This article is primarily for amateur and General Radio Service (GRS, commonly known as CB) radio operators. It provides basic information to help you install and maintain your station so you get the best performance and the most enjoyment from it. You will learn how to identify the causes of radio interference in nearby electronic equipment, and how to fix the problem.
What type of equipment can be affected by radio interference? Both radio and non-radio devices can be adversely affected by radio signals. Radio devices include AM and FM radios, televisions, cordless telephones and wireless intercoms. Non-radio electronic equipment includes stereo audio systems, wired telephones and regular wired intercoms. All of this equipment can be disturbed by radio signals.
Interference usually occurs when radio transmitters and electronic equipment are operated within close range of each other. Interference is caused by:
1. Try to prevent interference problems before they happen.
2. Be sensitive to interference problems and try to solve them asquickly as possible.
Each part of a radio station must be installed properly and checked to make sure it meets all technical standards. GRS stations must use approved equipment that meets Industry Canada Radio Standards Specifications (RSS-136).
If you make any modification or adjustment to the installation, you should verify the station's technical performance again.
Antenna
The antenna can be a major source of
interference. If the length of the antenna
is not properly matched to the transmitter, it can sometimes generate
unwanted signals that are transmitted more effectively than the
desired operating frequency.
Location
To reduce the potential for problems,
the radio station's antenna should
be located:
Installing a transmitting antenna in an apartment or on a balcony is not recommended.
If you are using a vertical antenna,
choose a ground plane type that
has three or four radials. This will help minimize the signal
received in neighbouring homes. A mobile antenna, which
is designed to use a car's body as the ground plane, should
not be used as a base station antenna without an appropriate
ground plane.
A
good quality coaxial cable will convey the transmitter's radio
signal to the antenna with a minimum loss of signal strength.
At the same time, it will not radiate signals directly. Coaxial
cables are generally made of two concentric conductors
separated by an insulating material and covered by
a weather-resistant outer jacket. RG-8/U cable, approximately
one centimetre in diameter, is often used. To shield
the centre conductor properly, the braided metal sheath should have
at least 95 percent coverage.
All radio frequency (RF) connectors must be properly installed according to the manufacturer's instructions. Pre-made cables are recommended for GRS use. These cables already have their connectors installed and come in lengths that minimize problems from standing waves (or reflected waves). Standing waves reduce the level of the transmitted signal and make it more likely that the cable will transmit unwanted signals. In severe cases, standing waves may damage the transmitter or cause it to malfunction.
Location
Find out where other cables are located
before deciding where to install
the antenna and its feed line. The transmitting antenna's cable must
not run close to, or parallel to, nearby telephone, cable television or
television antenna cables.
Handle with care
To prevent standing waves, take care
not to puncture, flatten or bend the
cable too sharply when you are installing it. To prevent wind damage,
make sure the transmission line is properly secured so it does
not blow around. To prevent rain from penetrating the connectors
and cable, make sure all outdoor connections are well sealed.
Grounding
All radio station equipment must be
properly grounded, separate from the
ground for the house's AC electrical distribution system. If the grounding
is not kept separate, reflected radio frequency energy in the
form of standing waves can be conducted into the electrical wiring
of the building and into neighbourhood power distribution lines. Do
not overlook this potential source of interference.
The Canadian Electrical Code contains regulations concerning the grounding of telecommunication devices. You can obtain a copy of the code (Catalogue No. CSA C22.1-1990) by writing to the Canadian Standards Association, 178 Rexdale Boulevard, Rexdale, Ontario M9W 1R3. Tel: (416) 747-4044; Fax: (416) 747-2475.
Cabinet
Every radio device should be enclosed
in a box or cabinet, preferably the
original one designed for the apparatus. The cabinet provides proper
shielding of the electronic circuits and components. All parts of the
cabinet should be held firmly in place, with tightly fastened screws.
Power line filter
Radio transmitters designed for fixed-base
use are equipped with internal
power supplies that have filtering to prevent radio frequency energy
from entering the power lines. Do not use a mobile transceiver in
the home if it is powered by an ordinary battery charger. These chargers
are not normally equipped with power line filters and may allow
unwanted radio signals to be conducted into the power line of the
house. If you want to use a mobile transceiver as a base station, make
sure you also use a proper filtered power supply.
Microphone
Over-modulating a transmitter can result
in unwanted radio signals that
disturb adjacent radio channels. Be particularly careful if you are using
an amplified microphone'monitor and adjust the transmitter's modulation
level regularly. Some transmitters have built in modulation meters
which make monitoring easier. If your transmitter doesn't have a
modulation meter, you can use an oscilloscope to set the initial modulation
levels of an amplified microphone or the microphone gain of
a transmitter. The microphone
cable, like all metal wires, can also act as an antenna. If
the connection between the sheath and the connector has deteriorated,
or if the connection is corroded, the microphone may pick
up radio signals that will distort or reduce the quality of the transmitted
audio signal.
Standing wave ratio
meter
If you have a standing wave ratio (SWR)
or power meter installed on the
RF cable at the transmitter's output, the cable to and from the meter
should be as short as possible and of good quality.
If you experience interference or get complaints from your neighbors, you should first ensure that your radio equipment is operating properly. If the transmitter is the source of interference, there are three possible causes.
Poor installation of
the equipment<
Check that the antenna and all equipment
were installed correctly and
are still in good condition. Make sure your antenna is in a good location.
An intense transmitter
signal
Follow the steps described below to
test the components of your equipment.
If everything is working properly but you still experience
problems, consider decreasing the power level of your transmissions.
Unwanted signals
Any signal other than the desired narrowband signal is called an unwanted signal, or spurious radiation. Spurious radiation includes harmonic radiation. Harmonics occur when signals are produced at two or three times the station's operating frequency in addition to the desired signals. If the harmonics fall on another locally used frequency, such as a television channel, they are likely to cause interference. The diagram above shows how a signal from a GRS station may interfere with television reception.
The chart below shows the television channels that may be affected by various radio stations.
Affected
TV Channels |
Interfering Radio Station |
Affected
TV Channels |
Interfering Radio Station |
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54 | 174 | ||||||
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60 | 180 | ||||||
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66 | 186 | ||||||
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72 | 192 | ||||||
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76 | 198 | ||||||
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82 | 204 | ||||||
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How to check your equipment for unwanted signals
Follow the steps described below to:
- check your equipment for the source of unwanted signals; and
- correct the problem, if possible.
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1. Some transmitters may radiate unwanted signals from their cabinet or conduct them through the power lines. To check for signals being conducted through the power lines, test the transmitter using a shielded dummy load. The diagram at left shows the correct set-up for the test. If you still have interference while transmitting into the dummy load, the source of the unwanted signal is definitely the cabinet or the power line. Proper shielding and grounding may solve a problem in the cabinet. If the problem is in the power line, a power line filter should be installed. Various types of power line filters are available from radio supply shops and electrical parts distributors. The diagram at left shows how to construct your own filter. 2. If the transmitter set-up includes any auxiliary RF devices, such as a standing wave ratio (SWR) meter, switches or antenna tuners, temporarily remove them from operation. Test the equipment with no auxiliary RF devices installed. Test again as each device is installed. This method will help you find the exact source of the interference. If any device is the cause of the problem, try grounding it or remove it permanently. |
3. If
the RF devices are not causing the interference, try temporarily installing
a low-pass filter in-line between the transmitter output and
the antenna. A low-pass filter blocks all signals above its stated
cut-off frequency, with no change to the desired signal. For example,
certain models of low-pass filters allow frequencies
of up to 30 MHz to pass through to the antenna,
but they block (or greatly reduce) unwanted harmonic
signals. The diagram at left shows how to install
a low-pass filter.
If the low-pass filter reduces the interference, then the cause is harmonic radiation. Adjusting the transmitter's internal circuits may reduce the unwanted signals. You may need to call an experienced technician to make these adjustments.
Use
a spectrum analyzer, a calibrated field
intensity meter or a frequency selective voltmeter
to measure unwanted radiation.
The diagram at right shows the
proper set-up to use this measuring
equipment.
If adjusting the circuits does not solve the problem completely, it is recommended that you permanently install a low-pass filter in the transmitter antenna feed line after all the other accessories. Remember, use the shortest possible coaxial cables to link the transmitter and low-pass filter to the SWR meter, and make sure the filter is properly grounded.
Double-shielded cable, such as RG-214/U, or some types of RG-58A/U, may be used. However, because of its small size and flexibility, double-shielded RG-58A/U cable is sometimes preferred for interconnecting (patch) cables.
4. Take steps to reduce the signal level received by the affected electronic device. For example, try reducing the transmitter output, changing the antenna type or the location of the antenna. If a specific piece of electronic equipment is still experiencing interference, then the equipment likely does not have enough shielding or filtering to prevent it from picking up unwanted signals.
Coaxial cable: Round cable in which one wire is centred inside and insulated from a metallic jacket. RG-8/U is the most widely used for interconnecting a transmitter to its antenna; its impedance is normally 50 ohms.
Dummy load: A device used at the end of a transmission line to convert transmitted energy into heat, so no energy is radiated outward or reflected back to its source.
Field intensity meter: Device used to measure the signal strength (or field strength) of a transmitted radio signal.
Filter: Electronic device that lets desired signals be transmitted but blocks undesired signals.
Frequency-selective voltmeter: Device that measures a circuit's voltage levels at preselected frequencies. The instrument can also be connected to a calibrated antenna to measure signal strength.
Ground/Grounding: Connection to the earth by means of a low resistance conductor.
Harmonics: Undesired transmissions that occur at multiples of the original frequency. For example, harmonics of a station transmitting at 27 MHz (GRS) may occur at 54 MHz (frequency X 2) or 81 MHz (frequency X 3).
Modulation: Variation of the amplitude (voltage), frequency or phase of a radio wave, in order to carry information such as voices, pictures, music or data.
Oscilloscope: Device that shows the variations in amplitude (voltage) of a signal as it is being transmitted.
Radiation: Another word for transmission of radio waves into the air.
Sheath: The outside metallic braid of a cable.
Signal: Radio frequency (RF) energy which carries information such as voices, pictures, music or data.
Spectrum analyzer: Device that measures the frequency components of a radio signal. It provides a visual image of how the amplitude of a radio signal varies in relation to its frequency.
Standing waves: Undesired effects that occur when two or more waves of the same frequency are present at the same time. This may happen, for example, when the transmitter, transmission line or antenna are not properly matched to each other.
SWR meter: Device used to detect and measure the relative size of the standing waves in an antenna lead.
Transceiver: A radio transmitter and receiver combined in one unit, including a switching arrangement between the two.
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