Radio Frequency Interference

1
Radio Frequency Interference

• ARRL Book Section 3.5

2
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)

3
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 Im
  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)

4
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 dont
  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

5
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)

6
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)

7
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 dont 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 didnt short cut they would have
  put them there in the first place

8
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)

9
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
  dont cause interference
• These are called part 15 devices
• If you radio station is getting interference from
  a neighbors 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 cant 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)

10
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 radios wiring is
  picking up noise from the vehicles electrical
  system (T5D10)
• One of the reasons Chris Pixton dislikes my plug
  the radio through the cigarette lighter trick.