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Digital Mobile Radio Hotspots May Be Interfering with Satellite Uplinks, AMSAT Reports

09/05/2018

This week, AMSAT News Service (ANS) cited an August 27 report from AMSAT Vice President-Operations Drew Glasbrenner, KO4MA, saying that a digital mobile radio (DMR) signal has been interfering with the AO-92 (Fox-1D) satellite’s 435.35 MHz uplink frequency. Glasbrenner said hotspots, repeaters, terrestrial simplex, and “anything not satellite” should never transmit in the segments 145.8 – 146.0 MHz or 435 – 438 MHz by international band plan. Well-known satellite enthusiast Patrick Stoddard, WD9EWK/VA7EWK, told ARRL that one DMR hotspot operating on the AO-92 uplink frequency in the St. Louis area has shifted to another frequency. But, he added, “I think there are still issues, since not all hotspots will report their frequencies and positions to websites such as BrandMeister or via APRS, where they appear on other sites such as http://aprs.fi.”

“There are others surely operating near satellite uplinks,” Stoddard added. “For many, the 435 – 438 MHz satellite subband is a big piece of quiet real estate in a busy part of the 70-centimeter band for weak-signal work, repeater links, amateur TV, and other possible uses.”

Stoddard points out that FCC Part 97 addresses Amateur Radio operation in these segments, although regulations in many other countries may not be as detailed. §97.3(a)(7) defines Auxiliary Stations as, “an amateur station, other than in a message forwarding system, that is transmitting communications point-to-point within a system of cooperating amateur stations.”

Stoddard said this would include remote bases, EchoLink and IRLP nodes, and hotspots used for digital voice modes, as well as stations using these hotspots and nodes. Auxiliary stations may not transmit in the 145.8 – 146.0 and 435 – 438 MHz satellite subbands (among others in the 2-meter and 70-centimeter amateur bands), per §97.201(b).

  • §97.3(a)(40) defines a repeater as, “an amateur station that simultaneously retransmits the transmission of another amateur station on a different channel or channels.” Stoddard said that because most hotspots operate on a discrete frequency, they would not qualify as repeaters, even if they operate like a repeater, per §97.205(b).

  • §97.101(a) stipulates, “In all respects not specifically covered by FCC Rules, each amateur station must be operated in accordance with good engineering and good amateur practice.” Stoddard remarked, “Whether the hotspot is interfering with a satellite downlink in a particular area, or it is interfering with the satellite uplink affecting a much larger area, this would not be good amateur practice.”

  • In addition to subbands where hotspots are not permitted, Stoddard said, §97.101(b) is also relevant. It states, “Each station licensee and each control operator must cooperate in selecting transmitting channels and in making the most effective use of the Amateur Service frequencies. No frequency will be assigned for the exclusive use of any station.”

Stoddard noted that frequencies used by satellites are usually incapable of being changed and are coordinated in advance of launch, while hotspots typically are frequency agile, and the frequencies used by those systems can be changed to avoid potential interference to satellites and other stations. — Thanks to AMSAT News Service, Patrick Stoddard, WD9EWK/VA7EWK 



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