Note: This page was prepared in 1998 to address a problem with the Phonex PX-421 model of wireless modem jacks manufactured by Phonex. Within days of receiving the initial reports of interference, Phonex discontinued that model and redesigned the product to use frequencies that do not affect Amateur Radio. The information here applies only to that model number and should not be taken to imply that all wireless modem jacks represent are a potential interference source to Amateur Radio. The PX-421 and all subsequent Phonex products comply with all FCC rules. Phonex has recalled virtually all of the old devices that were sold. Most PX-421 have also been removed from homes and have been replaced with new models that do not pose an interference potential to Amateur Radio.

At this point, this data is primarily of historical interest. As such, however, the experiences outlined here do demonstrate that carrier-current devices that are operated at the FCC Part 15 limits can cause harmful interference to sensitive Amateur Radio operation. Manufacturers of similar products should consider the nature of all over-the-air radio services in their selection of the operating frequencies of unlicensed devices. Regulators may find this case history useful in setting future limits and regulations for unlicensed devices of all types.

ARRL also notes that all involved parties in this matter addressed their responsibilities fairly.

In 1998, ARRL received reports of local interference from noisy and somewhat "drifty" carriers near 3.520 MHz. They can range anywhere from 3.510 to 3.550 MHz. Most of the initial reports were initiated on the Northern California Contest Club reflector and forwarded to ARRL HQ by Brad Wyatt, K6WR, then the ARRL Pacific Division Director. Harmonics of these signals have also been reported as high as 20 meters. An informal investigation in the Greater Hartford, Connecticut, area revealed several local, non-amateur signals, including some carrying FM voice transmissions that appeared to be telephone conversations. One signal heard in a residential neighborhood literally pinned the S meter at 60 dB over S9. These reports include interference ranging from S7 to 60 dB over S9. By any standard, the S7+ carriers monitored on popular ham frequencies constitute harmful interference.

The interference was being caused by model PX-421 wireless modem jacks, manufactured by the Phonex Corporation, 6952 High Tech Drive, Midvale, Utah 84047, (801) 566-0100, http://www.phonex.com/. The PX-421 jack is no longer being manufactured and the current wireless jacks do not pose a threat to Amateur Radio.

Many of the PX-421’s were installed with the digital cable converter for digital cable television. Some were also sold retail, or used with satellite-TV installations. They use the electrical wiring to make a convenient connection from the cable box to the telephone line to transmit billing information. These devices are also being sold in some computer retail stores. They are first cousins to "wireless telephone jacks," also being marketed in retail stores.

These tests and reports from the field demonstrate that the PX-421 wireless modem jacks pose a serious problem for reception on the lower part of the 80-meter amateur band and possibly on other bands because of harmonics. Likewise, the devices also are very susceptible to interference from HF signals. The ARRL Lab has received many reports of interference from these devices.

Wireless jacks are "carrier-current" devices, meaning that they conduct their RF signals over the power lines. They are full-duplex transceivers that use the ac electrical system to connect a remote telephone to the phone system, using FM signals conducted by the electrical wiring.

There are two types of jacks, very similar in design and function that are being made by Phonex and marketed under several brand names - wireless modem jacks and wireless telephone jacks. The PX-421 version of wireless modem jacks uses 3.52 and 8.27 MHz for their full-duplex operation. These are the ones causing the interference problems on some HF amateur bands. They are also very susceptible to interference from HF transmitters, especially those that operate on the 80-meter amateur band. The current production models of wireless jacks do not operate in the ham bands and do not appear to pose any specific problem for Amateur Radio.

The wireless telephone jacks, marketed under GE, RCA/Thompson and Radio Shack brand names, operate on 3.025 and 6.436 MHz. They are not likely to cause any interference problems to Amateur Radio, although they are also susceptible to interference.

In PX-421, the 3.52 MHz carrier is on all of the time, even when the phone lines are not in use. The 8.27 MHz signal appears only when the modem is in actual use. The 3.52 MHz signal is generally modulated only when the modem is in use. Analog phone signals from other phones in the house usually do not appear on the transmitter carrier, although the ARRL has received reports of telephone conversations near 3.52 MHz.

The modulation is narrow-band FM (NBFM) with a maximum deviation of +10KHZ and -25KHz (not symmetrical for very loud audio). The transmitter output impedance is approximately 15 ohms. The open circuit voltage is approximately 2.4 Volts RMS across the line and the short circuit current is approximately 150 milliamps. This would result in a maximum power of approximately 100 milliwatts into a matched load.

Under some circumstances, the unit gets into a mode where it produces a broadband interference that falls off at about 10 dB every 5KHz on either side of the signal. This is different than "normal" where it produces a T8 or T7 unmodulated note.

They are permitted, within certain limits, by FCC Part 15 rules on most frequencies, even on the ham bands. However, the rules state the operators of Part 15 devices must ensure that they do not cause interference to other radio services.

Identifying Units by FCC ID Numbers

There is an "FCC ID" number on wireless jacks. This is a Part 68 registration number, indicating the unit meets the appropriate standards to be used on a telephone system.

The 3.52-MHz units carry a registration number similar to HMTCHN-24794-KX-E and are sold as under various model numbers under several brand names. They were also used by TCI cable, now AT&T, in conjunction with their digital cable systems.

The 3.025-MHz units carry and ID number similar to AAOIND-25424-RJ-N and are also sold under various model numbers under several brand names, including RXC/Thompson, GE and Radio Shack.

According to the FCC registration number, the device is manufactured overseas by PT LINTAS ELEKTRONIKA DINAMIKA INDAH. It was registered with the FCC on 4/14/98.

The ARRL Lab has obtained samples of the PX-421. As expected from information we received from Phonex, these units all operate near 3.52 MHz. (Again, the wireless telephone jacks and the newer wireless modem jacks operate on non-amateur frequencies.) The Lab verified the extent of the problem; these devices result in serious interference to the lower part of the 80-meter amateur band. From tests done in the ARRL Lab, we've determined that these devices also are very susceptible to interference from 80-meter signals (and perhaps other bands).

Ed Hare, W1RFI, installed a PX-421 in W1HQ, the ARRL HQ operator’s club station. It is located on the ground floor, appropriately right near the Lab. The antenna is on the HQ-building roof, about 3 stories up. In this configuration, he found an S9 + 10dB signal, about 50 dB higher than the digital noise from all the HQ staff computers and local-area networks. When W1INF was placed on the air with 1500 watts, or W1AW bulletins came on, the devices suffered total blanking. At 100 watts, the interference consisted of very strong and objectionable key clicks.

Dave Sumner, K1ZZ, ARRL's Executive Vice President purchased a PX-421 at a local computer store. It operated on 3.538 MHz and gave an S9+20 signal on receive antennas located about 100 feet from Dave's house. It was also easily susceptible to about 50 W on the low end of the 80-meter band.

ARRL HQ also did some informal "field testing" as well. ARRL staffer Rick Lindquist, N1RL, checked out the situation in several neighboring towns served by AT&T with his HF mobile station tuned to the low end of 80. In those towns, Rick reports that every several blocks he heard a carrier--sometimes multiple carriers--in the vicinity of 3.52 or 3.53 MHz. At one location, he reported a signal that pinned his S meter at S9 + 60 dB! ARRL Lab Supervisor Ed Hare, W1RFI, reports that using a 15-foot wire antenna at his QTH, he heard a signal at about S9 +10 dB. One evening, he heard actual telephone conversations on this frequency.

Reports from the Field

These devices would be problem enough if they met FCC Part 15. A "legal" signal at 30 uV/m 30 meters from the source would result in a S9+15 dB signal under some circumstances. (Amateur Radio would still be protected by the "non-interference" provisions of Part 15.) However, the ARRL Lab has received a report done of tests done in the field by a Professional Engineer that indicate that some of these devices are producing some very strong signals, apparently over the permitted limits.

These field reports include interference ranging from S7 to 60 dB over S9.

How ARRL Contacted the Involved Parties

To gather more information about these products, the ARRL Lab has been in communication with a number of its professional contacts. Ed Hare first contacted one of his contacts on the Board of Directors of the Society of Cable Telecommunications Engineers. He introduced Ed to the Senior Engineer of AT&T cable. Ed also called Phonex and discussed the problem with their engineering and management staff.

Both companies were extremely cooperative in resolving the problem, however. Ed Hare summed it all up pretty nicely. "When I started receiving the reports that indicated that this is a widespread problem, I knew ARRL had to act quickly," he said.

Phonex

Part 15 requires that these devices be tested by the manufacturer (Verified) before they are marketed. According to a spokesman from Phonex, "All Phonex wireless products are tested to meet applicable FCC Part 15 regulations. Verification of compliance is performed by an independent test facility certified by the FCC to perform Class B certification testing. Site test performed for verification to FCC Part 15 Sec. 209 are performed in adherence to ANSI standards under the direction of the Phonex Corporation Engineering Department. Although the Phonex Company has complied with required FCC regulations, the ARRL has identified a potential interference problem on the low end of the 80-meter band," said Phonex Senior Engineer Scott Bullock, KK7LC. "We have several hams in our organization, and we do not want to cause any interference to any amateur band."

The good news from all this is that Phonex has responded appropriately. Within days of receiving the first reports from hams, Phonex discontinued the PX-421 jack and redesigned their product to operate outside the amateur bands under Part 15. They also made engineering changes to prevent the units from transmitting a dead carrier when the unit is not in actual use. They said that if one of its units causes interference, he said, Phonex will retune or replace it. Hams can contact Phonex Customer Service at 800-437-0101. They have been working with AT&T to implement this and have sent letters to their distributors, explaining the problem and Phonex's solution.

AT&T

An AT&T spokesman said that his company is a responsible corporate citizen and intends to comply with FCC rules. He said AT&T plans to eliminate the 3.52 MHz PX-421’s installed "as expediently as possible" by replacing them with 3.3 MHz units or by running a hardwired telephone connection. AT&T will use nothing but the redesigned product in the future. In the meantime, hams experiencing harmful interference they believe is related to these devices should contact their local AT&T office. AT&T has initiated a system-wide recall of the PX-421, and at this time, most of the systems have none of those products still deployed in the field. (Some of the retail and satellite-TV installations are still heard.)

AT&T and Phonex Respond to Amateur Radio Operators of 80 Meters--Update

Immediately after AT&T and Phonex were contacted by the ARRL regarding the report that the PX-421 may cause local interference with some HF transmitters operating at the 3.53 MHz frequency, action was taken to address this problem. An update of progress is listed below:

• The same week of the ARRL report, Phonex directed its offshore manufacturing site to change frequency to 3.3 MHz to avoid any further possibility of interference to Amateur Radio.

• Production of the redesigned model was ordered just three weeks after the first ARRL call.

• AT&T local offices began the process of responding to complaints of interference; replacement is also underway as part of normal maintenance and field calls.

• The new modem jacks arrived in the USA beginning in mid-March 1999 just a few months from the first report from the ARRL and hams from the field. This is actually quite fast by industry norms.

• AT&T has initiated a system-wide recall of the old PX-421 product
  
  
  

The voluntary cooperation of AT&T, Phonex and ARRL has been an important part of the ability of all three organizations to focus resources on the issues.

Most of the PX-421 jacks have been sold to AT&T cable for use with the control unit for their digital cable installations. A much smaller number have been sold in computer stores. PX-421’s may also have been used by some installers of DSS satellite systems.

Ths memo, originally from TCI, may be useful to hams. ARRL had obtained TCI's permission to reproduce that memo here. This memo states:

-----Original Message-----

 From:Scheffler, Bill

 Sent:Friday, January 15, 1999 2:49 PM

 To: Degenhardt, Jim; Harlin, Mike; Harrigan, Mark; Hershey, Earl;

 Murphy, Dan; Page, Jeff

 Cc: Werner, Tony; Smith, Lauri

 Subject: Phonex Wireless Phone Jacks

 Importance: High



 This communication is to enlist your assistance in tackling a 

 problem that needs immediate attention and to advise you that 

 we will be contacting all of the Cable Systems in your Division 

 with Digital customers to address the issue described below.



 Some of the Digital installs that have been done required the use 

 of a wireless phone jack.



 The Phonex wireless phone jacks that are in use today have the

 potential to interfere with certain amateur radio users. Even

 though the device complies with FCC part 15, they can still 

 cause interference. Under part 15, we as the operator need to

 suspend operation of these devices if they are causing 

 interference. The ARRL, (an organization associated with Ham

 radio operators) will be releasing an article this week. The 

 HAM operators have a tremendous network of users and as good

 corporate citizens it is extremely important that we work 

 toward insuring that our cable operations do not interfere 

 with the spectrum they are licensed to use.



 The approach that we need to take is as follows:



 1.0 Phonex is immediately changing the frequency on all new 

 devices from 3.53 MHz to 3.3 MHz. This new frequency will not

interfere with the HAM operators.



 2.0 It is extremely important that if we get complaints from 

 HAM operators that we get the call routed over to someone

 knowledgeable so that we can find the location of the 

 interfering device and get it removed from the system.



 3.0 We need to start a program to migrate the 3.53 MHz devices 

 out of the systems and replace them with either the 3.3 MHz 

 devices or a copper extension. I would suggest that we do this 

 via churn and if we are running a service call in a home with 

 one of the offending devices, (even on an unrelated issue) 

 that it be changed out during that visit.



 4.0 The new devices that operate at 3.3 MHz are identifiable 

 by a bar code on the back. The 3.53 MHz devices do not have 

 a bar code.

Note from Ed Hare/ARRL: This latter point has proven to be in error. The 3.52 MHz devices do have a bar code. -- Ed Hare.

 5.0 Chuck Schultz will be working with Phonex on the logistics 

 of getting the recycled devices retuned.



 Expect a spike in activity over the next period of time as the

 Following article goes out to most HAMs.

Hams who have problems with their local AT&T offices should first try to resolve the problem locally. A bit of polite persistence, or perhaps asking to be transferred to a supervisor should help resolve any misunderstandings with customer-service personnel. (In such a large undertaking, such things are inevitable.) Hams can also print information from this Web page, especially the TCI memo, to help the local offices better understand the problem. Ed Hare has agreed to act as a liaison between the amateur community and AT&T at the corporate level. If a ham has a problem that he or she is not able to resolve with the local office, they should contact Ed Hare. Ed will either help the ham with information, advice or forward the contact immediately to AT&T corporate, who will help the local office better understand how to correct this problem.

Ed says, "AT&T and Phonex have been very cooperative throughout this process, but they asked if ARRL would be willing to help channel problems to them as appropriate. I have volunteered ARRL to help resolve any problems that might crop up, ranging from helping hams to understand the issue to explaining to AT&T and some newcomers that not every tuneup signal on 3.53 MHz is a PX-421, to helping hams resolve any problems they may encounter with local AT&T offices by forwarding a summary of those problems to the corporate engineering staff at AT&T.

"While AT&T expects that they will become involved with a few problems that are not actually caused by AT&T equipment, we are trying to minimize the impact of this on them by being careful that they are not asked to do things that are not under their control, such as to correct a problem on another nearby frequency that is not related the model PX-421 or to correct a problem from a non-AT&T installation, such as a DSS satellite system or a neighbor using a PX-421 with a computer system.

Ed said, "I have taken this approach so we, too, can demonstrate the good faith that Phonex and AT&T have shown by first trying to work with the information published by ARRL and within the processes that Phonex and AT&T have set up. I am in this for the long haul, though; we have a good chance at a success story that we can build into similar good relationships with other cable companies and Part-15-device manufacturers. Those who say Dave Sumner's Feb 1998 QST editorial will probably get the idea that ARRL is in this for the long haul, too, and is looking for big picture solutions to the general noise problems that are eating away at our bands. This one may serve as an important stepping stone in that direction.

"Hams can contact me with questions or help in resolving problems. I would also like to receive all reports of interference from any type of Part 15 device. I am considering how we can best approach the FCC and want to have as much information available about the impact of Part 15 devices on the Amateur Radio Service."

Ed Hare summarized this by saying, "This is still progressing reasonably well in most areas. While we would all like to see this problem corrected overnight, there was a lot that had to happen, and still needs to happen, to get that happy ending that we all hope to see. This problem was rather large -- covering the US from coast to coast and all involved parties are making progress that is much faster than I usually see these things go. Phonex and AT&T have been forthright and responsive from day one. In their memo, TCI was pretty clear -- TCI expects that they need to fix the problem and are taking steps to ensure that their service organizations are well informed about the problem and the solutions. AT&T has assumed responsibility for the system when they acquired TCI.

Harmful Interference

There has been a bit of confusion among hams and some of the AT&T staff about these modem jacks. In general, these devices are permitted under Part 15 regulations, subject to certain limits. These limits, however, can create some pretty strong signals in the ham bands! Part 15 also has a requirement that Part 15 devices not cause harmful interference to radio services. It was on that basis that ARRL, Phonex and AT&T addressed correcting the problem.

What constitutes harmful interference? Part 15 is rather vague:

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 radiocommunications service operating in accordance with this Chapter.

In practical terms, "harmful interference" is in the eye of the beholder. In legal terms, it would mean whatever the FCC determined it meant, if they had to become involved. I think that few would argue that S9+ signals are harmful interference. The FCC would probably not find that a signal just audible above the ambient noise was harmful interference. They would probably not find that a signal that can be heard on an unused frequency in the 80-meter band is harmful interference. As a service, we probably don't want to put that to the test.

Teamwork

Ed Hare credits teamwork with resolving the wireless modem problem. Many hams were also involved, notably several member of the Northern California Contest Club, the hams of Phonex, Past ARRL Pacific Division Director Brad Wyatt, K6WR; current ARRL Pacific Division Director Jim Maxwell, W6CF, ARRL's cable industry contact Ron Hranac, N0IVN; ARRL Vice President Hugh Turnbull, W3ABC; and Roanoke Division Vice Director Dennis Bodson, W4PWF. Turnbull and Bodson, both members of the ARRL RFI Task Group, helped with the policy decisions to determine how ARRL should proceed.

"Both companies have been refreshingly cooperative," said Hare, who--as his W1RFI call sign reflects--is the League's point man for interference issues. "If every RFI problem that involves Amateur Radio could be fixed so quickly, I would probably be out of a job. To me, this cooperation and teamwork is the real strength of ARRL--acting as a coordinator for the best of our best as we collectively solve the problems of Amateur Radio," Hare concluded.

He continued, however, "In most areas, AT&T is making progress, although a bit slower than hams would like. In other areas of the country, the progress has been very slow, with no perceptible improvements. I have recently been in contact with AT&T corporate engineering staff about the need to have 'voluntary cooperation' proceed at a reasonable pace. I encourage hams to keep me appraised of the progress AT&T is making in their areas so that ARRL can judge how much additional encouragement needs be offered at the national or regulatory level."

 

An article on the subject appeared in the December 24, 1998 issue of the ARRL Letter. An article appeared in the March, 1999 QST "Happenings" column.

 

Amateurs with questions about this topic or those who encounter any problems getting their problems resolved with AT&T or the manufacturer can contact Ed Hare, W1RFI, the ARRL Laboratory Supervisor, 225 Main St., Newington, CT 06111, tel: (860) 594-0318, email: w1rfi@arrl.org.

 

AT&T can be contacted through their local customer-service centers.

 

 

This audio recording (RealAudio) was supplied to ARRL by Jim McCook, W6YA. In some crowded residential areas, there can be a lot of signals heard simultaneously. This is a recording made when Jim tuned from 3.505 to 3.550 MHz.

 

Wireless jacks are unlicensed devices that are permitted, with certain limitations, under Part 15 of the FCC rules, which defines them as "unintentional radiators." But because they use the ac wiring to conduct their signals, they must comply with the general limits for "intentional radiators" under Part 15. These limits permit unlicensed, radiated signals on HF of up to 30 uV/meter--even on the ham bands. But, these devices also are required by Part 15 to not interfere with licensed users of the spectrum.

They are carrier-current devices, meaning that they conduct RF over the power lines. As such, they are classified as "unintentional radiators and are subject to various Part 15 limits on conducted and radiated emissions. Under Part 15, such devices are "Verified," meaning that the manufacturer is required to test the units for compliance with all provisions of Part 15. The manufacturers are not required to submit their test results to the FCC, but must keep them on file. Once they have been tested and passed, they may be marketed.

Carrier-current devices have the specific requirement that they be tested in three "typical" installations.

The following definitions in Part 15 lay the groundwork for the ensuing discussions:

Section 15.3 Definitions.

(f) Carrier current system. A system, or part of a system, that transmits radio frequency energy by conduction over the electric power lines. A carrier current system can be designed such that the signals are received by conduction directly from connection to the electric power lines (unintentional radiator) or the signals are received over-the-air due to radiation of the radio frequency signals from the electric power lines (intentional radiator).

(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 radiocommunications service operating in accordance with this Chapter.

(o) Intentional radiator. A device that intentionally generates and emits radio frequency energy by radiation or induction.

(z) Unintentional radiator. A device that intentionally generates radio frequency energy for use within the device, or that sends radio frequency signals by conduction to associated equipment via connecting wiring, but which is not intended to emit RF energy by radiation or induction.

Part 15 is a morass of complex, interwoven regulations. Carrier-current devices are unintentional radiators. However, carrier-current devices that operate below 30 MHz are generally subject only to the requirements for radiated emissions. However, there are no specific limits for radiated emissions for unintentional radiators for frequencies below 30 MHz, so Part 15 stipulates that carrier-current devices that operate below 30 MHz are subject to the radiated emission limits for intentional radiators.

The permitted radiated levels are described in Part 15:

Section 15.109 Radiated emission limits.

(a) Except for Class A digital devices, the field strength of radiated emissions from unintentional radiators at a distance of 3 meters shall not exceed the following values:

Frequency of Emission (MHz)	Field Strength (microvolts/meter)
30 - 88	100
88 - 216	150
216 - 960	200
Above 960	500

***

(e) Carrier current systems used as unintentional radiators or other unintentional radiators that are designed to conduct their radio frequency emissions via connecting wires or cables and that operate in the frequency range of 9 kHz to 30 MHz, including devices that deliver the radio frequency energy to transducers, such as ultrasonic devices not covered under Part 18 of this Chapter, shall comply with the radiated emission limits for intentional radiators provided in Section 15.209 for the frequency range of 9 kHz to 30 MHz. As an alternative, carrier current systems used as unintentional radiators and operating in the frequency range of 525 kHz to 1705 kHz may comply with the radiated emission limits provided in Section 15.221(a). At frequencies above 30MHz, the limits in paragraph (a), (b) or (g) of this Section, as appropriate, continue to apply.

and

Section 15.209 Radiated emission limits, general requirements.

(a) Except as provided elsewhere in this Subpart, the emissions from an intentional radiator shall not exceed the field strength levels specified in the following table:

Frequency (MHz)	Field Strength (microvolts/meter)	Measurement Distance (meters)
0.009 - 0.490	2400/F(kHz)	300
0.490 - 1.705	24000/F(kHz)	30
1.705 - 30.0	30	30
30 - 88	100 **	3
88 - 216	150 **	3
216 - 960	200 **	3
Above 960	500	3

and:

Section 15.31 Measurement standards.

(d) Field strength measurements shall be made, to the extent possible, on an open field site. Test sites other than open field sites may be employed if they are properly calibrated so that the measurement results correspond to what would be obtained from an open field site. In the case of equipment for which measurements can be performed only at the installation site, such as perimeter protection systems, carrier current systems, and systems employing a "leaky" coaxial cable as an antenna, measurements for verification or for obtaining a grant of equipment authorization shall be performed at a minimum of three installations that can be demonstrated to be representative of typical installation sites. (Emphasis added -- Ed.)

Intentional Radiator Limits

The radiated limits for Part 15 devices that can be operated legally on any frequency, including Amateur Radio frequencies, are actually fairly high. Assuming far field conditions, an isotropic antenna should pick up about -58 dBm from a 30 uV/m field, or about 15 dB over S9, assuming S9 to be 50 microvolts into 50 ohms (-73 dBm at the receiver input terminal).

In addition to these absolute maximum requirements, Part 15 also requires that devices operated under Part 15 must not cause interference to authorized radio services. If such interference does occur, Part 15 requires the operator of the Part 15 device to take whatever steps are necessary to reduce or eliminate the interference.

Section 15.5 General conditions of operation.

[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 a Commission representative that the device is causing harmful interference. Operation shall not resume until the condition causing the harmful