Simple RF filter for telephone line

This article describes a simple RF choke which was part of the solution to preventing HF RFI to a telephone modem that was "permanently" dialled to an ISP.

The modem (Maestro Jetstream) located on the same premises and the amateur radio station was subject to RFI when transmitting on the 7MHz band. Telephone services are delivered by aerial cables.

On inspection, it was discovered that the modem board appeared to have provision for RF bypass capacitors on the telephone line input circuit, but they were not installed. The manufacturers oversight was fixed. Although this improved the situation, it was still subject to an unacceptable level of interference.

The RF choke shown in Figure 1 was made by winding 10 turns of flat modular cable around a piece of ferrite rod. The coil was wrapped in a couple of layers of heatshrink adhesive tape and then heatshrink plastic served over that. The nylon ties held the assembly intact while the heatshrink was heated and the adhesive melted.

Figure 1: External view of the finished RF choke.

The result was effective, no further line dropouts being attributed to RFI.