In early 2005, Josh Demmitt arrived at a federal prison camp, in Sheridan, Oregon, to serve a thirty-month sentence for starting a fire outside an animal-testing facility at Brigham Young University. The nineteen-year-old received a warm welcome from his fellow inmates, who greeted him with coffee and cigarettes, advice on procuring vegan meals, and a pocket AM/FM radio.

The radio provided hours of welcome distraction for Demmitt, who had come from Sheridan’s adjoining detention center, where, he says, he spent weeks without a radio while confined to a small cell for at least twenty-three hours a day. The radio was unlike any Demmitt had seen outside prison, with a transparent plastic body that revealed the landscape within: a single AA battery rested at the bottom of its circuit board, while its antenna—one and three quarter inches of copper wire coiled around a small ferrite bar—peeked through a white Sony logo, just above the AM/FM dial.

The pocket analog radio, known by the bland model number SRF-39FP, is a Sony “ultralight” model manufactured for prisons. Its clear housing is meant to prevent inmates from using it to smuggle contraband, and, at under thirty dollars, it is the most affordable Sony radio on the prison market.

That market consists of commissaries, which were established by the Department of Justice in 1930 to provide prisoners with items not supplied by their institutions; by offering a selection of shampoos and soaps, they shifted personal hygiene costs to inmates, while distractions like playing cards eased tensions among the nation’s growing prison population. More than half a million inmates each week shop at commissaries stocked by the Keefe Group, a privately held company that sells items to the Federal Bureau of Prisons, and twelve out of fourteen privately managed state departments of corrections. A sample commissary order form lists items like an I.B.M. typewriter ribbon, hair dye, RC Cola, Sensodyne toothpaste, chili-garlic sauce, Koss CL-20 headphones, and a “Sony Radio.”

Commissaries often carry other, bargain-brand radios, but according to former inmates and employees of the Bureau of Prisons and the Keefe Group, who spoke on the condition of anonymity, America’s federal prisoners are most likely to own a Sony. Melissa Dolan, a Sony spokesperson, confirmed in an e-mail that selling portable radios in American prisons has long been a “stable business” that represents “sizable” sales for the company. Of the models available, the SRF-39FP remains an undisputed classic, still found on commissary lists an impressive fifteen years after its initial release, making it nearly as common behind prison walls as Apple’s iPod once was outside of them, despite competition from newer devices like digital radios and MP3 players.

But sheer availability doesn’t explain its ubiquity. The SRF-39FP is the gold standard among prison radios in part because it runs on a single AA battery, and offers forty hours of listening time—longer than an iPod Classic. Digital models can require twice as many batteries, like the Sony SRF-M35FP, which runs on two AAAs. Federal inmates are particularly attuned to battery life because they are allowed to spend just three hundred and twenty dollars each month on commissary goods; more cash spent on batteries means less for snacks, stationery, clothing, and toiletries.

The importance of radio battery life in prison communities cannot be overstated; the devices are relied on for more than listening to music, hearing about local news and weather, and watching television (TV sets in common areas often use transmitters to broadcast sound on a dedicated frequency). A study conducted at San Vittore prison in Milan, Italy, found that “in a place where privacy is constantly denied, radio becomes a vital tool for building and maintaining one’s private self.” Some inmates even had a term for using their radio to create a bubble of personal space: “I headphone myself,” one said.

There is also a bit of prison culture itself at work in the story of the SRF-39FP. Radios like the one that was loaned to Demmitt are usually left behind by inmates who have reëntered the free world. Some prisoners believe that it is bad luck for radios to leave prison with their owners, while others believe that taking them simply violates the “convict code,” according to former inmates like Demmitt and Steven Grayson, author of “The Unauthorized Federal Prison Manual.” Whether radios are abandoned as a matter of solidarity, convenience, or good karma, they pass from inmate to inmate, serving one sentence after another. The durable, analog SRF-39FPs have been changing hands in this manner for a decade and a half, which adds up to a lot of radios in circulation.

This practice helps explain the relative rarity of the SRF-39FP outside prisons. A unit in good condition can fetch up to double or triple its retail value among enthusiasts and collectors like Gary DeBock, a co-founder of the Ultralight Radio Group. According to DeBock, the outside supply depends upon stock siphoned from the California prison system and sold on auction sites like eBay.

DeBock is a member of the “DXing” community, whose hobbyists attempt to pick up distant radio or television signals, including those from amateur or pirate radio stations. (“DX” is shorthand for “distant stations.”) DeBock’s fascination with the SRF-39FP began when he realized that it could receive AM signals from places as distant as Japan and Korea at his home in Puyallup, Washington. “Since then, I’ve probably had more exposure to the SRF-39FP than anyone else who has managed to stay out of prison,” DeBock said.

Others in the online DXing community argue that the SRF-39FP is superior to virtually every other pocket analog radio, praising it for its large tuning thumbwheel, over-all sensitivity and audio quality, and, above all, its reputed indestructibility. Electronics and radio collectors also marvel at features that are normally associated with professional equipment rather than consumer goods: in particular, an exceptional single-integrated-circuit receiver that insures reception in remote locations—or deep within heavy prison walls. In fact, the SRF-39FP was one of the first radios to use the breakthrough CXA1129N integrated-circuit chip, considered by DeBock to be the primary innovation among Sony pocket radios; it helped make the SRF-39FP the smallest and most sophisticated in a line of pocket radios that had launched two decades earlier, in the late nineteen-seventies.

In recent years, Sony has opted to shift its prison-radio lineup away from analog, focussing instead on digital models like the SRF-M35FP. Last year, the Bureau of Prisons decided that it was time to further upgrade prison tech. Following a successful test at the same West Virginia federal prison camp where Martha Stewart spent five months for lying about a stock sale, prison officials began selling MP3 players that allow inmates to download songs at terminals in prison commissaries.

A Bureau of Prisons spokesperson said that the MP3 program wasn’t expected to make money in its early years. Price is one reason: the MP3 player sold in federal prisons costs roughly three times as much as an SRF-39FP, and downloads can cost up to a dollar and fifty-five cents per song. Limited song selection is another reason; the Bureau of Prisons prohibits songs deemed explicit or likely to incite the inmate population. (JPay, a company that provides services to inmates, boasts that, with its catalogue of ten million songs, “no other music service in corrections offers as many tracks for download.”) However, despite modest expectations for the technology upgrade, the Bureau of Prisons spokesperson Ed Ross said that more than fifty per cent of federal inmates have already bought MP3 players. It seems inevitable that the MP3 player will soon completely eclipse radios like the SRF-39FP in American prisons, just as they did outside, but for now both devices are woven into prison life.

Josh Demmitt left his Oregon prison camp in May, 2007, two and a half years after he had arrived. While his radio didn’t come back with him to suburban Utah, within a matter of weeks he bought his first iPod.

Joshua Hunt is a freelance journalist who has reported on business, technology, and sports in Japan for the New York Times.

Illustration by Matthew Hollister.