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TV and Telephones Interferance

 
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TV and Telephones Interferance
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stylzmovement
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Joined: 11 Oct 2003
Posts: 142

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Post TV and Telephones Interferance Reply with quote
I will like to get some user experience on cleaning up TV and telephones interference caused by transmitters...
Fri Apr 13, 2007 3:38 pm View user's profile Send private message
Norwalk Electronics
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Joined: 01 May 2006
Posts: 7
Location: Connecticut USA

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Interference from transmitters to telephones almost always requires installing an RFI filter at the phone receiving the interference to correct the problem. At VHF frequencies grounding the transmitter has little effect on RF and is mostly for lightning. Filters at the transmitter only reduce harmonic content and also have no effect on telephone interference.

The problem is today's phones are very sensitive to RF and even FM transmitters can cause a severe hum on nearby phones. The wires going to the phone act as an antenna and pull the RF into the phone where it's rectified to cause hum with FM and full demodulation in many AM cases. You must decouple the wire from the phone as close to the phone or inside the phone.

In some cases simple snap on ferrite cores available from Radio shack will work. Other times you have to install a .001 mfd capacitor in parallel with the line on the phone side of the core/choke to provide a low impedance path to short the RF and force it to drop across the choke. In rare events some phones just refuse to cooperate in RF environments and are even discretely labeled to inform you they must accept interference. You may need to change the phone.
Fri Apr 13, 2007 10:10 pm View user's profile Send private message Visit poster's website
whiplash
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Joined: 17 Apr 2006
Posts: 19
Location: Australia
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Post Reply with quote
And TV reception interference is usally caused by harmonics. I found this out the hard way:

I forgot to check the local frequency guide for TV stations so I happily began broadcasting on 98.7MHZ. I noticed one of the commercial TV stations reception went crazy on my TV at home. I thought it was just because the transmit antenna was close to my TV.
After a while my friend rung me (lives about 5km away) and said that each time i began broadcasting this tv channel was was going crazy.
Checked the frequency guide and sure enough
QQQ9 Commercial 196.26MHZ
4ZR Radio Site Timbury Hills 7km E of ROMA

2nd harmonic of 98.7 = 98.7 x 2 = 197.4MHZ (TV as above 196.26MHZ, nuff said)
You can get a low pass filter (i believe) but this may or may not help. I had to change transmit frequency.

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Sat Apr 14, 2007 11:48 am View user's profile Send private message
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