Driving in my car to work, I can listen to NPR on 89.9. However, upon arriving at work and going inside, 89.9 is a rap channel, and it comes in very clearly. Why is that, and is there anything I can do to the stereo to make it change 89.9 back to NPR? The approximate distance between the stereo and my car is about twenty yards.
The stereo can’t be moved, but it does have one of those wire antennas that can be moved around a bit.
You need a directional antenna, and aim it at the NPR transmitter. Unfortunately, your wire antenna doesn’t qualify - you need to get one from Radio Shack.
FM receivers benefit (or suffer, as the case may be) from what’s called the capture effect, in which a stronger signal at the antenna completely captures the tuner and you only hear the one station. It only takes a small change in local signal strength to switch between them. This is unlike AM, where multiple signals can all be heard simultaneously. As beowulff says, a directional antenna is your best bet here.
Switching the receiver to mono can also make a big difference - give that a try if you can.
I wouldn’t imagine you need full stereo for All things Considered most days.
Since the question has been factually answered, I’ll just throw this in. Most of NPRs content is podcast. Since I discovered this fact I haven’t listened to NPR live in a couple of months now.
But then you have to have a ‘pod’… Too geeky… 
I burn most of the podcasts I care about to CD-RW and pop them into my car stereo.
Or just a computer, which gitfiddle probably has at work anyway.
Personally, I’m still a little puzzled by a total change within 20 yards. Even if there’s an abrupt threshhold where one signal completely swallows the other, you’d think that there’d be random effects (fluctuations in the power supplies, meteorological conditions, etc.) that would change the relative strengths slightly and thus move the dividing line back and forth a bit.
Not as geeky as reading this website.
How does the tuner figure out that there are two distinct signals if they are on the same frequency? Wouldn’t it receive the sum of the two carrier waves?
ETA: I know just enough to be stupid about this. It just occurred to me that with amplitude modulation it would add the two waves since they are truly on the same frequency, but with frequency modulation things are more complicated but I still don’t know how it distinguishes which signal is which.
It’s probably environmental; the construction of the building is such that it attenuates the NPR station much more than the rap station, for example. Local geology and architecture can affect things, too–I’ve encountered odd standing wave patterns where moving just a few inches makes the difference between recieving and not, due to reflections from hills, mountains or buildings.
I should note that this building (and the tiny street it’s on) are also the only place in my city that have little or no signal on my cell phone. This is especially true back in the lab that I’m working in.
I actually listen to NPR segments at home every day on my computer (through RSS feeds), but at work there’s actually someone in the lab (I’m working as a lab tech at a pharmacy) who is always back there with me. I don’t want to listen to my MP3 player, because we do talk. I’ve started to get her hooked on NPR, which is wonderful, but the station we do get in the lab (89.3) only has The World and All Things Considered.
I’d just like to integrate a little The Take-Away and Talk of the Nation (my favorite) into the mix of what is becoming a repetitive set on the music channels, and 89.9 is the channel that offers them.
I could probably put the podcasts on my mp3 player and plug my mp3 in as the auxiliary on the stereo, if I can find that cable I had.
Perhaps someone in your building has a satellite radio with an FM transmitter? It would emit a strong signal with a very short range. Those transmitter usually are programmed in that frequency range. Oftentimes, my NPR station is “overridden” in the car by a satellite radio from the next car over…
It does receive the sum. In an FM receiver, the first step of detection is called a limiter. This amounts to what happens when a guitar amplifier is turned up way past 11. The strongest part of the summed signal drives the limiter to saturation, removing the parts added by the weaker signal(s). It takes very little difference in amplitude for the stronger signal to totaly dominate the limiting process. The limiting process is what accounts for the absense of noise (static) on other than weak FM signals.
Another way to look at is would be to add a small sine wave to a large sinewave. Unless they are close to equal, the zero crossings will be controlled by the period (frequency) of only the larger signal…so the frequency of the combined signal only really depends on the larger one. Since it is only the frequency of the FM signal that is carrying the audio information, that is all you hear.
Are we talking about two different receivers here? I suspect we are. Radios vary widely in their abilities. My own guess, from personal experience, is that the rap station’s transmitter is nearby and the stereo’s front end is being overwhelmed, while the car radio is managing to reject that signal. (I’m doubtful that the two stations are on the same frequency, since it would be rare to find a rap station in the middle of the educational band!)
Anecdote:
Once upon a time, probably ca. 1985-86, I was driving from Winston-Salem to Columbia, where I lived at the time. Hunting around for a new radio station as I got close to Charlotte, I found Charlotte’s 104.7, which at the time was a top-40 station, or I probably wouldn’t have been listening to it.
Somewhere around halfway to Columbia, either the stations were playing the same song at the same time or the same ad at the same time or my attention wandered long enough for me not to notice an abrupt segue, but imperceptibly, I was suddenly listening to a different 104.7.
Creeped me right out, it did.
There’s an educational band?
I’m roughly equidistant from two NPR stations that both broadcast on the same frequency, for the duration of my commute. Most of the time the radio flutters uselessly between the two, though there are hills and other geography that seem to get it to settle on one or the other for extended times.
So I got a Sirius radio to listen to NPR with.
Now it flutters uselessly depending on how heavy the trees overhead are.
Effectively, frequencies below 100 MHz are reserved for NPR and college stations. This seems to be longstanding FCC policy, but I don’t know if it has the force of law.
It could be a college station staffed by rap fans. College stations are typically so strapped for staff they’ll let anyone on who will spin disks, regardless of musical diversity*, and I wouldn’t put it past one to be somewhat misconfigured so its signal bleeds outside the lines and overwhelms weaker stations nearby. (I don’t know enough about FM to be sure about that last.)
*(My local college station is usually just an old iMac with a bunch of MP3s on its hard drive playing songs at random. That makes college radio effectively ‘classic rock’ with a larger and more diverse playlist, straying into more recent ‘alternative rock’ and other genres on occasion. Individual live DJs can play damn near anything they want as long as it doesn’t frighten the FCC narcs in the street, though, and while I doubt we have any all-rap shows it’s certainly possible.)