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The Myth of Radio Spectrum Interference
Selanit writes "Just came across a fascinating article on Salon about a technologist who claims that there is no such thing as "interference" in the radio spectrum. He argues that interference is a symptom of inadequate equipment, not a fact of nature, and that with improved transceivers we could open the spectrum up to high-quality broadcasts by anyone. Reference is made to the GNU Radio Project. Neat stuff." We've posted other stories about this. I wonder if the "color" meme will catch on.
Reed's article is based on the observation that Maxwell's Equations are linear (for most materials) and that, therefore the waves pass through each other without modification (again, unless you're in pretty exotic environments --- early universe, etc.) The problem with interference arises because of imperfect spectral content and non ideal antenna response for both transmitters and receivers. Interference is like being at a party: There are a lot of people talking, and your ears hear in all directions, so you have to be near the person you're trying to talk to.
... the reason that it works is that your eyes have very fine angular sensitivity (high quality antennas) compared to your ears.
For a variation on this theme, there's an interesting moment in a movie (Frankie and Johnnie?) where there's a terrific racket in a diner, impossible to understand anything, but a cook and a clerk are communicating easily --- by sign language. Consider also those occasional TV images of the Wall Street pit traders flinging gang signs at each other
Spectral purity and antenna quality limitations can be overcome --- by money. You can build higher quality receivers and transmitters, bigger antenna installations but it costs money and space in fairly unavoidable ways.
Reed is also wrong from a regulatory level. It's not just the FCC that you'd have to work with, but the ITU. Those pesky radio waves have this interesting habit of leaking over borders on the ground, and pretty much everywhere down here from satellites.
There are pretty good reasons to pick on modern broadcasting: crappy content, media concentration --- but "broadcasting" is not one of them. Those great big transmitters permit the use of very dumb receivers with poor sensitivy. The very simplicity and asymmetry of broadcast provides tremendous economic and technical appeal, and I'd be amazed if it ever went away.
Far more interesting is the glacial progress of DTV in broadcast.
Unfortunately, this is not true.
Suppose a city has two stations, one on 1600 kHz and one on 900 kHz. Let's add a station on 700 kHz, ok? Let's put him near the 1600 kHz station, since we don't want these damn antennas cluttering up the whole city. No problem with "well made equipment", right?
Now consider that near to both the 1600 and 700 antennas is a large, old, steel-framed building, containing tens of thousands of rivets and metal-to-metal joints. Some of these joints have some corrosion. Consider that there may be several such buildings. Why is this a problem?
Each joint is a potential non-linearity. Each joint is capable of taking the 1600 and 700 signal and creating the sum and difference signals and re-rediating them. The sum is 2300 kHz, outside the AM broadcast band. The difference is ... 900 kHz. The same frequency as an existing station.
Now consider if you live inside one of these buildings. You used to listen to the station on 900 kHz. Now you hear a wonderful mixed babbling of both the 1600 and 700 kHz stations -- and your radio has nothing to do with creating the problem.
Let's go one step further. These same non-linear conductors will cause sum and difference issues with single-frequency signals, too. The new station on 700 kHz will sum with itself and cause a signal on 1400 kHz. And it's even worse. The actual result will be signals on every multiple of 700 kHz well up into the shortwave bands. (If the non-linearity created a perfect square wave, you'd get only the odd harmonics, but these aren't perfect and you get even harmonics, too.)
Can't happen, you say? Yes, it can, and does. I've lived with this problem for the last 4 years from two nearby stations. It has finally gone away, since one of them moved their antenna location a mile further away, but before they did that, they made a lot of the spectrum useless here.