Textbook destruction February 17, 2011
Posted by mareserinitatis in electromagnetics, humor, pets.Tags: antennas, macrocat, textbooks
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There are two books generally used for classes on antenna theory: Stutzman and Thiele or Balanis. They’re both awesome books. S&T doesn’t cover as many topics, but I find it’s easier to understand right out of the chute. Balanis seems to make a better reference. While we seem to use both books fairly regularly, I haven’t had the inclination to get the most recent copy of Balanis, so we’re still putzing around with the second edition.
Mike was talking to Layne, one of the undergrads who works for him. Mike mentioned that our copy of S&T recently fell victim to Macrocat’s bladder issues. Layne responded, “Ouch! That’s $150! It would’ve been better if he hit Balanis because you can get a second edition of that for $40.”
Somehow, I don’t think we’re going to be able to convince the cat to take cost into consideration when he’s wrecking our stuff. On the other hand, I think I would’ve preferred if he’d hit Balanis because then I’d have an excuse to upgrade.
So what weird ways have your textbooks been destroyed?
Polarize Me December 14, 2010
Posted by mareserinitatis in electromagnetics, engineering, physics, research, Uncategorized.Tags: antennas, polarization
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The first time I saw Rush in concert, they were on their Counterparts Tour. My favorite song from that album is “Animate”, which begins with the words, “Polarize Me.”
I think part of the song could be talking about an antenna. (Hey, if Brittany Spears can do semiconductor physics, can’t Rush do electrical engineering???)
Polarize Me
Most people have at least a passing familiarity with polarized light. Polarized light and antenna polarization are working on the same concept here: polarization deals with electromagnetic waves and the way they’re ‘oriented’.
Light and other electromagnetic waves in free space will travel as a transverse electromagnetic (TEM) wave. This means that the electric field will be orthogonal to the direction of propagation, and the magnetic field will be orthogonal to both of them. We can imagine a light (or any other electromagnetic wave) looking like this:
In this image, ‘k’ indicates the direction in which the wave is propagating; in this case, it is traveling from left to right. The infinitely thin electric field (in blue) is shown be be vertical, and the magnetic field (in red) is horizontal. All three are orthogonal and imbue us with that sense of happiness that comes from observing neat and easy rectilinear behavior.
The thing is, the EM wave doesn’t have to operate this way. We could just as easily have the electric field in the horizontal plane and the magnetic field in the vertical plane. Or, if you want to be really crazy, maybe the electric field is sitting at 15° while the magnetic field is at -75°. No matter how much we rotate our wave, as long as our electric field stays within that plane over time (and the magnetic field stays in its plane), then the wave is said to be linearly polarized.
Now what happens if the fields start to rotate over time? We’re going to only look at the electric field. Just remember, however, that the magnetic field is also there and it’s always orthogonal to both the electric field and the direction of propagation.
Anyway, our electric field starts rotating over time, but each time we check it, it has the same strength or magnitude. We can imagine that the edge of the field would trace a circle, if we are looking at the wave along the direction of propagation. This would be a circularly polarized wave. Mathematically, we can take the wave at any point and decompose into parts – a vertical part and a horizontal part.
In this image, you can image that the vector A could be created by adding horizontal vector of two units to a vertical vector of three units.
If it’s traveling in a circle, it’s intuitive to think about our electric field vector as made up of a horizontal vector and vertical vector. As the field makes one complete circle, we can add how much each direction contributes to the overall field. We would see that half the field would be horizontal and half would be vertical.
For the sake of completeness, there other things we can discuss, like elliptically polarized-waves as well as right- and left-handedness, but those are extraneous to the point at hand. We’ll save them for another day.
Sensitize Me
A monopole antenna is basically a straight wire or metal bar that is connected to the ground (or a ground plane). A good example is the antenna on a car. Suffice it to say that because it is the shape of a line, it will create a linearly polarized EM wave. In fact, the electric field polarization will be in the plane created by the direction of propagation and the antenna itself.
Have you ever wondered why antennas stick straight up from cars? It’s because the broadcasting antenna is also vertical:
You see, the broadcasting tower is vertical and creates a vertically polarized wave. If this wave will create a current on the receiving antenna (i.e. the one on your car), it will work best if it is also vertical. It can be at an angle, but then the signal it receives won’t be as strong. And if the antenna is horizontal, then it won’t be able to see the electric field at all, and your radio won’t pick up anything.
Let’s now imagine that the radio tower were able to put out a circularly polarized wave. If our car antenna is still a good old monopole (or whip antenna, as some people like to call them), then it would only pick up the part of the field that is vertically polarized. Half of the signal is in the horizontal polarization, so we wouldn’t catch that part of the signal. Overall, it would look like we had half the signal strength as when our radio tower was vertical. If we want a more sensitive receiving antenna, it helps to make sure it is polarized in the same direction as our transmitting antenna.
Criticize Me
When you’re working with antennas that may have different polarizations, we can talk about the polarization mismatch factor. This is a numerical value we can plug into the Friis Transmission Equation to help us estimate how much power will be transferred from a transmitting antenna to a receiving antenna. There’s a fun vector equation which will give you the quantity, but I’ll skip that and say that it’s quite well known that he polarization mismatch for a linear to circular antenna (or vice versa) is 0.5.
So why did I spend all this time talking about polarization? Because I needed to rant, and I wasn’t sure you’d understand the point without the background.
I finally finished up a lit review I was working on, and one of the last papers I read annoyed me because they stated that when they went from a linearly to a circularly polarized transmitting antenna (they were using a linearly polarized receiving antenna), their received power dropped by half. I found this annoying because it’s bloody obvious to anyone who works in antennas (and hopefully to you, having read my long-winded explanation). It need not be stated in a paper. In fact, it shouldn’t be. It is one of those things you can and should do in the lab to verify that your testing procedure is set up correctly, but it certainly shouldn’t be reported in the results section of a journal article.
Now that I have that off my chest, I’m going to go listen to the rest of Animate.
homonymic abbreviations November 29, 2010
Posted by mareserinitatis in electromagnetics, papers.Tags: antennas, receive, transmit, ultra-wide band
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Also known as HA!
I am currently reading a paper on characterization of ultra-wideband antennas. I am amused by this paper because the authors of this particular paper have chosen to not use the words transmit and receive. Instead, they refer to the transmit and receive antennas using abbreviations.
Transmit = TX
Receive = RX
And yet, every time I see those abbreviations, I keep wondering why they’re talking about Texas and prescription medication.
Oh yes, and UWB (ultra-wideband) is the University of Washington, Bothell.
Seawater antenna September 4, 2010
Posted by mareserinitatis in electromagnetics, engineering.Tags: antennas, seawater
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Found this through someone on Twitter. It’s a really cool video of antenna that is created by spraying seawater through a pump to create a monopole. To feed the signal, they use a magnetic coil to induce currents on the stream.
Your bed may be killing you… July 28, 2010
Posted by mareserinitatis in electromagnetics, science.Tags: antennas, beds, electromagnetics, physiology, scientific american
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(Note: I posted a much more complete analysis of the paper here.)
Scientific American posted a brief review of some research stating there may be a correlation between metal bed springs and incidence of certain types of cancers, such as cancer of the breast and melanoma.
The person who wrote the SciAm article obviously didn’t read the original paper very well…or even the abstract.
In the Sci Am article, it is stated early on that there have been correlative studies showing that there is a statistical relationship between incidence of cancer and proximity to transmission antennas for radio and television.
My personal interpretation of this data is that people who live in cities are more likely to get cancer than those who don’t. This could very well be a lifestyle issue because, as we all know, correlation is not causation. And transmission towers are going to be considerably more dense in urban rather than rural areas.
The article makes the point that the amount of electromagnetic energy is so small as to be nearly irrelevant.
Anyway, the claim is that the springs in a person’s bed are capable of capturing and amplifying this electromagnetic energy, just like an antenna.
Specifically:
Thus, as we sleep on our coil-spring mattresses, we are in effect sleeping on an antenna that amplifies the intensity of the broadcast FM/TV radiation. Asleep on these antennas, our bodies are exposed to the amplified electromagnetic radiation for a third of our life spans. As we slumber on a metal coil-spring mattress, a wave of electromagnetic radiation envelops our bodies so that the maximum strength of the field develops 75 centimeters above the mattress in the middle of our bodies. When sleeping on the right side, the body’s left side will thereby be exposed to field strength about twice as strong as what the right side absorbs.
The solution, obviously, is that one should get rid of their metal bed springs.
Without even running numbers, I can tell you that this makes no sense. Antennas are passive objects. Amplifiers, however, are not passive and require the input of additional power. An antenna cannot amplify a signal. In fact, most antennas, including the one in your car, require an external amplifier so that you can listen to the signal.
Unless you’ve figured out a way to plug in your bed springs, I don’t think it’s going to do much to the signal other than capture it passively. I cannot generate a field any stronger than the one it is receiving…and generally, if it’s receiving, it’s not going to be transmitting anything.
The other problem I see, from an electromagnetics perspective, is that it just isn’t the right shape to work as a good antenna. If you look at the wire mesh in a box spring, that may work as a decent receiving antenna. However, your mesh is covered with springs and metal springs are basically going to act like inductors from an electromagnetics perspective. That is, if you have a time-varying field from the mesh, as it tries to move upwards through the springs, it will induce a current in the springs. This current, however, will generate a magnetic field which will oppose the current that induces it. If anything, I would think that it would reduce the fields around a person.
Of course, the easy way to check this would be to actually use an antenna and take measurements of the ambient field as you move toward the bed. Curious, I decided to check the original article to see if that was attempted.
It turns out that the SciAm article is completely wrong. The original article abstract states:
We found that people tend to sleep for longer periods on their right side, apparently to avoid disturbance by the heartbeat. This puts the left side farther away from the field-attenuating influence of the metal springs in the mattress; thus the left side will spend, on average, more time exposed to stronger combined fields from incident and reflected waves. This hypothesis may also explain why body parts farthest away from the mattress (trunk and upper arms for men; lower limbs and hips for women) have higher melanoma rates than the sun-exposed face area. The implications of this study should promote a critical consideration of population exposure to electromagnetic fields, especially during the night.
This is completely the opposite of what the review in SciAm stated. They are saying what I just said: the bed springs will actually reduce (attenuate) the field. They also posit that the real danger comes from the left side being away from the bed and thus exposed to EM radiation.
I still don’t know that I buy the bed-spring argument. One could argue that the safest place to sleep would be in a giant Faraday cage. Frankly, the fields transmitted are pretty weak (especially with the conversion to digital). Also, if you need to put an antenna on the top of (i.e. outside) your house to get a good signal in many cases (which needs to be amplified by the electronics in your TV), the fields are obviously attenuated by your house to some extent. Are you really going to get much electromagnetic energy hitting your body? I personally am inclined to think that there may be some other physiological factor that has to do with sleeping position but not necessarily emag.
And yes, the fact that Japanese use different beds may very well affect sleeping position which may, in turn, affect left-side prevalence in certain cancers. To assume that they must sleep the same way is a bit silly. (I say this as a side sleeper, but when I sleep on the ground, such as when I’m camping, I’m far more comfortable sleeping on my back.)
Regardless, the Sci Am summary of the journal seems to be, by all appearances, completely backwards in it’s understanding of the original research…or is doing a very creative interpretation.
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Tinsel teeth December 20, 2010
Posted by mareserinitatis in dental surgery, electromagnetics, humor, societal commentary.Tags: antennas, tinsel teeth
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I told you the other day about my harrowing dental surgery. The surgery is a precursor to the next step, which is braces. I’ve been putting all of this off as long as possible (let’s face it: braces aren’t cheap), but the baby tooth that was holding all of this together came to it’s unfortunate demise about 25 years after it’s intended end of operation.
So yeah…braces.
I am hoping that people will be more sympathetic to my plight now that I am an adult versus the typical middle schooler. (At least I hope so for their sake: I’m certainly not as passive now as I was as a 12-year-old.) I remember epithets like “tinsel teeth” being thrown around the hallways of my junior high school so many years ago.
One insult that took me a while to get: “You get good radio reception with those?”
I have to admit: I still don’t get it. Think about it. There are several issues with using braces as receiving antennas.
1 – The most obvious is that the length of the archwire is significantly less than the wavelength of most radio station signals. The archwire would probably be almost Hertzian in length. And Hertzian dipoles aren’t known for the great reception. Usually there’s a bit of active device action thrown in when using a Hertzian dipole.
2 – The second issue is that the archwires are laying horizontally. Most radio stations have a vertical transmitter. This means the braces are cross-polarized relative to the radio station signal. That’s going to knock any reception way down.
3 – It also ignores the fact that human tissue is quite lossy, so the signal will already be considerably attenuated before it reaches the inside of your mouth.
4 – While they are developing the concept of smart braces (which I really like), I don’t think they’re out yet. And while the braces may have cool electronics to move your teeth around at an optimal rate, I seriously doubt they have plans to turn them into dual-use devices that can tune in and amplify a radio signal.
5 – What kind of feed would you use? You can’t use the two archwires as dipole arms and examine the potential across one of the ends because, lying parallel to each other, they will fail to have much of a potential difference. Do they somehow feed to your brain through your incisors?
6 – How do you decode the signal? I got the feeling when this insult was being thrown around that electrical behavior was being confused with mechanical vibration. Mechanical vibration would undoubtedly provide something to the ears/brain that would be musical, but you generally need some sort of transducer to change the signal from electrical to mechanical energy. Again, I don’t think they’re standard on braces, and I don’t imagine that they have plans to make them that way any time soon.
I feel this thoroughly debunks the idea that braces are useful as radio signal receivers. However, I still have no idea what to say to anyone who calls me ‘tinsel teeth’. (And, in case you’re interested, I’m fairly certain that my dad will do exactly that.)