Shorter Waves: The 33cm, 23cm, and 13cm Amateur Radio Bands

2024

As it always has been, most Amateur radio activity in the United States is biased to the 70cm (440 MHz) band and below. The higher bands are used for Amateur TV and point to point data links primarily with some some mobile communication in the 33cm band. This page covers what amateur, commercial, and government users are doing in and around the 33cm, 23cm, and 13cm bands.

33cm Band: 902-928 MHz

The 33cm band is full of ISM (Industrial Science and Medicine) part 15 device transmissions. There are some narrow FM amateur radio voice repeaters and some digital activity (POCSAG pagers), but Meshtastic which uses non-amateur and unlicensed LoRa transceivers to forward packets in a mesh network has recently revitalized this band.

33cm Band Spectrum Usage in the United States

References

902-928MHz ISM Band

The 900MHz ISM band is highly active in cities. It is used for a litany of low datarate communications including remote meter reading, IoT connectivity (including Amazon project sidewalk), and Meshtastic. These are FCC part 15 applications with power limits as high as +36dBm for spread spectrum systems. Part 15 use of this spectrum should not be conflated with part 18 use of this spectrum. Part 18 is use in line with the original intent of the ISM bands: "Industrial, scientific, and medical (ISM) equipment. Equipment or appliances designed to generate and use locally RF energy for industrial, scientific, medical, domestic or similar purposes, excluding applications in the field of telecommunication. Typical ISM applications are the production of physical, biological, or chemical effects such as heating, ionization of gases, mechanical vibrations, hair removal and acceleration of charged particles."

This distinction is relevant for considering the prioritization of use of this spectrum. Part 15 use is currently responsible for most traffic in this band, but it is the lowest actual priority. A company called NextNav is petitioning the FCC to move around some of its existing licenses and change the band allocation to have a 5MHz 5G uplink channel at the bottom of the band and a 10MHz channel at the top of the band. This would total more spectrum than they currently own and interfere with existing part 15 use at the top of the band, so I am a bit skeptical that their petition will be approved.

900 MHz ISM Band Priority from NextNav FCC Petition

M-LMS stands for Multilateration Location and Monitoring Service which was intended to be a terrestrial addition to GPS for higher precision especially in cities. This service never really took off, and the restrictions on the licenses appear to make them incompatible with using this spectrum for 5G. The Electronic Frontier Foundation (EFF) characterizes NextNav's proposal as a "Callous Land-Grab." Because their proposal would expand the spectrum they are licensed to use without an Auction and because their proposed use case is incompatible with the original intention of M-LMS, I tend to agree.

23cm Band: 1240-1300 MHz

There is not much happening in the 23cm band. There are a few scattered amateur television repeaters and some occasional voice and data usage, but in most areas this is an empty band. While the phrase "use it or lose it" comes up in reference to the belief that using Amateur radio allocated spectrum will make it less likely for the FCC to reallocate this spectrum to other users, I actually think that the current light use of the 23cm band has protected it from action by the FCC to protect radars. If anyone could buy a Baofeng off of Amazon and talk to their Airsoft buddies thereby in some minority of instances causing interference to Air Route Surveillance Radar, I think the FCC would be motivated to restrict use. As it stands, the 23cm band provides quiet spectrum to Amateur operators who can deconflict their operation with Local radars.

23cm Band Spectrum Usage in the United States

References

Air Route Surveillance Radar: A Significant Source of interference

The AARL bandplan webpage notes that FAA radars are used in this band and that operation at those frequencies must be avoided to prevent interference. These radars appear to be CARSR (Common Air Route Surveillance Radar) radars operated by the FAA and ARSR-4 (Air Route Surveillance Radar) radars operated jointly by the FAA and Air Force in the JSS (Joint Surveillance System).

Map of CARSR and ARSR-4 Radars in Preliminary Report on SENSR (Spectrum Efficient National Surveillance Radar). Final Report

SENR (Spectrum Efficient National Surveillance Radar) appears to be a program to consolidate radar spectrum usage in the United States to make at least 30MHz in the 1300-1350 MHz range available for public auction. This program would satisfy the requirements of the Spectrum Pipeline Act of 2015 which required the FCC to identify at least 30 MHz of spectrum to put up for auction with the auction proceeds used to pay for moving equipment off of these frequencies. This effort is relevant because it may mean that radars currently operating in the 1300-1350MHz range may be reallocated to the 1240-1300MHz range. It seems that the repurposing of 1300-1350MHz is on hold (or will be subject to geographic restrictions) based on a 2023 report on the status of spectrum repurposing.

The exact frequency assignment of each radar is not publicly available as far as I can tell which requires local measurements to be made in order to avoid interference. There is some anecdotal information that CARSR radars are more sensitive than the older radars. In 2012, the FCC took action against an Amateur radio operator after his repeater inadvertently continued to operate after he was instructed to turn it off to stop interference. Aside from that, the only other reference to mitigating 23cm interference with radar is from the Southern California Repeater and Remote Base Association indicating that some repeaters in that area "moved up in the band" to prevent interference in 2012.

CARSR at St. Louis, MO FAA Long Range Radar Site

I stopped by the CARSR in St. Louis while home for Christmas. It is right in the middle of a residential neighborhood and, in addition to being impressively large, it looks a bit like something out of Fallout. It is probably a coincidence, but it seemed to use similar frequencies to the one in Seattle. I unfortunately messed up saving the captures I took with an Airspy plugged into my phone, but here is what I see from the Seattle CARSR while sitting at home:

Emissions from the Seattle CARSR Radar

The pulses of energy seen here are not individual pulses but rather many pulses averaged together as the radar rotates. If you run the FFT at full speed, you can see that the radar transmits an up chirp at the higher frequency before transmitting a paired pulse at the lower frequency; the two pulses are not transmitted simultaneously as the plot zoomed out in time would suggest.

Emissions from the Seattle CARSR Radar

13cm Band: 2300-2310 MHz & 2390-2450 MHz

The present 13cm band allocation consists of 15MHz of largely unencumbered spectrum and 55 Mhz of spectrum shared with WCS Uplink and WiFi. With the split between the low and high end of the band, it would be possible to operate a frequency division duplex (FDD) radio system with symmetric 5MHz uplink and 5MHz downlink spectrum. While no phones or filters exist to enable this today, you could operate a cellular network using this frequency allocation. So far, though, most amateur activity in this band is from High Speed Multimedia radio (HSMM) which is basically WiFi at ham frequencies. There is some of this at 33cm as well but none that I found at 23cm. There is also some amateur television and Earth Moon Earth (EME) contesting on this band.

13cm Band Spectrum Usage in the United States

References

According to Wikipedia, the FCC withdrew 55 MHz of Amateur Radio 13cm band spectrum and auctioned it off in 1997 to create the Wireless Communication Service (WCS) and Satellite Digital Audio Radio Service (SDARS) bands. This does not seem to be true, though. I couldn't find a copy of the relevant section of the federal register from before 1989, but in the 1989 copy, this spectrum is not allocated to amateur radio. In the 1997 DARS (Digital Audio Radio Service) order, the FCC makes no mention of this spectrum previously being used for amateur radio. It does, however, state:

In the band 2320-2345 MHz, the mobile and radiolocation services are allocated on a primary basis until a Broadcast-Satellite (sound)service has been brought into use in such a manner as to affect or be affected by the mobile and radiolocation services in those service areas. Permissible use of these bands include telemetry an telecommand transmissions associate with the launching and reentry into th earth`s atmosphere as well as an incidental orbiting prior to reentry of manned or unmanned objects undergoing flight tests.

— 1997 FCC DARS Order

That definitely suggests that 13cm band wikipedia article is incorrect about this spectrum recently being removed from amateur radio use. One entertaining aside from the 1997 DARS Order is that Cracker Barrel Old Country Store, Inc. was apparently a contender for this spectrum:

A band plan introduced by Cracker Barrel in its reply comments maintains that by using Time Division Multiplexing (TDM) technology, 30 channels of CD quality audio can be accommodated in 8.32 MHz, or 32 channels of CD quality audio could be provided in 8.32 MHz using Code Division Multiplicity (CDM) technology, and thus six operators (presumably six economically viable systems) could be accommodated in the 50 MHz initially allocated for satellite DARS. Cracker Barrel also contends that if all satellite DARS providers use the same error correction rates, then as many as eight satellite DARS licensees could be accommodated in the 50 MHz (i.e., each with a 6.25 MHz assignment) and each could offer at least 30 channels of CD quality audio. Cracker Barrel contends that its band plan does not require use of regional spot beams or a higher order modulation constellation to gain additional channels per MHz of spectrum. It asserts that by using 1/3 rate or 1/2 rate FEC as opposed to 1/4 rate as originally proposed by CD Radio and Primosphere, the bandwidth requirement for a 32 or 30 channel CD quality system could be reduced from 12.5 MHz to 8.32 MHz and 6.25 MHz respectively.

— 1997 FCC DARS Order

I did not dig into exactly what audio encoding Cracker Barrel had in mind, but what we have today is certainly not "CD Quality." Whenever I hear satellite radio today, the audio artifacts are painfully obvious. Sirius and XM satellite radio are the present operators in the SDARS (Satellite Digital Audio Radio Service) bands. The rest of the gap in amateur spectrum is consumed by LTE band 30. To borrow a phrase from Light Reading, AT&T is the WCS cellular band operator through "auction and acquisition."

Uncle Herschel's SDARS