MODEM/TELEPHONE RFI FILTER CONSTRUCTION
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.
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?
San Diego, CA (USA)
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.
John K. Browne
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.