Transcript Radio Frequency Interference

Radio Frequency Interference
ARRL Book Section 3.5
Interference
• Hearing something or signals getting where not suppose
to
– Things like hearing the radio on your telephone or telephone
calls on your TV
• Sometimes get complaints that radio is interfering with
neighbors television
– Since TV and amateur radio use different frequencies that
should not happen
– Can happen if your radio is putting out off frequency signals or
TV has poor shielding and tuning and tunes in anything in the
area
• TV and telephone manufactures are notorious for this problem
• Doppler shift does not cause interference problems (T5D02)
• The major cause of telephone interference is that the phone
was not built with proper interference protection (T3D03)
• The most likely cause of telephone interference from a nearby
transmitter is that telephones components are making it act
like a radio receiver (T5D03)
Fundamental Overload
• Quality of device design measured when very
strong but off frequency signals in the area –
proper shielding should block
– Strong off frequency signals can interfere
• If I can reject 99% of stray signal but the stray signal is 100
times stronger than what I’m listening to I will have
interference
• Called Front End Overload when interference results
from a strong nearby source (T3D11)
• Can also be called Fundamental Overload when one is
talking about a receiving device getting interference
from strong but off-frequency signals (T5D01)
Cable Television
• Can get interference with cables with bad
shielding
– If there is a break in the cable Radio signals can be
picked up by cable line or cable signals can radiate
out and be picked up by Radio (T3D07)
• Obviously the cable company needs to fix their cable
• Interesting problem BPL – trying to send data
over bare power lines
– Lines radiate signals notorious for interfering with HF
– Called unintentional radiator
– FCC is responsible for seeing that people don’t put
out signals on someone elses frequency
• Bush administration was pushing BPL but FCC lost a case
and was ordered to protect other peoples frequencies
What To Do If You Get a Complaint
• Check you station to make sure it meets standards
for good amateur practice (T5D08)
– This would be checking for things like proper connections,
grounding, antennas
• Make sure your station is operating properly and that
it does not cause interference with your own TV (T5D05)
– If radio is not operating properly it can put out stray frequencies
or harmonics
– Your TV is probably much closer – if your radio is putting out TV
signals you should have the problem
• To stop spurious emissions you can install a filter at
the transmitter (T5A06)
Types of Filters
• High pass filters (block all frequencies below a certain frequency but
let high frequencies pass)
– I wonder whether that is why they are called high pass filters
• Low pass filters (block frequencies higher than a certain frequency
but let low frequencies pass)
• Band Pass filter (allow only frequencies in a certain range to pass)
• Notch Filter (opposite of a band pass filter – blocks only a certain
frequency range)
• Snap on Ferrite Choke
– One of problems is that coax cable or a rain gutter can start acting like
an antenna and send out harmonics of an intended signal
– Chokes reduce or stop that tendency
• All of these types of devices can be used to correct radio
interference problems (T5D07)
Who Is Responsible to Fix?
• If your neighbors TV is getting front end
overload from your radio it is the TV sets
owners responsibility to fix (T3D02)
• Often people don’t have a clue how to keep
radio waves from getting into their equipment –
the trick is filters that eliminate frequencies art
suppose to be monitored
– If manufactures didn’t short cut they would have put
them there in the first place
Things to try
• If a TV is getting front end overload
from a 2 meter transceiver try a notch
filter that blocks out 2 meter amateur
band (T5A07)
• If a telephone is having a radio
interference problem install an RF filter
on the telephone (T5D04)
What if Your Radio is Picking Up
the Interference
• Overload will often be bursts of tones or unwanted
conversations suddenly popping in (T9B03)
• FCC allows household devices to put out weak signals
on other users frequencies provided they don’t cause
interference
– These are called part 15 devices
• If you radio station is getting interference from a
neighbor’s part 15 device, check to make sure your
station meets good amateur radio design standards
(if someone is using your frequency with an unlicensed
device you can’t filter it) work with your neighbor to
identify the device, if problem cannot be corrected
politely inform him that FCC rules require him to
stop using the device. (T5D09)
Other Amateurs Are A Great
Resource in Helping You Track
Down Problems
• If another operator tells you that he
hears a high pitched variable whine
from your mobile the most likely cause
is the radio’s wiring is picking up noise
from the vehicle’s electrical system (T5D10)
– One of the reasons Chris Pixton dislikes my
plug the radio through the cigarette lighter
trick.