 |
 |
|
|
|
|
|
|
||||||||||||||||
|
|
|||||||||||||||||
|
|
|||||||||||||||||
|
|||||
|
|
Additional information on this subject and related topics can be found in back issues of QST and the following:
Reprinted from March 1994 QST Lab Notes column. Copyright © 1994 American Radio Relay League, Inc. All rights reserved.
Q: Hello, again, Mr RFI-Fix-It. It's me, Joe Hamm. I really appreciate the advice you have given me about my other EMI problems. It seems I can't win, though. My new neighbor, Bruce, just came to talk to me about some interference problems he's having with his TVs (TVI). It seems that when I transmit on HF, I'm getting into his VCR and both of his TV sets. One is hooked up to cable, the other is hooked up to a "rabbit-ear" antenna. When I transmit on 2 meters, I'm tearing up channel 18 on the cable TV. I heard that TVI is the fault of the transmitter. Should I buy a new transmitter?
A: A: Don't buy a new transmitter just yet, unless you're looking for a good excuse to spend your money. If you haven't already read it, go to the ARRL "RFI-General Information" page. This will refresh your memory about the technical and interpersonal basics that are part of any RFI problem. It also contains a list of companies that sell filters and other helpful products, plus links to other RFI-related sites. I also suggest that you buy the ARRL RFI Book to get a more complete picture. Contact your ham-radio dealer or refer to the ARRL Publications Catalog
Let's see what we can do right now, though.
Interference can result from several different causes--electrical noise (such as a vacuum-cleaner motor), spurious signals from your transmitter, or the inability of the TV set to "tune out" a strong local signal. As an analogy to the last point, if you were to tune a TV set to channel 3, and saw channel 13 instead, you would probably conclude that the TV was broken. If you tune your TV to channel 3, and see a local HF transmitter instead, you could reach the same conclusion. That is, the TV is "broken" in the sense that it needs additional filtering or shielding to work near a strong RF source.
If you can demonstrate to Bruce (and yourself) that you are not interfering with an antenna-connected TV at your house, your transmitter will be off the hook. (It would still be a neighborly thing to offer your help to find a solution, however.)
Q: Well, I suppose I must tell you that when I transmit on HF, I interfere with the TV in our den. It is connected to an outside antenna, located about 30 feet from my beam. Does this mean the interference is all my fault?
A: Not necessarily. Let's run through the things you can do to find out. Start by installing a low-pass filter on your transmitter. This should be installed as closely as possible to the transmitter or amplifier output.
Q: I installed the filter and it helped quite a bit. I still see some interference on the video and I hear myself in the sound. Should I add another low-pass filter?
A: Most interference problems have multiple causes. The fact that you made things better by installing a low-pass filter tells you that you probably did have a problem with spurious signals from your transmitter. The next thing you can do is to look at your station ground. If you have no ground, add one. Grounding is not a cure-all for EMI problems, but under some circumstances, it can help.
I suggest that you install a low-pass filter permanently--whether you need one or not! You can always point to it with pride, explaining that you have installed a filter on your end. If the FCC is ever involved with your EMI problem, they will ask you lots of questions about your low-pass filter and station ground.
Q: I added a good ground and it didn't seem to make any difference. What's next?
A: It sounds as if we're dealing with a TV that is susceptible to overload from your fundamental signal. It's time to try the appropriate filters for the TV.
For an antenna-connected TV, the first filter to try is a high-pass filter. These are available from Radio Shack. Several other sources are listed on the RFI General Information page. Buy one and install it as closely as possible to the TV antenna input.
Q: Wow! The picture is clear as a bell, but I'm still getting into the sound. What gives?
A: A high-pass filter will attenuate the HF signal that's picked up by the antenna and sent to the TV inside the TV's feed line. But you can also pick up a lot of signal on the outside of the feedline. The signal that is not inside the feedline is called a common-mode signal.
You can filter the common-mode signal with a common-mode choke. Obtain an FT-240-43 ferrite core² (Don't use unknown material; it may not work.) Wrap about 10 turns of the feedline (either coax cable or twin-lead) onto the ferrite core, installing it just before the antenna input to the TV.
In some cases, you may also need to install a common-mode choke on the TV's ac-line cord. If necessary, you can also try an ac-line filter, available from Radio Shack (catalog #15-1111) and from the sources at TISFind.
Q: Hooray! It worked. My TV is clean as a whistle, except for that real weak channel we get from about 150 miles away. Can I fix that one, too?
A: Probably not. The FCC rules do not protect fringe-area reception. If you are outside that station's coverage area, you will just not get enough TV signal to be usable. If you, or your neighbors, are having problems receiving that station, it is not surprising.
Q: Well, I am ready to go help Bruce. Any other words of wisdom before I go over there?
A: Don't forget that you do not want to service your neighbor's equipment. Once you have determined that the problem is not your radio station, you want to be a good neighbor. You're a locator of solutions, not a provider of solutions. If you take the back off the TV, you may be blamed years later when the 25-year old clunker gives up the ghost.
There are a few other things you should watch out for. Make sure the TV is getting a good picture. An old, rusted TV antenna or feedline can be a direct cause of an interference problem by generating harmonics right there at the TV! A bad installation will result in a weak, snowy picture, much more prone to interference. Fix these things first.
Q: I'm over at my neighbor's house and he's right--things are a real mess. Where should I start?
A: Well, you just fixed the antenna-connected TV in your house, didn't you? Start doing the same things with your neighbor's TV, in the same order.
Q: You're right. We finally fixed it. Should I install the high-pass filter on the VCR and cable TV installation?
A: No. If the cable TV system is not leaking severely, there will not be any HF signal inside the cable to be filtered. Most of the time, cable TV is virtually immune to HF interference problems. If there is a problem with CATVI from HF signals, it is usually caused by a common-mode signal on the outside of the coaxial cable. If you tell your neighbor to buy a high-pass filter, and then you go back and tell them that specified the wrong filter, he may be a little upset!
Start by disconnecting the VCR. If you still have interference, add the common-mode choke. Install the choke where the cable attaches to either the set-top cable converter or to the back of a cable-ready TV. If this still doesn't cure the interference, you can try a common-mode choke and/or ac-line filter on the TV and/or the converter. If this doesn't do it, you could try a high-pass filter as a last resort.
Once you fix the cable interference problem, apply the same cures to the VCR. VCRs can be a bit harder to fix, primarily because the video and sound signals inside the VCR are in the HF range.
A cable-ready TV or VCR that doesn't respond to any of the above treatments is subject to direct-pickup interference. In this case, the only recourse your neighbor has is to contact the equipment manufacturer in writing. He could also call the Electronic Industries Association(3) to get the appropriate contact persons for the manufacturer.
If you are able to determine that the converter box is subject to direct pickup, or that a cable leak exists, contact the cable company for assistance. Often the cable repair personnel are not familiar with all of the technical issues surrounding a cable-television interference (CATVI) problem. If that's the case, there are two things you can do: call the cable company and discuss the problem with the repair supervisor or the system engineer. The ARRL has written a two-part article about CATVI that was published in Communications Technology, the journal of the Society of Cable Television Engineers. The cable company will recognize this publication! This article has been reprinted in the ARRL RFI Book.
Q: Well, it took quite a combination of filters, but I managed to fix most of Bruce's problem. The only thing that's left is my 2-meter interference to cable channel 18. Why is that problem still there?
A: In a nutshell, cable television makes use of frequencies allocated to over-the-air services. Cable channel 18, or channel "E" in CATV parlance, is from 144-150 MHz. The amateur 2-meter band falls within that channel. Your 2-meter signal is clearly leaking into the system. The hard part is finding out where.
Start by determining if the signal is leaking either in (ingress) or out (egress). In most systems, the video carrier will be on 145.25, plus or minus 25 kHz. If you hear a strong carrier on that frequency, the cable system is probably leaking. The cable company will appreciate your calling this to their attention. This type of leakage can be very expensive if they fail their cumulative leakage tests or if they're cited for violations of FCC rules. If their signal is leaking out, you can be sure your signal is leaking in. In the case of cable-plant leaks, it is the cable company's responsibility to cure the problem.
Even if the leak from the cable system is below the FCC leakage requirements, the interference could still be caused by a leak in the cable system. An interfering signal that's more than 40 dB below the video carrier level can still cause interference! If the leak is not severe, the cable company may try to tell you that they don't have to fix it. Don't be fooled. The cable company must adhere to several regulations about picture quality. FCC regulations 76.605(a) state, among other things, that the carrier-to-noise ratio must be at least 40 dB and the carrier-to-coherent disturbances ratio must be at least 47 dB. These regulations, and the ARRL CATVI package, will help your cable operators to understand their responsibilities.
Unfortunately, it is not all black and white! The leak could be in the cable system, the cable company's set-top converter, or a customer-owned cable-ready TV or VCR. The cable company is responsible for the first two, but cannot be held responsible for the design of the TV. If the TV leaks, Bruce will have to contact the manufacturer.
Sometimes, a common-mode choke does help. Try one on the incoming cable, just before it contacts the first piece of electronic equipment. Try a common-mode choke on the ac line connected to each piece of equipment, too. Other than that, there is no filtering you can do. If you filter the 2-meter signal, you will be filtering cable channel 18, too!
The manufacturer and cable company can get involved in a real finger-pointing contest in some situations! There is an easy way to determine if the cable operator is "off the hook." Ask the cable operator to demonstrate good performance on channel 18 using a cable-company supplied set-top converter. If they can, the leak is clearly in the cable-ready TV. If they can't show that their system is working well, they have a bit more work to do. Suggest that they buy the ARRL RFI book and contact the ARRL RFI Desk here at Headquarters.
Q: Thanks for all your help. Bruce is happy now. His dog Bucky even likes me again! Is it always this easy?
A: Unfortunately, no. The proper application of personal diplomacy and technical solutions sure make it a lot easier, though!
The ARRL members and HQ staff would like to thank the following people for their contributions to this information file: Ed Hare, W1RFI.
Send any additional information or changes to tis@arrl.org.
73 from ARRL HQ.
Note: Reprints are available from the Technical Department Secretary, ARRL, 225 Main Street, Newington, CT 06111. Article copies must be prepaid $3 for ARRL Members, $5 for non-members. You may order by telephone (860-594-0200) and pay by credit card. Reprints can only be sent by mail and cannot be sent by or attached to e-mail.
This information was prepared as a membership service by the American Radio Relay League, Inc., Technical Information Service, 225 Main St., Newington, CT 06111 (860) 594-0214. Email: tis@arrl.org (Internet). ARRL HQ is glad to provide this information on the Web free of charge as a service to League members and affiliated clubs.
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.
If you have any questions concerning the reproduction or distribution of this material, please contact:
