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Wireless Modem and Telephone Jacks

  • Overview-

    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.

  • About Wireless Jacks+

  • ARRL Testing+

  • Manufacturer and Cable Company Responses+

  • How to Fix the Problem+

  • More Information+

  • +Listen to a Recording of PX-421 Interference+

  • Background on Part 15 of the Applicable FCC Regulations+

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