Cordless phone consistently experiencing RFI

Question

I've tried several cordless phones (most recently KX-TG570), and all seem to frequently pick up terrible RF interference. I have a neighbor that is a ham enthusiast, with large antenna masts on the first rooftop, and it does seem to coincide with his use periods. I'd prefer not to bother him about it, but if I could solve it another way, that would be great.

I've tried wrapping the phone cord in a loop to act as a filter. I also asked IP about it, and they said I never upgraded to VOIP yet. Would that really help this case? I would think the cordless hand units themselves may pick up the RF interfernce and maybe not just the physical rj11 line.

I read a few other threads about internet interference, but in my case I'm more concerned about phone line RFI here.

Answer 1

First, let me say that most hams would be delighted to have such a polite neighbor! I would encourage you to contact your ham neighbor. Most hams are more than pleased to help neighbors with interference problems, rather than give the hobby a bad name. Also many hams that are active in HF (the ones with large antennas) are good at hunting down interference problems. If you would keep a record of the times that you have interference, your ham neighbor could compare those times to his log book, which would be a help for troubleshooting.

The interference is likely in the telephone line, or the power cable to the cordless phone base. The first thing that I would try is to get two snap-on ferrite beads (which can easily be found for sale with an internet search), one for the phone cable and one for the power cable, and then wrap each cable around a ferrite bead several times. Good luck!

Answer 2

If the interference is coming into your telephone via the RJ-11 cable, then the type of ferrite material you use to suppress the interference will matter. Engineers refer to the type of ferrite as the "mix", meaning what type of ferrite material is mixed to make up the device. These mixes are simply given numbers that are common place in the industry.

There are two mixes that will help knock down interference from ham radio and CB transmissions: type 75 and type 31. The type 75 material is excellent at suppressing frequencies in the 150 kHz to 10 MHz range. In ham speak, this would cover the 160 meter through 30 meter bands. The type 31 material provides broad coverage from 1 MHz to 300 MHz. This would include CB and the 160 meter to 1.4 meter ham bands.

Ferrite materials comes in a variety of physical shapes but the two that are most usable for your application are the toroid (doughnut looking) and the split sleeve type. Either type will work.

When installing the ferrite, plan on several turns through the core so select a core with a large enough hole for this purpose. The reason is that, in general, the effectivity of the choking action goes up as the number of turns squared. So for example, 4 turns is 16 times more effective than 1 turn. Space out the turns evenly on the core, wrap them tightly around the core, and do not overlap turns.

If you are not certain which core material to use, both can be applied to the same cord. Just place one after the other, winding each one individually. Alternatively, talk with your ham neighbor. Most hams are more than willing to help a friendly neighbor with any interference problems. At the very least, your ham neighbor can let you know what bands he or she is operating on so that you can select the best ferrite mix for the situation.

Answer 3

Good answer, but it should be noted that ferrite mixes are not created equal. The choice of the mix depends on the frequency of the interference. What works on the 80 meter ham band (3.8 MHz) might not work if the interference is on CB (27 MHz).

A great resource is K9YC's site and specifically, this PDF there. There is no better information on common-mode RF choke design anywhere, either online or in print.

Ferrite-core chokes almost always trump air-core chokes. See G3TXQ's excellent information here as to why this is true, and note how air-wound chokes have a much narrower effective bandwidth than ferrite chokes.

Answer 4

In my experience, the RFI in a telephone system almost always comes thru the RJ-11 cable which is acting as an antenna for the RF generated by ham operators or CB operators (or, anyone else doing similar things). Thus, filtering that interference using an RF Ferrite Core Chokes is the method to try.

Too bad Radio Shack no longer exists (or, at least all the stores in our area are long gone closing up last year sometime). Many years ago, when such things were actually cheap, I bought about a half-dozen RFI Filters sold at Radio Shack Stores. They have RJ-11 adapters on each end so you do not thread or wrap your cable around a ferrite. You merely plug your RJ-11 cable into one end and the short end cable on the other side (about 3 inches long) you plug into the base of your phone or base station. Radio Shack gave these a simple name, the only identifiable marks are the Radio Shack trade mark and then the name "Telephone Interference Filter".

I still have one left. I used them on my own phones that suffered from my own RFI and it stopped the interference cold without any problems even when I operated at about 1200 watts RF. My neighbors on each side of my house let me know I was getting into their phones so I went over and installed a filter for each of their phone base stations -- stopped the interference, no more problems.

These things work great by my experience.

I have searched on-line but could not find the exact product even though I did find a radio shack on-line catalog. I do know this. They are designed for filtering 3 to 30 MHz RFI (says so right on the filter) and they use a number 43 ferrite mix which I know to be common for the HF frequencies.

Judging by the dimensions of this Radio Shack filter, I am betting it contains one type 43 ferrite mix of the clamp around style (that RJ-11 cable passes thru) of about 1 inch long. I could open it up to confirm this but that would require destroying this rather well sealed container.

Answer 5

Talk to the Ham Op next door. We are more than happy to help cure a problem, and as a Ham myself, I have these Filters all over the place,"Hams have RFI also".

73 de kg0tr

Answer 6

Ferrites can certainly be effective in blocking RF. But so can capacitors which short RF energy to ground. Place them on incoming lines and anywhere rectification of the RF signal could be occurring. Both methods can be used in stubborn cases.