Although I live in Baltimore, I sometimes like to listen to WCBS, an all-news radio station (880 am) from NY. There are times when I’m listening to WCBS that I will hear WBAL, an all news Baltimore station (1090 am) in the background. I might be able to understand this if the broadcast frequencies were close together on the dial, but they aren’t. Can anyone explain this to me? 
Not really, but -
Somehow your receiver is generating an intermediate frequency signal from the WBAL transmission when you are tuned to WCBS.
I have no idea as to your electronics background so I’ll include a little primer on AM receivers.
All broadcast receivers use the heterodyne system to produce a single frequency, called the IF (Intermediate Frequency) signal which is then amplified and the audio modulation is detected and amplified, producing the sound out of the loud speaker.
The IF is produced by a device called a “mixer” which is a device that multiplies the incoming signal by a locally generated oscillator signal producing an output the frequency of which is the difference between the two mixer input frequencies. Unfortunately the mixer also produces a host of other frequencies which includes every difference and sum between the two mixer inputs and sums and differences of the sums and differences and of all harmonics of the inputs.
The receiver contains filters that are supposed get rid of all but the desired difference frequency, the IF. I can only suppose that some idiosyncratic quirk in your particular receiver is allowing an IF to be generated at a low level by the Baltimore station.
I should add that since you live in Baltimore that signal will be a lot stronger than the New York signal and if you are fairly close to the Baltimore station antenna, the signal from WBAL might possibly be slightly overloading your receiver input which is the mixer. If that’s the case almost any weird effect can take place.
I can’t come up with any combination of image frequency that might explain this. So I am just guessing that since WBAL is a 50,000 watt trasmitter sitting in your back yard, I would not find it surprising that such a strong signal is muscling its way into your radio’s detector stages with utter disregard for whatever frequency your front-end might be tuned to.
If the WBAL occurs in the background only when the receiver is tuned to WCBS then it can’t be direct detection of the WBAL signal because of overload. That situation would result in WBAL always being present. In this case it has to be somehow tied to the frequency of the receiver local oscillator.
I suspect that if you take the m[sup]th[/sup] harmonic of the local oscillator and the n[sup]th[/sup] harmonic of WBAL, both strong signals, that somewhere in the range of m’s and n’s you will find a difference at or very near the intermediate frequency.
All of the input frequencies and their harmonics and all of the possible sums and differences are always present in the mixer which is a non-linear device by definition. In the presence of a strong signal like that of a nearby antenna some interesting things can happen.
There are just dozens of reasons that can cause this effect. Old time R&E publications used to carry stories about DXers and hams that managed to snare an extremely weak signal, even from a completely different band, due to various unusual effects.
E.g., If the path of the signal from the NY stations to you passes directly over the BAL antenna, then you get a weird modulation effect there that allows you to pick up a combined signal. Some roving might help determine if this is the cause.
But, like I said, dozens of weird reasons can cause this.
O.K. I’ll give it a try
AM=Amplitude Modulation
AM is generally in a low frequency range that is measured in “KiloHertz” or (Khz). The signals that it uses are on a very long bandwidth that take up a lot of space
when transmitting data. Some of these bandwidth signals are so long that sometimes they tend to overlap one another (which is why back in the 1970’s almost every popular Radio station in the country all went over to the FM or Frequency Modulation) dial. This (FM) operates on a much higher frequency and is measured in the MegaHertz range (Mhz). The signals that FM uses have a far shorter bandswidth than AM and can transmit far more data thus making it better for both transmitting and recieving data signals.
I hope the cleared things up a bit
I don’t know how likely this is, but in theory a defective (or old) radio elsewhere in your house or a neighbor’s is tuned to the other station and radiating the signal on its IF band (which is the same for all radios) and your IF amplifiers are picking it up and amplifying it. I think you would need some fairly sophisticated equipment to test if this is in fact the case. But if that is the case, you should also hear when tuned to no station.
Not really.
It is true that the higher the frequency, the more data you can transmit using it, but that’s not really important for this discussion. The AM and FM radio bands are arbitrarily limited in what you broadcast on them by the FCC.
The reason AM signals “overlap” is due to the fact that the signal is modulated on the amplitude of the wave. When you add signals together, you add their amplitudes together, and there’s no easy way to seperate them again. If you have two AM stations on the same frequency, their signals will simply add together and you’ll hear both.
FM, on the other hand, works by shifting the frequency of the carrier back and forth. Changes in the amplitude don’t matter so much for FM. A receiver is going to lock onto the strongest signal and track its frequency changes, ignoring any weaker signals, which is why FM doesn’t have all of the noise and interference common to AM.
That alone would probably be reason enough for a lot of music stations to switch to FM, but there were other reasons as well. First, AM is set up as a single channel, so you can’t have stereo. Second, the FCC allows for more bandwidth to be transmitted on the FM band, so the audio signals there sound better just because they aren’t as limited in frequency.
All of these things combined made all of the music stations switch over to FM.
As for the OP, my first instinct is to go with what Dave Simmons said. The only other thing that comes to mind is that in the real world, when you lay out circuit boards and such, you end up creating all of these little tank circuits just from the layout of the tracks, etc. It’s possible that the receiving circuit has a couple of minor resonnances, which is allowing some of the other frequency to get into the demodulator. Just a WAG on my part.
Not necessarily. 455 kHz is pretty standard, but a few manufacturers bravely set out to be different. For instance, Telefunken used 460 kHz in its AM receivers, and Sony used 710 kHz in at least a few of theirs. There’s probably other oddballs out there, as well.
In the US, 455 kHz is strongly recommended and a narrow band around that frequency isn’t used for any (legal) transmitting so as to avoid problems with interference.
Very few, I’m tempted to say no, US receivers have RF preselection as shown in the block diagram cited by Attrayant so any signal at the IF that gets to the antenna will pass right through the receiver ciruitry.