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Television Interference (Radio transmitter interference) This is oftein caused by the shortcommings of the TV set and the antenna. A good introduction to the subject is provided by that UHF AM Television is a wonderful example which illustrates many principles. What is presented here about TV also applies to other radio and nonradio equipment. In order to diagnose the exact cause of the problem the answers to the following questions will be of great help. This type of EMC , problem is a combination oftein of both Technical and Human issues.
1. When does the interference occur 2. Do you hear any weird or unexpected sounds from the TV set If you hear either human speech or music from the TV set then it is likely that the transmitter is using Amplitude Modulation . This is caused by a non-linear electrical connection within the TV set acting as a nonlinear mixer which Demodulates the AM. AM is still used for Longwave and Mediumwave broadcasting, and sometimes in communications equipment used by services such as the Fire Brigade , Military or Police . If you hear something which sounds like a weird animal quacking or is an unintelligable human voice then it is likely that the transmitter is using Single-sideband Modulation . SSB (single side band) tends to be used in applications where long range communication is needed, here the non-linear device is mixng frequencies but because the carrier has been supprressed it is not possible to restore the original audio with such a simple demodulator. 3. What does the picture look like 4. If a human voice is heard then does it use a Callsign Herring bone patterns are a sign of harmonics of the radio transmitter that are either radiated by the transmitter or are generated inside the TV set. For instance if a callsign can be found, then it is oftein possible for the authoritys to discover the person responsible for the transmitter. Beware that in the case of Pirate , Iliegal and poorly regulated radio services (such as CB Radio ) that operators are likely to make up callsigns at random which are then never registered with the state. Still even if you think that the call sign is a ''random'' ilicit one, make a note of it as it may help with the identification of the transmitter. You have a choice in many countries, you can either fix the problem yourself or you can contact the authoritys. In the UK contact . A COMMON METHOD Summary The owner of the TV set should do everything in blue, you might need help with the choice and use of the filters. You might want to ask the local aerial owner for help, ''bear in mind that they do not have to help you particularly if their equipment is not related to the interference you are suffering from, any help which they provide you with is out of the goodness of their hearts. Treat them nicely''. The owner of the aerial/transmitter should do everthing in magenta Search for the transmitter If you can look around the neighbourghhood for any new aerials or for any existing large or unusual looking antennas. It is best to approuch the residents at this address in a Polite and Reasonable way. It is best to go and visit them, and tell then the following.
Testing to see if a transmitter is responsible for the interference Please see Radio Transmitter Design for a discussion of common defects in transmitters. Also make sure that all aerials are installed correctly, check that Balun s are being used if required. Improving the resistance of the receiver to out of band signals It is important to make the system as simple as possible, additional gadgets such as external speakers on TV set offer new paths for radio signals to enter the receiver. The connections and cables associated with these devixes may in time need to be filtered or screened to imporve the imunity of the system. But at first they should be removed to make the system as simple as possible. Braid-breaker s If an electronic device with a long coaxial aerial lead is be disturbed by a signal from a HF transmitter then the braid of the coaxial should be considered as a path of entry of RF into the affected device. One additional feature which can assist a briad breaker is the use of a RF ground, a quarter wave length of wire will act as a RF ground even when it goes nowhere. This is because the quartwave transforms the infinite impedance into a zero impedance. By adding a Braid-breaker filter to the system common mode signals will be attenuated while unbalanced signals will be allowed to pass unhindered. This is also something which is added to long leads in Military electronic equipment which is intended to continue to function even after being subjected to a nuclear electromagnetic pulse ( EMP ) and also would increase the likelihood of an electronic device surviving a near-by lightening strike (direct lightening hits are very hard to protect against). For UHF TV a wide range of good designs exist, these include a ferrite RF choke on the downlead and some networks of passive components. RF Choke s on leads This is a well used method for filtering many leads on computer and commuications equipment. In common with the briad breaker the inteniton is to suppress common mode signals. One popular method of making a RF choke is to use a large ferite bead where the cable passes through the hole. Such chokes can oftein be seen on the power connection leads of laptops and on the video signal leads of computer mointors. The addition of RF chokes can oftein reduce greatly the RF current flowing through a TV set and hence improve the imunity of the installation. Filters in the aerial lead If the receiver's input is routed via a device which rejects the unwanted signals then this additional filtering can improve the imunity of the installation. It is the case that the additonal filter will improve the situation. When a transmitter needs to be colocated with a receiver where the frequncy difference is very small (''for example 145.000 and 145.600 MHz'') the Cavity filters can be used on both the transmitter and the receiver to improve the purity of the transmitter and the imunity of the receiver. One design for VHF/UHF TV which also has the ability to filter out 144 MHz, 432 MHz and 1296 MHz signals has been created by a Russian (UA9FAD). A design for UHF TV which is designed for the six meter band (50 MHz) has been pubsihed by the UK six meter group. [http://www.uksmg.org/filter.htm One design for FM radios (band II) has been created by a German (DF9CY) [http://www.df9cy.de/df9cy_bcbpf.htm] Rusty Bolt Effect It is possible that a ''horror'' known as the rusty bolt can be present. This is a situation where the output of a transmitter is clean, the receiver is able to reject the transmitter's frequency but some object is able to absorb RF energy and then emit it again with either Harmonics or Intermodulation products present. To solve this the best thing to do is to locate the ''rusty bolt'' and either remove it or render it harmless. OTHER TYPES OF EQUIPMENT Here is an overview of other items of equipment. FM radio sets (88 to 108 MHz) Treat the problem in the same way as for a TV. In the case of FM band II radios look out for signals close to the common Intermidate Frequency of 10.7 MHz entering the IF system and then being reporduced. One HF band used by radio hams is at 10 MHz. Mediumwave and Longwave radio sets Because of their low frequencies bear in mind that Harmonics are not likely, check for excessive Field Strength . Audio equipment Concentrate on the entry of RF into the device, when the source of the interference is below 50 MHz (six meters wavelength) then consider the leads feeding external speakers as a possible Dipole Antenna . As a first attempt at a cure, add a RF Choke to the mains lead near to the unit by using a snap on ferrite bead, or by winding the mains lead on a ferrite ring. Secondly add similar RF chokes to the speaker leads, these will reduce the RF current flowing through the audio equipment. 'Singing manhole covers' and the like If you hear a report of a singing or talking manhole cover, a fridge with radio 4 coming out when the door is open or some other object which you would never expect to act as a radio reciver then do not instantly think that some one is telling a Lie or that they are Insane . Likewise if you hear such a thing, stop and think for a moment: you may be perfectly sound in body and mind. Do the following 1. Stop and think calmly 2. Think, is there a strong AM transmitter anywhere nearby. Examples of this might include a national Longwave or Mediumwave radiotransmitter. If there is one nearby then it might be responsible for the audio effects. 3. Try to disturb the object which is ''singing'', for insatnce if it is a manhole cover then try lifting it to see if that stops the sound (make sure you do not fall down the hole). If you do not have the tools to lift it then you could try stamping your foot on the cover (make sure that the cover is not too weak to tolerate this treatment). When you put it back down (or stamp on it) you may have cured the problem by changing the way in which the two surfaces touch. If it is a talking fridge try moving it to a new site. What you are likely to have is a rusty bolt effect in a system where electrical energy can be converted into sound. |
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