Television interference
From Wikipedia, the free encyclopedia
Television interference (TVI) is a term for anything that can adversely affect the signal reaching the TV or the TV's ability to function correctly. It is usually an indicator of electromagnetic compatibility problems.
In this page only conventional UHF (or VHF) AM TV will be considered. Satellite TV tends to be FM TV and operates at circa 6 or 10 GHz (microwaves). While this page is mainly concentrated on UHF AM TV, many of the principles can be applied in cases where other devices are being troubled by poor reception. Also the advice on radio transmitter interference can be helpful in cases of non-radio equipment such as Hi-Fi units and stereos.
Many different things can cause TVI, a list of the main types of TVI are shown below.
[edit] Types of TVI
[edit] Strong signal
It is possible to also get a bad picture if the signal strength of the TV transmitter is too high, for instance in the Bromley area of south east London the signal from the TV mast is so strong that it may cause the TV's front end to be overloaded. Check for this by inserting an attenuator inside with the TV aerial connection. If you start by trying with 10 dB, and then move to 20 dB then this might provide a cure.
In that part of London the TV transmitter (Crystal Palace Transmitter) and the Croydon Transmitter tower which has VHF pager transmitters are both such strong sources of radiowaves that FM only UHF (432 MHz) and VHF (144 MHz) radiosets can become overwhelmed when they are attached to a beam aerial which is pointed at the Crystal Palace and Croydon towers respectively. The receiver of the FT-290R2 which is a 144 MHz multimode (FM/CW/SSB) radio is more able to cope with such strong out of band signals.
So this problem of overloading is not confined to TV sets only. One of the reasons these FM radio sets have this shortcoming is the fact that they often use two diodes which are wired across the front end of the front end. These two diodes act in the same way as the rusty bolt, the receiver then experiences a great array of mixing (intermodulation) products some of which fall upon the frequency which the radio is tuned to.
It is vital that before you attempt to start to locate an unwanted source of radio signals that you first check that the unwanted signal is not being generated in the front end of your radio set. By adding either an attenuator or a band pass filter to the radio its front end can be protected from these out of band signals.
