur bands are shared bands, i.e. are shared by amateurs with other radio services. This is an expanding trend and more sharing to make optimum use of scarce spectrum is likely in future. Other services may legally operate in some amateur bands or amateurs in theirs depending upon the ITU and local Radio Regulations. The amateur bands are not identical in all ITU Regions or in all countries. The NZART Administration Liaison Officer can provide complete details on the situation pertaining to any particular amateur band.
Sharing by amateurs with other services may be on an equal, a primary, or secondary basis. The full significance of these arrangements can be found in the International Radio Regulations. With secondary sharing, amateurs are permitted to operate subject to not causing any interference to stations of the primary service.
In general, stations of other services operate on fixed frequencies and lack the frequency-agility of amateur stations to change frequency to avoid interference. It is incumbent on the radio amateurs to do the courteous thing and to move to another frequency so all can operate satisfactorily.
Some amateur bands are exclusive to the Amateur Service, world-wide. It is important that non-amateur stations in these exclusive bands have prompt action taken against them. Unfortunately there are historical and regulatory reasons why action has not been effective against some of these intruders. But all intruders must be continually recorded and reported. The bands available to New Zealand radio amateurs are shown in "Schedule 3" attached to each amateur radio licence. The Notes to this Schedule give details about the sharing of certain bands.
Intruder stations should be reported to the NZART Monitoring Service Coordinator so that further observations can be made and a report sent from the NZART Administration Liaison Officer to the New Zealand administration for action.
The NZART Monitoring Service Coordinator should be advised of cases where harmonics of transmissions from other services are identified in amateur bands. Care must be exercised in identifying harmonics in these cases. The simultaneous use of a second general coverage receiver with a different intermediate frequency (hence different image-frequency response characteristic) is recommended.
Interference may be experienced from a strong local transmitter (of another radio service) to the reception of amateur transmissions when the amateur's receiver is geographically very close to and hence in a strong field from the transmitter. This should be considered in the same way as BCI or TVI -but with the amateur on the receiving end. The usual investigations for receiver front-end overload, fitting of wave-traps, and so on should be followed. If these measures are not effective, seek assistance from the NZART EMC Officer
It is important when reporting interference from stations in another radio service that the source be positively identified. The callsign, location, frequency and transmission content of the offending station are usually required.
D. Interference by Electrical and Electronic Apparatus (other than radio transmitters) to the Reception of Amateur Transmissions
This interference originates from equipment that is itself not subject to the Radiocommunications Regulations. Examples are welders, motors, industrial and domestic appliances, digital electronic gear, light dimmers, fluorescent lighting, and so on -too numerous to tabulate.
Some of this equipment utilises radio frequency generators -known as ISM, Industrial, Scientific and Medical -on fixed frequencies. One example is 27.12 MHz. Typical applications are diathermy and RF heating. The specification and operating licence for such equipment may set the bandwidth and radiation limits.
Interference to amateur transmissions from these sources must be treated individually, there is no general cure. In persistent cases the NZART Administration Liaison Officer may be able to assist, but full details must be provided. The NZART Administration Liaison Officer is also able to provide assistance in cases where an individual approach to an Electrical Power Supply Authority has been unavailing.
Details of electro-magnetic interference and successful cures developed by radio amateurs should be sent to the NZART EMC Officer. Such experience is useful to other amateurs and to assist negotiations for improved regulations to cover the subject.
The radiation of harmonics from domestic television receivers and monitor sweep circuitry is a persistent source of interference to amateur operations on some bands. The radiation levels can vary from model to model and sometimes set to set. Again it is an every case-on-its-merits problem. Some radiation may be mains-borne and additional RF chokes (mains lead wound on a ferrite core) may reduce it. A braid breaker in the TV aerial lead may also assist. Shielding the inside surface of a TV receiver cabinet has been known to help. When you purchase a TV receiver you should question the supplier on this point and arrange for a selection of sets to be tried in the home. Pressure on manufacturers is needed to get them to heed the problem. In cases where excessive radiation levels are suspected, complaints to the local MED about a neighbour's TV set are justified.
NZART has appointed an EMC Officer and his advice on EMC problems should be sought. NZART supports: Better standards for the limitation of electromagnetic radiation. Firmer policing of the interference regulations. Speedier action in dealing with breaches of the regulations. More resources being made available to the MED to deal with the problem. Improved type-approval and testing of all devices with a potential for interference. In this regard, the standards and statutory provisions made in some European countries should be noted. To this end NZART commends and supports New Zealand involvement in CCIR, CISPR and similar bodies.
New Zealand Standards and Electrical Wiring Regulations refer to the fitting of suppressors to various appliances and devices with the purpose of minimising the radiation of or the transmission by the supply mains, of interference to adjacent radio receivers. The effectiveness of such devices is sometimes in doubt and additional suppression may be needed. Care in suppressing live mains circuits is needed. A radio amateur is not qualified to make changes to electrical mains wiring so assistance should be sought from suitably qualified persons. The value and voltage rating of suppression components must be carefully selected for safety reasons. For the level of permitted radiation refer to the various interference regulations and standards. The regulations and hazards of mains wiring must be respected.
It should be compulsory for motor vehicles with electric ignition systems to be subjected to radio interference radiation tests at the time of each warrant of fitness or certificate of fitness check. A vehicle found to not comply with the appropriate standard should be required to undergo servicing to correct the situation.
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