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| TVI - Television Interference Many of us have come across TVI at sometime. The lucky ones have their neighbour come knocking on the door, and the less fortunate, get a visit from the radio inspector! When you have this problem, something is not right and you need to fix it soon. No good saying, it must be the TV that is at fault! Although you might be correct in saying so, there is a lot that we can do to prevent this from happening in the first place. If you do not cause TVI on your own TV, there is a good possibility that it is not your fault. If that is the case, well then you have the oppertunity to still help your neighbour solve his problem! But first, let us look at some of the causes of radio and television interference. Whenever you make any changes to your antenna or equipment, always do a test and see if there is any TVI. An antenna which is not matched properly or a poor connection on your coaxial feed, or even moisture in the cable itself can all be a cause of severe RF and TVI interference. A low pass filter will not solve any of these problems, you need to trace the cause and fix it. With normal operation, it is wise to have a low-pass filter, so as to reduce normal radiation and harmonics from causing problems. A good earthing system is important, and also try using ferrite rings on any badly screened audio lines, such as on processors,phone-lines etc. High-pass filters are cheap to buy or you can make your own. And it would be wise to give that neighbour a call and put one on his tv. Believe me, having good neighbours is most important to any radio operator! Here are some tests you can do to find the cause: 1. Check the vswr of your antenna, which should be carried out each time you are about to operate. But watch it! Sometimes the fault can be intermitting because of a bad connection somewhere on your feedline. Check for corrosion in cable and connections. Fit RF choke and Low Pass filter. 2. Try using a different power supply, such as a battery. There is always the possibility of RF feedback through the mains power line. Fit mains RF filter and ferrite rings to power cables. Use High Pass filter on TV receiver. 3. Check all plugs for poor soldering or corrosion and look for moisture in the cable. Water or moisture in coaxial cable is one of the most common causes of TVI. Your VSWR meter reading might even give you quite a normal reading, so be sure to open up your connecting plugs and check. Replace coaxial cable and resolder all plugs. 4. Sell that high-power amplifier !!!! ... buy or make a better antenna. 5. Get rid of the rat's nest of wires behind your radios. Keep all connecting cables as short as possible. Have one good earth cable and use 1/4 wave radials for the freq/s you operate, as a counterpoise earthing. ...........You can certainly do without any of the last two items! IMPORTANT: Remember, if you have a low-pass filter inserted, it should be the last item in series between your radio and the antenna. Your VSWR meter, antenna tuner etc, must be inserted between the filter and radio. Also important is to keep all patch leads as short as possible. A good earth system with 1/4 wave "counterpoise" wires, for the frequency you operate, will go a long way to eleminate many TVI problems. Radio Note Book by Ben 43AX101 |
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