The land line telephone line in most houses is now used for dual purposes – to carry traditional telephone voice service but also to carry DSL for internet connections. The long phone lines running in the walls of most houses act as excellent “antennas” for picking up radio frequency interference for any electronic device connected to them including telephones, cordless phones, answering machines, computer routers, etc.
Since the phone lines are physically long, they generally pick up lower frequency interference (< 10 MHz) better than higher frequency (>10 MHz) interference, but in some cases they pick up all frequencies due to strong signal overload.
Many “victim” devices have two ports of entry for RFI – the power supply cord (AC line in house also acts as an “antenna”) and the actual phone line wall connection. Both ports need to be protected otherwise the RFI may enter through the unprotected port. The ferrite filter should be installed on the wires as close to the victim device as possible. Simple snap on filters with multiple turns usually suffice to choke off the RFI. For plain old telephone service (POTS) phones which have the power and the signal in one line, select a single filter, on the phone line to the wall, for the RFI frequency range from the choices below.
Here are some examples:
High Frequency Snap On Filter – FSB31-1/2-5 turns for 10-30 Mhz
Low Frequency Ring Filter – FT240-43 – 14 turns for 3-14 MHz or use FT240-31 for 1.8-10 MHz or F240-77 for .1-15 MHz interference
If you know the interference frequency, choose the appropriate filter from the above examples – you will need two for each victim device – 1 for the power cord (AC or DC) and the other for the phone line connection to the wall. If you have interference from a range of frequencies (80-6 meter ham operation for example), then use a high frequency filter in series with a low frequency filter on the same lines (You will need for filters for the device – 2 on power line in series, 2 on phone line in series).
To determine the number of filters you need, count the number of devices getting interference and select the appropriate number of filters (either hi and low) needed. If you are not sure of the frequency of interference, use a single FSB31-1/2 snap on on the power line and another on the phone line of each device. If one is not sufficient to suppress the RFI, add another in series with the first on each line.
Individual snap on ferrites FSB31-1/2 are HERE or order the RFI-Telephone kit below which has a box of 10 FSB31-1/2 useful for up to 5 devices and also useful for other RFI affected devices.
Individual ring filters are HERE – remember to select the correct frequency range of the interfering RFI or just use F240-31 for general use and vary the number of turns for different frequency ranges.
Part # RFI-Phone (10 FSB31-1/2 and instructions) $50
Computer DSL/Cable Modem/Router RFI kits are HERE