Train tracks wreak havoc with my radio transmitter
Not quite halfway between Fargo and Minneapolis is an old, rather battered looking train bridge that runs over I-94. It would be one of those minor things I notice and forget about 3 seconds later on most trips, except that something unusual happens every time we get close to it. The bridge is a major source of EMI (electromagnetic interference). I haven't had time to sit and figure out the specifics, but when I'm listening to the radio during the drive, I start getting static in my reception about a half mile from the bridge. It increases in intensity until we're very close to the bridge, at which point it completely swamps my reception. After passing the bridge, it slowly decreases until 1/2 a mile to a mile away.
It's annoying, to say the least.
Initially, I assumed that the tracks, being very long conductors, were absorbing the electromagnetic wave passing from the radio tower and thus preventing the signal from reaching my car antenna. What puzzled me was that it didn't matter which side of the tracks the radio station or I was on. I got interference from either direction and any radio station to which I happened to be listening.
A few months ago, I bought a radio transmitter. This is a small transmitter that allows you to play your mp3 player or portable CD player (or heck, a portable cassette player, if you happen to still own that piece of archaic technology...yes I do, and thank you very much for asking) and pick it up on your radio. Because the transmitter is attached to the dashboard of my car and only has to be received by the antenna of my car, I figured that the nasty train bridge would no longer plague my travels.
I was wrong.
The bridge has the same effect when I'm listening to my iPod as it does when I'm listening to the radio. This can mean only one thing: the train tracks are actively radiating electromagnetic energy in the FM band. (Sorry, I never listen to AM.)
I know that there is a lot of work done on track EMI at low frequencies. For instance, solar storms can cause pulsations that will turn train tracks into large wires because of their huge length. If you think about things like your cable TV, you use a wire to transmit a signal from the cable company to your TV. My understanding is that some of the signaling for trains uses the actual tracks as the cable. This is a problem because, unlike the cable TV wire, the train tracks are not shielded from external electromagnetic radiation. Although the frequency of the communcation signal is a bit higher than what you would see from a magnetic storm, the pulsations can still jumble that signal.
However, I have yet to figure out why the tracks would be emitting noise in the FM band as the frequency is orders of magnitude larger than both train signals and magnetic storm pulsations. My nearest guess is that some way down the path, it must be close to some sort of radio tower and happens to be absorbing some of that energy. As that energy passes down the track, the signal suffers from wave dispersion (i.e. the waves get separated out by frequency, the higher frequency waves traveling faster), and then re-emitted in their new, non-coherent form as white noise. If this is the case, I can't imagine that the radio tower is terribly power efficient.
Back to the iPod, I have finally found a way to keep it from being affected by the tracks. The tracks, like all useful railroad tracks, lay flat on the ground. We can imagine that this will behave like a simple dipole antenna.
A dipole antenna, as shown above, basically consists of two wires that are co-linear. The vertical cylinder in the picture represents the wire that is 'feeding' electricity to the wires. Usually, the electricity is an alternating current (AC), which causes the electrons (or electric field) on the wires to move up and down the wire, changing direction as the feed current changes. Going back to basic electromagnetics (although I'll spare you the equations), we know from Ampere's law that a current in a wire will produce a magnetic field. Because we usually are using AC, this magnetic field will be a time-varying magnetic field (i.e. one that changes in time). We know that a changing magnetic field will give rise to a changing electric field thanks to Faraday's Law. Finally, this will end up resulting in our favorite picture of an electromagnetic wave:
In most representations of a transverse electromagnetic wave, as in the picture above, you see that the E-plane (or electric field) is one plane. Likewise, the magnetic field is orthogonal (or perpendicular, in normal speak) to the electric field. In reality, it doesn't have to be this way: the fields must always be perpendicular to each other, but they don't have to be confined to a single plane. However, it turns out that when you use a dipole, this is a pretty good representation of how the fields would look, if we could see them. The fields come out strongly polarized - this is, each field will be confined to a single plane (which is vertical for the electric field in the picture and horizontal for the magnetic field). This is the same principle used in polarized glasses (which you need for some of those nifty new 3D movies). Light waves will move up-and-down (which we call vertical polarization) or side-to-side(horizontal polarization). One lens of the 3D glasses blocks the vertically polarized wave while allowing the horizontally polarized wave to pass through, while the other lens does the opposite. Each eye will then receive a different image.
When working with antennas, we're mostly concerned about the electric field, and it turns out that the polarization of the electric field of the dipole will be in the same plane as the antenna. In other words, if you have a dipole antenna that stands up (like, for instance, the radio antenna on your car), it will have an electric field with vertical polarization. If your dipole is laying flat, you will have a horizontally polarized wave.
It turns out that your best reception of the wave happens when the receiving antenna is parallel to the one that's transmitting the wave. If a radio station has a tower standing up (which most of them do), you'll get much better reception of the signal if your car radio antenna is also standing up. If it is, instead, run across the top of your windshield, as an example, you probably won't get much of anything. That's because the field is 'moving' up and down, and will encourage the electrons in the antenna to move up and down. If the antenna is lying flat, the electrons don't have far to move, and they won't create much of a potential difference across the antenna feed. This behavior is called cross-polarization.
This ends up being the solution to my iPod problem. The signal is actually not being received by my car antenna. The receiving antenna, it turns out, is actually the cable between my iPod and the transmitter in my car. When we get near the train tracks, I have my older boy pick up the iPod and hold it so that the wire is more or less straight up and down. This means that my iPod cable is cross-polarized relative to the train tracks and won't pick up a signal from them.
It's annoying, to say the least.
Initially, I assumed that the tracks, being very long conductors, were absorbing the electromagnetic wave passing from the radio tower and thus preventing the signal from reaching my car antenna. What puzzled me was that it didn't matter which side of the tracks the radio station or I was on. I got interference from either direction and any radio station to which I happened to be listening.
A few months ago, I bought a radio transmitter. This is a small transmitter that allows you to play your mp3 player or portable CD player (or heck, a portable cassette player, if you happen to still own that piece of archaic technology...yes I do, and thank you very much for asking) and pick it up on your radio. Because the transmitter is attached to the dashboard of my car and only has to be received by the antenna of my car, I figured that the nasty train bridge would no longer plague my travels.
I was wrong.
The bridge has the same effect when I'm listening to my iPod as it does when I'm listening to the radio. This can mean only one thing: the train tracks are actively radiating electromagnetic energy in the FM band. (Sorry, I never listen to AM.)
I know that there is a lot of work done on track EMI at low frequencies. For instance, solar storms can cause pulsations that will turn train tracks into large wires because of their huge length. If you think about things like your cable TV, you use a wire to transmit a signal from the cable company to your TV. My understanding is that some of the signaling for trains uses the actual tracks as the cable. This is a problem because, unlike the cable TV wire, the train tracks are not shielded from external electromagnetic radiation. Although the frequency of the communcation signal is a bit higher than what you would see from a magnetic storm, the pulsations can still jumble that signal.
However, I have yet to figure out why the tracks would be emitting noise in the FM band as the frequency is orders of magnitude larger than both train signals and magnetic storm pulsations. My nearest guess is that some way down the path, it must be close to some sort of radio tower and happens to be absorbing some of that energy. As that energy passes down the track, the signal suffers from wave dispersion (i.e. the waves get separated out by frequency, the higher frequency waves traveling faster), and then re-emitted in their new, non-coherent form as white noise. If this is the case, I can't imagine that the radio tower is terribly power efficient.
Back to the iPod, I have finally found a way to keep it from being affected by the tracks. The tracks, like all useful railroad tracks, lay flat on the ground. We can imagine that this will behave like a simple dipole antenna.
A dipole antenna, as shown above, basically consists of two wires that are co-linear. The vertical cylinder in the picture represents the wire that is 'feeding' electricity to the wires. Usually, the electricity is an alternating current (AC), which causes the electrons (or electric field) on the wires to move up and down the wire, changing direction as the feed current changes. Going back to basic electromagnetics (although I'll spare you the equations), we know from Ampere's law that a current in a wire will produce a magnetic field. Because we usually are using AC, this magnetic field will be a time-varying magnetic field (i.e. one that changes in time). We know that a changing magnetic field will give rise to a changing electric field thanks to Faraday's Law. Finally, this will end up resulting in our favorite picture of an electromagnetic wave:
In most representations of a transverse electromagnetic wave, as in the picture above, you see that the E-plane (or electric field) is one plane. Likewise, the magnetic field is orthogonal (or perpendicular, in normal speak) to the electric field. In reality, it doesn't have to be this way: the fields must always be perpendicular to each other, but they don't have to be confined to a single plane. However, it turns out that when you use a dipole, this is a pretty good representation of how the fields would look, if we could see them. The fields come out strongly polarized - this is, each field will be confined to a single plane (which is vertical for the electric field in the picture and horizontal for the magnetic field). This is the same principle used in polarized glasses (which you need for some of those nifty new 3D movies). Light waves will move up-and-down (which we call vertical polarization) or side-to-side(horizontal polarization). One lens of the 3D glasses blocks the vertically polarized wave while allowing the horizontally polarized wave to pass through, while the other lens does the opposite. Each eye will then receive a different image.
When working with antennas, we're mostly concerned about the electric field, and it turns out that the polarization of the electric field of the dipole will be in the same plane as the antenna. In other words, if you have a dipole antenna that stands up (like, for instance, the radio antenna on your car), it will have an electric field with vertical polarization. If your dipole is laying flat, you will have a horizontally polarized wave.
It turns out that your best reception of the wave happens when the receiving antenna is parallel to the one that's transmitting the wave. If a radio station has a tower standing up (which most of them do), you'll get much better reception of the signal if your car radio antenna is also standing up. If it is, instead, run across the top of your windshield, as an example, you probably won't get much of anything. That's because the field is 'moving' up and down, and will encourage the electrons in the antenna to move up and down. If the antenna is lying flat, the electrons don't have far to move, and they won't create much of a potential difference across the antenna feed. This behavior is called cross-polarization.
This ends up being the solution to my iPod problem. The signal is actually not being received by my car antenna. The receiving antenna, it turns out, is actually the cable between my iPod and the transmitter in my car. When we get near the train tracks, I have my older boy pick up the iPod and hold it so that the wire is more or less straight up and down. This means that my iPod cable is cross-polarized relative to the train tracks and won't pick up a signal from them.