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Te Weraiti (Bay of Plenty) TOPO50: BD36 558.87 097.54 WGS84: 175.9072617E 37.8237783S
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What does interference look like?


Normal signal

Weak signals

Ghosting (Multi-path)

Radio Transmitter Interference

Co-channel interference (same frequency)

Electrical interference – power lines

Electrical interference – household appliances

 


Normal signal

A normal television picture should look like this:

[image] Normal signal.

 

Weak signals

Which services can be affected?

  1. UHF TV
  2. Digital TV
  3. FM radio
  4. AM radio

 

What does it look or sound like?

On television (analogue)

TV picture looks “snowy and grainy”.
[image] Weak signal on television (analogue). [image] Weak signal on television (analogue).

On television (digital)

TV picture disappears, locks, or goes into little squares

On radio (AM and FM)

Radio reception becomes noisy or distorted.

 

 

What causes weak signals?

  • The signal may be reduced by an obstruction blocking your antenna from the signal e.g. trees, hills, severe weather, etc
  • Multi path signals causing cancellation of the primary signal
  • You are too far away from the transmitter
  • Your television is faulty, or antenna system is a low gain type or faulty
  • On Radio, AM radio reception of weak and/or distant stations at night is prone to fading and distortion, caused by the simultaneous reception of both wanted and unwanted signal

 

What can I do about weak signals?


Ghosting (Multi-path)

Which services can be affected?

  1. UHF TV
  2. Digital TV
  3. FM radio
  4. AM radio

 

What does it look or sound like?

On television (analogue)

You will see multiple images i.e. images to the right or left of the primary image. The pictures are superimposed. Severe ghosting may result in your picture rolling or tearing.
[image] Ghosting on television (analogue). [image] Ghosting on television (analogue).

On television (digital)

TV picture disappears, locks, or goes into little squares.

On radio (FM)

A common symptom of this problem is a harsh edge or loud scratching noise distorting the sound, often accompanied by the stereo light flashing on the radio receiver. It may also cause a flutter effect on car radio FM reception when the vehicle is moving.

On radio (AM)

There are two types of effect noticed.  The first effect is where the audio sound fades away into a hiss.  The second effect is where the sound becomes very distorted and unpleasant to listen to.  In both cases the sound continuously varies between good and bad states.

 

What causes ghosting?

Ghosting happens when part of the signal comes directly from the transmitter, while another part has been reflected from a hill, a building or some other large object in the locality. The reflecting obstacle can be located in any direction from the antenna.  On AM radio, in the morning and evening, the changing ionosphere causes the audio to slowly drop in and out of phase causing the distortion.

 

What can I do about ghosting?

 

Radio Transmitter Interference

Which services can be affected?

  1. UHF TV
  2. Digital TV
  3. FM radio
  4. AM Radio

 

What does it look or sound like?

On television (analogue)

You will see a course or fine RF pattern completely covering the screen. The pattern will normally be diagonal but may vary between vertical and horizontal. These lines usually form a zigzag wavy pattern which may vary as the signal is modulated.

[image] Radio transmitter interference on television (analogue). [image] Radio transmitter interference on television (analogue).

On television (digital)

TV picture disappears, locks, or goes into little squares

On radio (AM and FM)

A common symptom of this problem is a buzzing, rasping type noise or you may hear distorted voices or other audio sounds, often accompanied by the stereo or signal light flashing on the radio receiver.

 

What causes RF interference?

Transmitters on the same or similar frequency to the one you are receiving i.e baby minders, cordless phones, Amateur or personal radio transmitter, oscillating amplifiers (like audio or radiating aerial amplifiers), etc.

At times you may receive interference from a powerful near by transmitter that is over loading your receiving equipment i.e taxis, carrier, etc. This effect is normally only temporary.

 

What can I do about radio transmitter interference?

 

Co-channel interference (same frequency)

Which services can be affected?

  1. UHF TV
  2. Digital TV
  3. FM radio
  4. AM radio
  5. Other radiocommunication services, including landmobile

 

Radio engineering processes determine the geographical separation which permits radio services on the same frequency to operate without interference.

These processes are based on calculations taking into account how well the frequency propagates (travels) through the atmosphere. For distances beyond line-of-sight, propagation generally depends on refraction and scattering in the ionosphere and troposphere which is sufficiently consistent to allow planning of VHF and higher frequencies.

However, vertical layering of moisture content and temperature in the atmosphere (inversion layers) can occasionally cause signals to travel hundreds or thousands of kilometres further than usual. An inversion layer (or duct) is most commonly observed over high pressure regions and may affect radio signals for several hours to several days. The phenomenon is commonly referred to as anomalous propagation and is more likely in hot, dry weather in late summer.

Disturbance to television viewing or radiocommunication services from this cause are observed infrequently in New Zealand and reception returns to normal as atmospheric conditions change. If you are experiencing reception problems, this mechanism should be one of the interference factors considered.

Disturbances due to anomalous propagation cannot be resolved through the normal interference location and resolution process. If co-channel interference due to anomalous propagation is harmful and persistent, the factor needs to be considered in the engineering process used by the radio transmitter licensee.
 

What does it look or sound like?

On television (analogue)

Usually evenly spaced horizontal bars of varying width over laying the whole picture i.e. the picture is usually still present behind the opaque bars. In severe cases the unwanted television picture will appear in place of the wanted picture.
[image] Co-channel interference on television (analogue). [image] Co-channel interference on television (analogue).

On television (digital)

TV picture disappears, locks, or goes into little squares.

On radio (FM)

With FM radios, and in particular radios fitted in moving vehicles, the primary signal suddenly disappears and is replaced by the unwanted signal. This is often noticed as both signals alternatively taking over from each other on a random basis as the vehicle is moving. There is often scratchy distortion preceding the change over.

On radio (AM)

With AM radios the interference appears as beat note or loud tone mixed in with the audio.

 

What causes co-channel interference?

  • Two or more television signals on the same frequency appear at your television receiver input. Note that there are hundreds of different television transmitters sharing the same frequency at hundreds of locations around the country.
  • Under certain weather conditions (often during hot dry weather in late summer) strong signals can be received from distant transmitters operating on the same frequency as your local station.

 

What can I do about Co-channel interference?

 

Electrical interference – power lines

Which services can be affected?

  • UHF TV
  • Digital TV
  • FM radio
  • AM radio

 

What does it look or sound like?

On television

 

It appears on the television set as two to three horizontal, speckled bands, which can drift up and down the screen. In severe cases it may remove the colour from the picture or cause the picture to tear and or roll.
[movie] Watch movie of electrical interference - power lines on television (analogue) [5MB mpg]. [image] Electrical interference - power lines on television (analogue).

 

On radio (AM and FM)

On the radio, the interference is characteristically heard as a buzzing noise affecting mains and battery operated radios. AM reception is more prone to power line interference than FM reception.

 

 

What causes power lines interference?

A voltage arc between hardware located on the structures holding up the power lines. At times this arc is due to a fault in this hardware, like a cracked or loose insulator. Weather conditions are often a factor in this type of interference, which may be continuous, or come and go during windy weather, and may cease after rain. It often affects a number of homes in the neighbourhood.

 

What can I do about electrical interference - power lines?

 

Electrical interference – household appliances

There are two primary types of interference created from house hold appliances. One is caused by electrical motors/thermostats and the other is from microprocessor/switch mode power supply controlled devices.

 

 

Electrical interference - Electrical motors/thermostats

Which services can be affected?

  1. UHF TV
  2. Digital TV
  3. FM radio
  4. AM radio

 

What does it look or sound like?

On television

The effect of the interference on your TV is a dense band of long, dash like, black and white or coloured flecks that can appear anywhere on the screen. It may be accompanied by a buzz in the sound from the television speaker.
[image] Electrical interference - Electrical motors/thermostats on television (analogue). [image] Electrical interference - Electrical motors/thermostats on television (analogue).

On television (digital)

TV picture disappears, locks, or goes into little squares.

On radio

Often a rough static or buzzing noise. Appliances using electric motors cause a whine or buzz at a pitch which varies with the speed of the machine. AM reception is more prone to appliance interference than FM reception.

 

 

What causes household appliances interference?

  • Television interference can often be caused by domestic electrical appliances, either in your own home or in that of a near neighbour.
  • Interference is also commonly caused by faulty thermostats in appliances. The interference occurs every time the switch contacts of the thermostat open or close. For this reason the interference occurs briefly (a few seconds to several minutes), then recurs after a much longer interval (for example, five to forty minutes).
  • The interference often occurs only when such an appliance is in use, so noting the times when interference occurs will help you with its identification. Note that many appliances have a standby mode and while they appear to be off are in fact still operating.

 

What can I do about electrical interference - household appliances?

 

Electrical interference - Microprocessors/Switch-mode Power Supplies

Which services can be affected?

  1. UHF TV
  2. Digital TV
  3. FM radio
  4. AM radio

 

What does it look or sound like?

 

On television (analogue)

The effect of the interference on your TV is an even pattern of black and white or coloured flecks which are often diagonal in movement but may vary form horizontal to diagonal in a random manner. At times the pattern may form into two wide bands. The pattern may vary between all effects over a period of time. The picture is normally present behind the pattern in all but severe cases where the picture may tear or roll.
[image] Electrical interference - Microprocessors/Switch-mode Power Supplies on television (analogue). [image] Electrical interference - Microprocessors/Switch-mode Power Supplies on television (analogue).

On television (digital)

TV picture disappears, locks, or goes into little squares.

On radio (AM and FM)

A subdued hiss, buzz or hum may be heard and the radio signal may appear to fade out.  On AM an audio tone may also be heard.
 

What causes household appliances interference?

  • Television interference can often be caused by domestic electrical appliances, either in your own home or in that of a near neighbour.
  • Television receivers, videos, DVD players, satellite decoders, etc all have microprocessors and switch mode power supplies and thus often the source of their own interference.
  • Sewing machines, bread makers, energy saver lights, freezers, washing machines, security systems, etc all use microprocessor and switch mode power supplies.

 

What can I do about electrical interference - household appliances?

  • The interference often occurs only when such an appliance is in use, so noting the times when interference occurs will help you with its identification. Note that many appliances have a standby mode and while they appear to be off are in fact still operating.
Last updated 24 January 2011