7c.1 Identify the
forms of interference caused by amateur radio and other radio
transmissions: patterning on the TV screen, loss of colour, voice on TV
sound, radio, telephone or audio systems. Recall that interference to
digital televisions is different; the picture may freeze, become jerky
or disappear.
There are various types of interference caused by amateur radio and other radio transmissions.
1. patterning on analogue TV screens
2. loss of colour on analogue TV screens
3. breakthrough of amateur sound on TV sound, radio receivers, telephones, Hi-Fi sound systems
4. Freezing, pixelation, jerkyness and disappearance of digital TV signals,
Patterning of the screen of an analogue TV caused by a nearby transmitter.
Pixelation caused by interference to a digital TV signal.
7c.2 Recall
other sources of interference and their effects: arcing thermostats and
vehicle ignition systems, electric motors in vacuum cleaners, fans,
drills, sewing machines etc, computers and peripherals. Recall that
this give rise to spots on TV or a buzz on sound radio. There are other sources of interference
1. arcing thermostats
2. vehicle ignition
3. electric motors in domestic appliances such as vacuum cleaners, drills, sewing machines, etc
4. Computers and their peripherals
The results of these types of
interference to analogue TVs are spots or white dashes on TV screens. On radios
they producing a buzzing sound.
7c.3 Recall that direct pick-up in affected devices tends to be independent of the transmitted frequency. The
pickup of interference by domestic appliances is often independent of
the frequency in use. This is because many electronic devices will have
diodes and other semiconductors that will demodulate the signal, just
like a simple crystal set. In addition because the interference is not
filtered, the interference may be picked up over a wide range of
frequencies.
7c.4 Understand
that masthead and downlead TV amplifiers are broadband, amplifying a
wide range of frequencies, including amateur frequencies. Understand
that this can result in overloading of the amplifier and/or the TV
input.' A
common cause of TV interference are the masthead and downlead
preamplifiers used in TV distribution systems. These are often
broadband devices which will amplify any radio signal, including
amateur signals. This causes the amplifier to be overloaded producing a reduction in the quality of a TV picture.
7c.5 Recall the
use of ferrite ring filters for minimising unwanted RF on aerial
downleads and mains leads to affected equipment.
Recall and understand the use of high-pass filters to reduce the level of HF and VHF amateur transmissions into TV systems.
Understand the use of mains filters to reduce RF, electric motor and thermostat interference to TV, radio, and audio systems.
Methods used to reduce the effect of amateur interference include:
1. Winding the coax to the TV round a ferrite toroid. This forms a
high impedance which reduces the amplitude of radio signals entering the TV.
2. Likewise interference entering via the mains cable can be reduced by wrapping the mains cable round a ferrite ring.
3. A
high pass filter placed inline with the incoming signal can reduce the
strength of HF and VHF. The higher TV frequencies, usually above 400MHz
will pass through, but signals below 200MHz will be attenuated.
4. Mains filters can be used to reduce RF, electric motor and thermostat interference to TV, radio and audio systems.
Ferrite rings placed at Y1 Y2and Y3 used to remove HF interference from the TV coax. X1, X2 ferrite rings used to remove mains radio interference.
In-line mains filter
AKD TVI RF UHF TV/VCR HIGH PASS INTERFERENCE FILTER
7c.6 Understand that transmitting into a dummy load is a good test for
any unwanted RF being conducted out of the transmitter along its power
supply leads and into the mains. To
discover if you are producing interference via the mains or via an
antenna, then one method is to connect your transmitter to a dummy
load.
If the interference disappears then it is being broadcast from the antenna.
If the interference remains, then it is being transmitted via the mains.
