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July 2000 Issue
Feature

Signal Leakage & Harmful Interference
A Ham Radio Perspective
By

• Leakage limits
• Damage-control options
• Other considerations
• Possible situations
• Small problems need fixing
• Another scenario
• Case study
• Part 15 devices
• Stay tuned
• The ARRL Connection
• Ham Bands In the RF Spectrum
• Bottom Line

Ron Hranac

The Federal Communications Commission’s signal leakage rules, found within Part 76, have been with us for many years. If you’ve been in the industry long enough, you no doubt remember mid-1990, the time when a new, tougher version of those rules, including such things as the cumulative leakage index (CLI), went into effect.

Our industry has largely done a good job of keeping leakage under control, and a nice side benefit has been better performance from our networks. Two-way operation has forced us to tighten our plants further as we battle reverse path ingress. This has in many instances resulted in even better leakage performance.

Even so, signal leakage is an ongoing battle, ranging from craftsmanship problems (the infamous loose F-fitting) to outlets sloppily installed by subscribers. To make matters worse, now and then I run across a situation where leakage monitoring and repair land on the back burner, and I’ve even seen a system or two that had no signal leakage program in place at all.

Thank goodness these are exceptions rather than the rule. Still, you’d think that by now compliance with Part 76 would be second nature -- and for the most part, it is -- but leakage gremlins probably always will be with us.

Leakage limits

§76.605(12) of the FCC’s rules is very clear about just how much RF is allowed to leak out of our systems. For frequencies less than and including 54 MHz, and over 216 MHz, the leakage limit is 15 microvolts per meter (µV/m) at a 30-meter (about 100 feet) measurement distance. For frequencies over 54 MHz up to and including 216 MHz, the limit is 20 µV/m at a three-meter (about 10 feet) measurement distance.

Most of us perform leakage measurements in the midband, so the latter spec applies. That means any signals leaking out of any part of the system -- headend, distribution network or drops -- over 54 MHz through and including 216 MHz must not have a field strength that exceeds 20 µV/m three meters from the system. Given that you probably make your measurements in the 108-150 MHz range, the maximum equivalent half-wave dipole level is in the -41 to -44 dBmV range, depending on the actual frequency of measurement.

If you operate a two-way network, you know that the FCC’s leakage limits are nowhere close to being tight enough for successful upstream operation. Every leak is a potential ingress point, where over-the-air interference can leak into the system and mess with your signals. Most two-way system operators must keep leakage at or below 5 µV/m to minimize ingress.

Damage-control options

But what happens if you get a call from a ham operator complaining about possible leakage interference to his or her radio communication? How do you handle that? Here are two checklists for you and your system staff.

What not to do:

• Tell the ham operator your system complies with the FCC’s signal leakage rules and your plant’s perfectly legal.
• Ignore the ham, hoping the complaint will go away.
• Don’t return phone calls.
• Get confrontational.

What to do:

• Take the complaint seriously.
• Respond in a timely manner.
• Work with the ham to resolve the interference complaint.
• Educate your customer service reps, installers and technicians about the seriousness of interference complaints.

Why two checklists? Because in a few interference cases with which I’ve been involved, system personnel did everything on the "what not to do" checklist. It’s much better to foster an atmosphere of cooperation to resolve interference problems. After all, ham operators are licensed to use the over-the-air spectrum, and signal leakage has the potential to disrupt amateur radio communications -- not to mention police, fire, ambulance, aircraft, broadcasters and other users.

Other considerations

Going the other direction, if your system leaks, it’s going to be susceptible to ingress. That means licensed and legal over-the-air transmissions can interfere with signals carried on your cable system. Ham radio-related ingress interference has the potential to be severe, given the right circumstances. One reason for this is that most hams are legally able to transmit with RF power levels up to 1,500 W. That kind of transmitter power, combined with a high-gain antenna located perhaps a few tens of feet from a loose F-connector, will cause all sorts of interference grief. It’s your responsibility to ensure that those legal signals don’t get inside your plant.

Besides, hams can be valuable allies when it comes to solving interference problems. Most have a good background in electronics and operate radio receivers that are far more sensitive than the signal leakage detection equipment that we use.

A few enjoy "fox hunting," where the objective is to use radio direction-finding techniques to track down hidden transmitters or interfering signals. In some countries, hidden transmitter fox hunting is a serious competitive sport, combining athletics and amateur radio.

So, if you get a call from a ham complaining about potential leakage-related interference, schedule a service call to see what’s going on. Just because you have a good leakage detection and repair program in place and you passed the latest flyover with ease, don’t assume there can’t be a problem leak.

In most cases when a ham calls about a potential leak, it’s because he or she is experiencing interference in the 2-meter ham band. More than likely, the culprit is Ch. 18’s 145.25 MHz visual carrier. Table 1 below shows popular amateur bands and their relationship to the RF spectrum.

Possible situations

Let’s go through a couple possible scenarios you might find when you visit the residence of a ham operator who is experiencing interference.

Scenario 1: You find one or more leaks in the vicinity of the ham’s house that exceed 20 µV/m. You repair them and the interference problem is taken care of. Congratulations. You’ve just scored a public relations coup and tightened up your plant in the process.

Scenario 2: You carefully check the neighborhood and find that while there is some low level leakage, no leak is greater than 20 µV/m. You tell the ham the system complies with the FCC’s regulations and drive back to the office thinking that’s all you have to do.

Obviously the first scenario has a happy ending. You eliminated leakage that was clearly in violation of Part 76, you solved the ham’s interference problem, and you probably also made a friend in the process. The second scenario has the potential to be a PR nightmare.

"Wait a minute, Hranac," you say. "In that example, leakage was below the FCC’s 20 µV/m limit. The system is in compliance, so the interference is the ham’s problem."

Not so fast. The system may indeed comply with §76.605(12), but what about §76.613? In case you’re unfamiliar with that section of Part 76, read on.

Small problems need fixing

Section 76.613 describes interference from a multichannel video programming distributor (MVPD).

(a) Harmful interference is any emission, radiation or induction that endangers the functioning of a radionavigation service or of other safety services or that seriously degrades, obstructs or repeatedly interrupts a radiocommunication service operating in accordance with this chapter.

(b) An MVPD that causes harmful interference shall promptly take appropriate measures to eliminate the harmful interference.

(c) If harmful interference to radio communications involving the safety of life, and protection of property cannot be promptly eliminated by the application of suitable techniques, operation of the offending MVPD or appropriate elements thereof shall immediately be suspended upon notification by the District Director and/or Resident Agent of the Commission’s local field office, and shall not be resumed until the interference has been eliminated to the satisfaction of the District Director and/or Resident Agent. When authorized by the District Director and/or Resident Agent, short test operations may be made during the period of suspended operation to check the efficacy of remedial measures.

(d) The MVPD may be required by the District Director and/or Resident Agent to prepare and submit a report regarding the cause(s) of the interference, corrective measures planned or taken, and the efficacy of the remedial measures.

This means that even a lowly 5 µV/m or 10 µV/m leak that is causing harmful interference must be fixed. What constitutes harmful interference? If the interference merely breaks squelch on, say, a scanning type receiver, it’s not considered harmful. But it is harmful interference if communication is seriously degraded, obstructed or interrupted.

Another scenario

Let’s look at a third scenario. You respond to the interference complaint and find a greater than 20 µV/m leak. However, it’s not coming from the feeder or neighborhood drops. It’s from the next-door neighbor’s cable-ready TV set. What to do? This has the potential to be a really nasty public relations nightmare.

If you confirm the leakage is from a subscriber’s TV set, there are several approaches. One way to reduce or eliminate the leakage interference is to see if the offending TV set is receiving too much signal. An in-line attenuator installed somewhere in the drop may be all that’s required to fix the problem. Another trick to try is installing a commercially manufactured common mode choke such as the Ghost Buster at the input to the TV set. The Ghost Buster was designed to reduce VHF common mode currents and has been shown to be quite effective.

If either of these fixes fails to work, you may have to tactfully explain to the subscriber that the TV set appears to be leaking signals that are interfering with over-the-air users and may have a technical problem that requires repair or modification by a local shop or perhaps the set manufacturer.

The odds are pretty good the subscriber also has been experiencing ingress interference from the ham next door -- along with pager transmitters and commercial two-way radios. An interim solution would be to provide the owner of the leaky TV set a plain converter so that all the set receives is the converter’s Ch. 3 output, rather than the whole cable spectrum. This will fix the problem most of the time.

If the owner of the offending TV is absolutely uncooperative, you do have one last resort. It isn’t an elegant solution, but it will get rid of the interference. You can legally disconnect his or her drop. Section 76.617 provides for that, which says in part, "In cases where excessive signal leakage occurs, the cable operator shall be required only to discontinue service to the subscriber until the problem is corrected."

I’d add one piece of advice to this whole interference business. Document everything. If any of this ever comes back to haunt you in the future, thorough documentation will show the steps you’ve taken to resolve the interference. Include copies of all correspondence and notes from all telephone calls and meetings.

Case study

Now let’s look at an interference complaint that went beyond the local level, but did have a happy ending. I’ll leave out names and places (it’s a true story), but remember, this could happen to you.

A ham operator was experiencing moderate to severe interference at 145.25 MHz, and neighbors who owned cable-ready TV sets connected to the cable system were experiencing Ch. 18 interference when the ham used his 2-meter transmitter. Sometimes the interference obliterated the pictures. Attempts to get the local system to fix the problem apparently were unsuccessful.

The ham took his complaint to the American Radio Relay League, which in turn forwarded the background information to me. I contacted one of the cable company’s corporate engineers and filled him in on the details. (See the sidebar "The ARRL Connection".)

A few weeks later, I was forward-ed copies of correspondence between the affected ham and ARRL headquarters. In that correspondence, a now quite happy ham operator explained that the system had sent technicians to his neighborhood to replace several nearby drops and that his received 145.25 MHz interference had dropped from greater than signal level 9 (S9) down to S3.

This represents an estimated 30 dB to 40 dB improvement because each "S" unit on many ham receivers corresponds to about 6 dB. At the same time, the ham explained that most of the interference to his neighbors’ TV sets had gone away. To close the loop, I advised my contact at the cable company’s corporate office of the successful outcome.

Part 15 devices

Another type of interference has nothing to do with signal leakage. It’s related to so-called Part 15 devices, which I wrote about in a column last year ("Work Together to Beat Over-the-Air Interference," May 1999).

As I mentioned in that column, Part 15 devices are electronic devices such as cordless telephones, wireless modems and even garage door openers that share over-the-air frequencies with licensed users of the same spectrum. Part 15 devices do not have to be licensed and aren’t supposed to cause harmful interference to licensed over-the-air users. When harmful interference does occur, it’s the responsibility of the operator of the Part 15 device to do whatever is necessary to reduce or eliminate the interference.

Just like Part 76 of the FCC’s rules, Part 15 includes a definition of harmful interference. It can be found in §15.3(m): "Harmful interference. Any emission, radiation or induction that endangers the functioning of a radio navigation service or of other safety services or seriously degrades, obstructs, or repeatedly interrupts a radiocommunication service operating in accordance with this chapter."

The rules are very clear about the operation of Part 15 devices, too. §15.5 details general conditions of operation, saying in part:

(b) Operation of an intentional, unintentional, or incidental radiator is subject to the conditions that no harmful interference is caused and that interference must be accepted that may be caused by the operation of an authorized radio station, by another intentional or unintentional radiator, by industrial, scientific, and medical (ISM) equipment, or by an incidental radiator.

(c) The operator of a radio frequency device shall be required to cease operating the device upon notification by an FCC representative that the device is causing harmful interference. Operation shall not resume until the condition causing the harmful interference has been corrected.

You might be inclined to ask what this has to do with cable. Plenty, if your system uses Part 15 devices such as wireless modem or phone jacks. If you use these devices or have installed them in subscribers’ homes, you are technically the operator of the devices.

As of a year ago, some 160,000 or more Phonex Corp. (6952 High Tech Drive, Midvale, UT 84047, ) wireless modem jacks had been installed in several cable systems, and while many have since been removed or replaced because of interference complaints, a lot of them still are in service. The Phonex modem jacks, which are frequently sold under a variety of brand names including Phonex, GE, RCA/Thomson and Radio Shack, use the home’s electrical wiring to extend a phone outlet to areas of the home where hard-wired phone outlets are not available. These devices are very convenient to use for connecting digital set-tops to the cable subscriber’s phone line.

The problem? The early Phonex modem jacks transmit signals at 3.53 MHz and 8.27 MHz in full duplex operation. There have been numerous instances where cable subscribers’ home electrical wiring radiates the 3.53 MHz signal at levels sufficient to cause harmful interference in the 80-meter amateur band. When this happens, the cable operator is responsible for finding and fixing the problem.

The modem jacks operate at a frequency that is outside the range of most test equipment commonly available in cable systems, which means direction finding must be done with a portable shortwave receiver. Unless, that is, you fancy packing around a spectrum analyzer to track down the 3.53 MHz signals. This is where a good relationship with local hams will be invaluable.

Stay tuned

In the second installment of this article, I’ll present information on how to build your own 80-meter ferrite rod-style direction-finding antenna -- a conventional half-wave dipole at this frequency is a bit unwieldy, considering it’s more than 100 feet long. I’ll include tips on how to use the antenna in conjunction with a portable receiver for doing your own fox hunting. The test bed? My own neighborhood, where I can receive several interfering carriers around 3.53 MHz.

The ARRL Connection

Ron Hranac recently was appointed to the American Radio Relay League’s RFI Task Force, in part because of his involvement with the ARRL and Society of Cable Telecommunications Engineers in the area of amateur radio interference issues. Dovetail Sciences Corp.’s Robert V. C. Dickinson has a similar role via the National Cable Television Association’s Engineering Committee.

These two individuals -- both of whom are licensed amateur radio operators -- serve as liaisons between the ham radio community and cable TV industry when signal leakage and interference problems cannot be resolved locally. If you get a call from "corporate" urging you to sort out an interference problem with a ham operator in your system’s service area, it’s likely because the interference complaint was, for whatever reason, taken to ARRL headquarters.

ARRL staff in turn contacted Hranac or Dickinson to "pull the appropriate strings" at the cable operator’s corporate office. If this happens to you, don’t feel too badly. The complaint could have gone to the Federal Communications Commission.

Table 1: Ham Bands in the RF Spectrum

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