ARRL Technical Information Service Cable Television Interference Information File: catvi.txt Updated: March 25, 1997 Author: Michael Tracy, KC1SX (email: tis@arrl.org) Prepared as a membership service by the American Radio Relay League, Inc., Technical Information Service. For your convenience, you may reproduce this information, electronically or on paper, and distribute it to anyone who needs it, provided that you reproduce it in its entirety and do so free of charge. Please note that you must reproduce the information as it appears in the original, including the League's copyright notice. If you have any questions concerning the reproduction or distribution of this material, please contact Michael Tracy, American Radio Relay League, 225 Main St., Newington, CT 06111 (email: tis@arrl.org). ----------------------------------------------------------------- CATVI EMI/RFI advice from ARRL Headquarters. In a CATV installation, the usual form of interference is as a result of the common-mode signal on the cable, ie the signal present on both the inside and the *outside* of the cable. Some TVs or VCRs are very susceptible common-mode interference. The best way to eliminate this common-mode problem on a 75-ohm TV antenna or CATV system is to use an Amidon FT-140-43 or FT-240-43 ferrite core (or equivalent). Wind about 15 turns of the incoming coaxial cable onto the ferrite just before the point where it connects to the first electronic equipment -- set top converter, VCR or TV antenna or cable input. The ARRL TIS "EMI/RFI Package" (on the web at http://www.arrl.org/tis/info/rfigen.html) lists several sources for these cores. Another way to eliminate this common-mode problem on a 75-ohm TV antenna or CATV system is described in the March 1989 QST Hints and Kinks column. The write-up describes how to use two 4:1 transformers and a 300-ohm high-pass filter to effectively eliminate the common-mode HF signal that is causing interference. If you would like a free copy of this Hint and Kink, send an 8X12 SASE to the ARRL Technical Secretary and ask for a copy of the column. Any amateur that is involved in an EMI/RFI problem should also contact the local ARRL section Technical Coordinator (TC). The TC will know of a more local Technical Specialist and or local club TVI Committee that might be able to offer some assistance. The ARRL Section Managers (SM) are listed in the front few pages of any QST, a quick phone call to the SM will locate the TC quite quickly. ARRL HQ staff can also look up the TC in the database. Education is also important. The ARRL Handbook for Radio Amateurs and Bill Orr's Radio Handbook both contain a chapter on EMI/RFI. There are also two good books devoted entirely to the subject - ARRL's "Radio Frequency Interference - How to Find It and Fix It" and the "Interference Handbook" by William Nelson, WA6FQG (SK). The first book covers nearly every aspect of interference problems, while second book is THE book to buy for any *electrical* interference problems, especially for power line noise interference. Both books are available from ARRL Headquarters. Ensure that your own house is "clean". You should be able to watch an *antenna*-connected or cable-installed TV while your transmitter is operating. This is usually enough to satisfy the FCC that your station is not radiating any harmonics, and it is a real powerful point to be able to tell your neighbors that if they can't watch TV at their house while you are on the air they can come over to your house and watch while you are operating. When troubleshooting EMI, simplify the problem. Instead of dealing with 3 TVs, 2 VCRs and a satellite system all hooked up together, work at first only with the cable drop and one cable- ready TV or the cable-company converter box. After that is all properly debugged, add the other components back to the system one at a time, and if the problem recurs at least you know which piece of equipment is the culprit. VCRs are notorious for their susceptibility to fundamental overload. These basic principles apply to any interference troubleshooting. Grounding the shield of the cable outside the house can be effective, especially if used in conjunction with a high-pass filter AND common-mode choke right at the TV antenna terminal. The HF common-mode (translate: signal on the *shield* of the coax) will take the low-impedance path to ground rather than go through the high-impedance filter. CATV grounds should only be installed by cable-company repair personnel. Either use the filter described in the March 89 QST Hint and Kink, or use a conventional high-pass filter with a common-mode choke made from a ferrite core or rod. It is a bit big (2.4" o.d.), but I would use a FT-240-43 core, with about 15 turns or so. Ocean State Electronics, POB 1458, Westerly, RI, (401) 596- 3080, has them available for about ten dollars. If you have a souce of "free" or inexpensive ferrite material, then by all means, try making a choke with this material to see if it is effective or not. If it works, fine business, but if it doesn't, that may mean that it is not suitable material for RF. In that case, try the *correct material*. Get the ARRL EMI/RFI Package. It is free (except for the stamp) and will answer many questions. The bulk of the text is contained in the file RFIGEN.TXT available from the ARRL Information Mail Server (info@arrl.org). If your neighbor wants his or her cable TV problem fixed, it is better to have the local cable company send a technician out to resolve the problem. Send a large SASE to ARRL to request a copy of our "Cable Television Interference Package". This package is a reprint of a three part article series from the cable television industry publlication, Communication's Technology. This article series, written by ARRL Lab Supervisor and RFI expert Ed Hare, W1RFI, is a cable installer's guide to resolving interference problems. If you supply the cable company technician with a copy of this series, that should give them the information they need to fix the problem. The articles also refer to the cable company's regulatory obligations that require a cable company to maintain a "clean", closed system (one that doesn't radiate RF). *eof