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Iam writing to you in hopes of helping me solve this problem. We have a
customer that has an ISDN line. The ISDN line entering the house to the "D"
mark is clear (this was verified by Bell Atlantic in my presents). From the
"D" mark it enters a Shiva Access Port (router) via RJ-45 connector. The
router has 2 external ports (line 1 & line 2) which leave via RJ-11. The 2
RJ-11 are plugged into a shielded patch panel. The router also has an
Ethernet connection leaving via RJ-45 to and RJ-45 on a hub. From the
shielded patch panel the 2 RJ-11 lines leave via shielded Cat. 5 cable, to an
office approximately 80 feet away. One shielded cable is terminated in a
floor jack along with 2 RJ-45 data connections, and 1 Pots line. The other
shielded is terminated in a wall jack in the same office area with 2 data
RJ-45 connections and 1 Pots line. Only 1 ISDN line in the office is being
used, it is hooked up to a phone.
We are experiencing RF interference on the ISDN line at both termination
points. The interference is coming from an AM radio station (1150am) located
approximately 1 mile away. In hopes of correcting this problem I have
confirmed the Cat. 5 shield cable is properly grounded at one end. Also, I
have unplugged the RJ-11 immediately exiting the Shiva hub and I don't hear
the radio station. In addition, I have plugged into the shielded patch panel
and again no radio station. I only hear the radio station at the final
termination point in the office. Also, there is no RF interference on the
Pots line which is run side by side with the ISDN line. Also, I have
installed a phone jack (supplied by Bell Atlantic) with a suppressor, with no
success in stopping the interference. I have also plugged in a butt set and
various other phones into the jack and the interference is still there. I
have also placed a ferrite choke on, again no success.
I would greatly appreciated any help you can lend in solving this
I have the two filters (the latest versions optimized for approx. 1150
Khz.), packaged up and I will put them in the mail at the post office
early in the morning (Thurs).
There are a number of possibilities concerning your RFI.
1. Something after the shielded patch panel is picking up RF and feeding
it into the telephone.
2. Something after the shielded patch panel is picking up RF and feeding
it back into the shielded patch panel and the router.
When you listened at the shiva hub RJ-11, I presume you disconnected the
80 ft. line.
When you listened at the shielded patch panel I presume you had the 80
ft. lines disconnected.
This is why I suggest putting one filter at the RJ-11 output on the
shielded patch panel and one as close to the office telephone as
I would suspect any 80 ft.length of cable including the shield would
make an excellent a.m. antenna even if you were 50 miles from the
station let alone one mile. Even grounding one end will not stop it from
being an excellent antenna. It will still have an RF current and voltage
distributed over its length. However I think you said the problem was
there before you attempted to shield the line.
Be careful of long ground lines or shields connected to the chassis of
the equipment because it too can act as a huge antenna and feed RF into
That's all I have for now. Let me know what happens.
I would like to begin by thanking you for you help. As you know we had
serious RFI with an ISDN line coming into the building. The problem occurred
after the ISDN line exited the router as an analog line. We tried everything,
we thought (including other filters). We placed two of your filters on the
phone line (one at the router and one at the phone). They worked great!
Thanks again John for you support and good product!
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