How did spongebob squarepants hack my radio?
July 16, 2014 10:49 PM   Subscribe

As I was driving home the other day, listening to you do, my radio fuzzed out and an episode of Spongebob Squarepants took over. It played for about 2 blocks as I kept pace with the traffic nearby, then faded back to All Things Considered when I turned. Our NPR station is at 88.3 on the dial, and I can only assume that a nearby car had a kid watching Spongebob in it, but how did the not-so-soothing tones of Patrick and Squidwart supplant Melissa, Robert and Audie?!
posted by nerdcore to Technology (7 answers total) 5 users marked this as a favorite
FM transmitters for interfacing devices to car stereos that don't have input jacks often use the low frequencies (or people tune them there out of habit even if their transmitter can do the full FM range). Someone near you was using one powerful enough to overcome the signal strength of the NPR station.
posted by Candleman at 10:55 PM on July 16, 2014 [2 favorites]

The Spongebob fans had something like this and its carrier signal was overwhelming that of your local NPR affiliate. If you need more detail than that you'll have to specify further what you mean.
posted by contraption at 12:02 AM on July 17, 2014 [1 favorite]

To go a little bit beyond that, and explain why you didn't get a mix of both: FM receivers exhibit a phenomenon called 'capture effect', where a slightly stronger signal (above a certain threshold) will completely block out a weaker signal and render it inaudible.

(If, on the other hand, the signals are very similar in strength, the receiver will tend to jump quickly between one and the other - so-called "picket fencing".)
posted by Pinback at 1:18 AM on July 17, 2014 [2 favorites]

Back in 2006 NPR actually threatened XM Radio with a lawsuit because the FM transmitters they bundled with their radios were too strong and overpowering the lower FM frequencies where most of their stations were parked.

Knowing people on the inside, the problem wasn't exactly the strength of the transmitter but the fact that the modulator was putting the signal on the wrong wire on the car's power bus, which turned the car's external antenna into a small FM station. My bet is that the minivan's modulator has a similar cheap miswired device.
posted by JoeZydeco at 1:56 AM on July 17, 2014 [4 favorites]

Actually my memory is a little clearer now...most FM modulators send the signal to the ground plane of the car, which is the negative side of the power supply. The entire power/signal cable is shielded by a metal sheath which is supposed to keep the FM from leaking out. The suspect modulators put the signal on this shield instead of the ground wire, turning the entire cable into an emitter. This was how they were leaking signal and stomping on the reception of others...
posted by JoeZydeco at 2:38 PM on July 17, 2014

This is a phenomenon called Sporadic E propagation. It peaks in the summer and has a smaller peak in the winter. Many times FM stations from across the country can overpower your local station. A good explanation on Wikipedia.
posted by Grumpy old geek at 5:58 PM on July 17, 2014

Unless there was an FM transmitter on 88.3MHz about 500~2000km away that just happened to be transmitting Spongebob at stupendously-high* power levels, it probably wasn't sporadic E.

Tropo / atmospheric ducting maybe, but not sporadic E…

(* As in "fuck you, FCC!" levels of power, to overcome the path losses and still be ~6dB or more stronger than the local transmitter in order to capture the receiver & overcome picket-fencing.)
posted by Pinback at 2:31 AM on July 18, 2014

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