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RID – Radio Interference Damper

  • Ungrounded Modular RID – P31050A
    Ungrounded Modular RID – P31050A
  • RID – Radio Interference Damper
  • RID – Radio Interference Damper
  • RID – Radio Interference Damper
  • Proper RID selection for the input (telephone line) side of the KSU depends on wiring configurations.  When RJ-11 jacks supply single phone lines, use single line RIDs.
    Proper RID selection for the input (telephone line) side of the KSU depends on wiring configurations. When RJ-11 jacks supply single phone lines, use single line RIDs.


“Reduces annoying AM, FM Radio Frequency Interference (RFI) on your telephone”

UL Listing 100902

RFI occurs in the vicinity of a radio station transmitter tower. Generally, the telephone wire acts as an antenna that picks up the radio signal. The signal is then demodulated (changed to audio frequency) by non-linear components such as varistors, transistors, and data or communication line.

Ungrounded Modular RIDs

Here are the frequencies that are most often the source of problems on phone systems

  • AM Radio (.5 to 1.5 Megahertz)
  • CB & Amateur Radio (3 to 30 Megahertz)
  • CB & Plastic Sealing Equip. (26 to 60 Megahertz)
  • FM, Air & Business Band Radio (80 to 150 Megahertz)
  • Welding Noise (500 Hz to 150 Mhz)

First Things First – The Basics

The nice thing about these RIDs is that they are modular and only take a second to plug into the phone in many cases.  On some jobs, you may also need the RIDs at the dslam.  (See diagrams.)

  • Ground the spare pairs in any house cables you are using - this includes cables running between buildings.
  • Remove unused cables from the frame - if you are having RF problems, prewires will often cause problems by acting like an antenna. Try a longer or shorter mounting cord or handset cord – simply retuning the antenna to a different length will often help.
  • Before ordering RlDs, find out where the RFI is entering the system – through the CO lines, station cables or handset cords.
  • Before ordering RlDs, find out the frequency of the station you are hearing. This is very important – what works for one frequency won’t do anything for another! Ask the customer what station it is – often he’ll know only too well. Be careful of broadcast stations that are simulcasting on AM and FM, it gets confusing (call the station to check).

AM and FM Broadcasting Stations

This is the most often encountered form of RFI.  Luckily, it’s easy to fix.


Headsets seem to have a real affinity for picking up AM or FM radio stations.  You can be sure that you need one of our RIDS if you hear the RFI on the headset, but not the handset -  this is the most common scenario.  Every brand of headset seems to be equally susceptible to the RFI.  Just plug a RID into the handset jack on the phone and plug the headset adapter box into it.  We are at a 100% success rate with this problem.

If the phone or console has two jacks for the handsets (like for training), be sure nothing is plugged into the second jack as it can also pick up the RFI – even if you don’t hear it on that handset or headset.  If you need to have both jacks used, you may need a RID for each handset/headset.


A high percentage of RFI problems come in through the handset cord. There are two ways in which you can tell there is RFI coming in through the handset cord. First, move around with the handset, or bunch up the handset cord so that it is very short (3 or 4 inches). If the RFI gets louder or softer our RID should help. Make sure you know whether it’s AM or FM RFI!

Second, if the telephone has a monitor or speakerphone, listen for the RFI through the speaker. If you don’t hear any, it must be coming in through the handset cord.

CO Lines, Modems, Station Cables

If you hear RF on the CO lines at the Nl with a Butt Set, you have to get rid of it there before you go any further. Modems, fax machines and other single line devices may be more prone to RFI than other equipment (like headsets are more susceptible than handsets.)  This is due to the internal design of the device. RFI can cause corruption of fax and modem data and irregular function of special features on electronic telephones (all of these symptoms can also be caused by high or low loop current on the telephone line which can be corrected by a Loop Current Attenuator.)

It is possible that as a result of poor design or component failure, a telephone device can be the cause of interference.  This device may continue to operate even with a malfunction that causes interference to other devices on the line.  For this reason, it is important to “simplify” your diagnosis by putting a phone at the dslam )or NI) and stripping the system or line down as far as possible.  Replace the cards or punch down the cables one by one until the trouble reappears (standard troubleshooting procedures for system problems).

If you have another device of the same type you can try swapping them with each other to see if the trouble follows.  For maximum interference rejection, make sure your system ground is of the highest possible quality.  Proper telephone installations establish system ground at either the Protector Box or Service Entry.  Grounds other than known-to-be conductive cold water pipe, building frame ground or driven ground rods should be considered suspicious.  Good grounding goes a long way toward shielding your telephone wiring from intruding RFI  An upgrade in system ground at some sites can eliminate telephone RFI.  At other sites, a better ground can reduce interference noticeably requiring less work and/or fewer RIDs.

Beware of speaker or thermostat wire, mic cable or anything other than telephone cable. Old style Red/Green/ Yellow/Black “Jake” has no real twists to it, so it has no inherent interference rejection. Sometimes (rarely) picking other than a normal pair, like R/B or G/Y will help a little, both for RFI and crosstalk problems. A radio station prewired their own building for telephones using shielded audio cable right under their transmitter tower. Because there were no twists in the cable like telephone wire, every phone had loud RFI that was impossible to eliminate. You get best rejection with Level 5 computer type cable, because of the high number of twists per foot. Shielded phone cable seldom helps. Refer to illustrations shown above regarding placement of RIDs. An X marks each point you could try a RID based on the type of system you’re working on. Shortening the length of the antenna (station cable) by putting RlDs in the middle of the cable run is not always easy, but can be quite effective. Occasionally you’ll need RlDs on both handset and line cords, but not often. As at some AM & FM radio stations, you may need to cascade RlDs together to get both offending frequencies.


Welding noise is another hard one to get rid of, but it can be done! The same basic cable installation rules apply as with the RF Sealing Equipment.  Make sure that any unused cables or pre-wires are removed from the frame. If they remain bundled with the working cables, ground all of the pairs after removing them from the frame. Spare cables will act like antennas, picking up the welding interference and inducing it into the other cables

Welding noise seems to be picked up by the handset cord in many cases. The cure has been to use three of our handset RlDs cascaded together -the AM, FM and CB, which will notch out a huge portion of the RF spectrum. You know that you need the HANDSET versions of the RlDs if you can move around with the handset and the noise gets louder or softer. Welding noise is RF spread throughout the radio spectrum. We are batting 100% at getting rid of it, but it’s expensive since you have to use three RIDs on the affected phones.  This seems to work as close as 8 feet from the welder.  This applies to all kinds of phone systems, both analog and digital.