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I have terrible telephone interference from 3.8 mhz ham radio transmissions I have tried ferrite beads and .01 caps all over the wiring and in the phones. Also just tried some K-COM modular filters. Not too sucessful. Why would your filters do a better job.?
John K. Browne wrote: Thank you for the inquiry. Our filter attenuates 3.8 Mhz RF, four to five times better than the K-COM single line filter and 10 times better than their 2 line filter.
However this does not mean it will be more successful in your situation because from your description it is possible that you have something else going on such as RF rectification.
.01 Caps won't help unless you have place to ground them. Putting them across the two lines won't help because this is usually a common mode problem not a differential one.
It is also possible that another piece of equipment connected to the phone line is picking up RF and rectifying it and then re-introducing it to the phone line as an audio frequency rather than a radio frequency. If this is the case, the source of rectification should be located and corrected. An RF filter will not help after rectification has taken place. For this reason, it is best to simplify the problem first by disconnecting all other equipment (answering machines, fax machines , other phones, alarm systems etc.) from the phone line and try solving the problem on one phone at a time with a filter.
Don't forget, equipment that plugs into the ac line can be getting RF from the ac wiring so you might need a good ac line cord filter.
Another common source of RF rectification are loose screws at wire terminals and at other times it has been traced to a defective lightening arrestor. All telephone house wiring is supposed to have a lightening arrestor. It should be located somewhere near the entry box. Also, some of the newer telephone line entry boxes or customer telephone interface boxes (ones with "MFG Siecor USA" markings on them, for example) have diodes in them to limit signal amplitude, and these diodes are known to rectify RF. If all else fails you might try having the phone company clip the leads on these diodes.
It is also possible that the length of telephone wiring in your walls or attic is a resonant length or multiple of a quarter wavelength at 3.8 mhz (roughly 62 ft. or 134 ft.etc.) or for some reason it is acting as a very good antenna and feeding a lot of RF voltage into your phone. These filters only attenuate the RF, so it is still present in some small quantity and some telephones are exquisitly sensitive to small amounts of RF. (Have you tried different telephones).
Ferrite beads are only good if you know what type of material they are made of and what the characteristics are regarding frequency of attenuation.
Usually even with the best materials, one bead will not do the job on 3.8 mhz unless you put a number of windings through the bead.
K-KOM use a ferrite torroid with approximately 35 turns of wire. Our filters use two large beads of #43 material each wound with 50 turns. I hope this information is useful to you.
Sincerely,
John K. Browne
John,
I saw your web page on construction of an RF filter for a phone line, but I have a couple of questions? To make a filter for a two line phone do you need to add two more lines to the twisted pair, or add in another set of magnets and have two sets of magnets and twisted line pairs? Does adding more turns of wire on the magnets have any effect? Ian
San Diego, CA (USA)
Ian,
Thanks for the inquiry. The torroids are ferromagnetic material but they are not magnets in the sense of a magnet that attracts metal.
I twisted four wires together and made 2-line filters that seemed to work well until we discovered that a person on one line had a problem with crosstalk from the fax machine on the other line. It seems that there is some coupling between the pairs when it is done this way. Also you can not get as many windings on a core because the twisted quad is thicker than a twisted pair.
A much better 2-line filter is made by using two choke sets made of 2 beads each. Also the size of the box can make a difference on the higher bands. A bigger box allows for wider spacing of components. On 10, 15 and 20 meters, the filters work better if the torroids can be spaced further apart than the box size specified on my page. The box, that I used to show the inside of the filter, is actually of the next larger size polycase box and filters of this type work especially good on the higher HF bands. As soon as I look up the polycase numbers I will post them on the filter construction page.
The distance between the beginning of the winding and the end of the winding should be as great as possible or you get coupling from input to output.
The capacitance between windings adds up and comes into play when using a large number of windings. On 20 meters, for instance, 38 turns works fine but if you use 50 turns, it works ok but not as good as 38 turns per bead.
The more windings, the better it works at lower frequencies, up to a point. 50 turns per toroid works super on the am broacast frequencies. 40 turns is adequate for 160, 75, and 40 meters. I hope this helps you some.
Sincerely,
John K. Browne
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