MODEM/TELEPHONE RFI FILTER CONSTRUCTION




RFI Filter
Recent measurements show that: At 850 kHz. this filter attenuates RF by a factor of 6 times more than Radio Shack's HF filter. Their filter is identical to K-COM'S filter by the way.

At 1400 kHz. we get 7 times more filtering.

At 1.8 MHz it is 6 times better. The 50 turn version of this filter measures 8 times better (ie. there is 8 times less RF than with the Radio Shack (K-COM) filter).

At 3.5 MHz it is 6 times.

At 7 MHz this filter attenuates RF more than Radio shack's by a factor of 3 times

At 14, 28 and 50 MHz our filter works about the same as Radio Shack's.


The heart of the filter is the two section Nickel-Zinc Ferrite RF Super Choke which greatly outperforms conventional air-wound chokes.




Two toroid cores made of Amidon #43 ferromagnetic material are used to construct the choke. The toroids (Amidon #FT-50B-43)because of their dimensions, have the appearance of large ferrite beads.
#43 material is a Nickel-Zinc ferrite which has a very broad band of attenuation. Each core is bifilar (2 wires) wound with #30 enameled wire.


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 (Polycase P/N P-1521TX or Allied Electronic P/N 883-0900) and filters of this type work especially good on the higher HF bands.
When winding the individual toroids, 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

Also
If there are multiple pathways for the interference, just fixing one of them will not work. I have encountered this in the past. Sometimes you have to to filter the ac power line and the phone line simultaneously.
Of course, the choke doesn't have to be mounted in a box with connectors, to be effective. It can be installed in the wall behind the telephone outlet or installed inside of larger pieces of equipment.

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Is the radio enthusiast next door, or your neighborhood radio station, slowing down your modem and jamming your telephone? Then, you may need a K-Y RFI filter.

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