Antennas

I think its Ohm's law. The carrier has a field strength measured in volts/meter. The antenna impedance is 300 ohms, and some small voltage in microvolts is picked up to be amplified and demodulated.

Did you find this page on Wikipedia:

The first section, "Radio waves", has a decent enough introduction.

Tim

Electromagnetic radiation impinges on the antenna, invoking a small voltage. Do you know about the three finger rule? You should look for literature regarding this, resonance and impedence in antennas. This will then lead you to concepts such as gain and the decibel. In essence, the antenna is a transducer. On the one hand, it is very simple. On the other, there are many details that can take you years to learn.

It is unfortunate that when I tried to learn this from my first year college physics books, I was left with many questions. One of the other posters pretty much hit it on the nail, as near as I can tell. However, there are many details about this you should be made aware of if you are thinking to build your own. For instance, this equation is only an approximation for the length of an antenna:

Wavelength = speed of light in a vacuum (C) / frequency (F)

The correct length for an antenna will be somewhat shorter. Director and reflector elements will vary from this by a small percent. And then, there is a vast amount one can discuss about signal strength in various directions and front-to-back ratios in omni vs uni- directional antennas. A more thorough answer can be found in the ARRL "Technician/general class license manual for the radio amateur".

Dominic

OK, you got my curiousity up with that one - I've never heard of a "three finger rule" that has anything to do with antennas (or antennae, if you prefer) so I went googling. Of the four (Count 'em... FOUR) hits for "+antenna +"three finger rule"" I found, two involved situations of the "Oops - the GPS on my boat broke, the compass is out, and I need to get home. But how far away from home am I?" type. In this case, the "three finger rule" involved using three fingers and the known height of a distant object (such as a radio broadcast antenna, smokestack, or lighthouse tower) to get an estimate of the distance between you and the object.

Somehow, I doubt this is the "three finger rule" you're talking about...

There were two other hits that looked semi-promising, but they were XLS files, and I don't run (or even posess any) MicroSoft software, so couldn't read them.

Care to clarify?

The three finger rule is closely related, in particular, the orthogonal directions of a cross product (the two input vectors and their cross product), which is very important in E-M since the electric and magnetic fields are perpendicular.

Tim

Ahhh... OK. I "know" that one, but I've never seen/heard it named as such. The concept is nothing new to me, but referring to it as "the three finger rule" is something I've never encountered before.

Thanks!

I use the term "3 finger rule" because different physics books use diffferent hands. Some books describe a "left hand rule", while others describe a "right hand rule". They both work as long as you make sure to orient the 3 vectors correctly. Still other textbooks describe this as a "3 finger rule", which is independent of what hand you use. I should warn that there are some terrible mistakes in the Wikipedia, and much more clear descriptions of this rule can be found in many physics textbooks.

Dominic

Good, I think I am starting to understand this a little. But how can the AC current in the antenna be at say 550 Hz and 570 Hz at the same time. Thanks

-Kit

Errr, uhhh.... sorry you lost me. Not sure how we got to these two frequencies. Could you you send again, maybe re-word this?

Dominic

The antenna does get many different frequencies on it all the time, all from different sources. It is the receiver that will tune into the frequency you want to pick up. The rest of them are ignored. The different frequencies have there electromagnetic wave at different hights you might say or length's, so the length of the antenna elements are cut to best match the length of the wave created by the frequency you want to pick up. Most antenna elements are cut to a fraction of the frequency, usually 1/4 of it so the antenna is a manageable size. By cutting the elements to best match the frequency you want to pick up, then that frequency will have the highest voltage induced for your reciever to tune into. Hope this helps some. JTT

The "Left-hand rule" and the "Right-hand rule" are the same thing, but one uses conventional current flow, and the other uses electron flow. What you do is wrap your fingers around the wire, with your thumb extended. If your thumb is pointed in the direction of current flow, your fingers show the "direction" of the magnetic field.

The three-finger rule is used differently - that's for a wire moving through a fixed magnetic field, where one finger is the direction of current flow, the next finger is the direction of the magnetic field, and the third represents motion.

Applying either of these to an antenna seems somewhat of a stretch to me, since they're interacting with electromagnetic fields at a distance, but it's pretty much the same principle. The radio wave induces a current in the antenna, which causes a voltage to appear at the receiver end.

Hope this Helps! Rich

That's not hard at all - have you ever heard two instruments playing in harmony? It's exactly the same thing, except in current, not sound waves.

It happens all of the time in your radio antenna - it's picking up ALL of the signals from the air, but then you select which one you want to listen to, with the tuner.

Cheers! Rich

There is only one AC current flowing which may be the sum of several signals. But if the antenna is tuned, it will accumulate energy at the tuned frequency and the AC current will be predominately at the tuned frequency. Sort of like pushing someone on a swing. You give a slight push at the right time and the swing goes higher and higher, which is similar to the AC current in the antenna going higher at whatever frequency it's tuned to.

-Bill

I thought the three finger rule was invented by Bill Gates to restart hung up programs.....

I always wondered why transmitting antennas act like receiving antennas and vice versa. "Reciprocity" certainly works, but I've never seen a good explanation of the process.

Here's my own explanation of antennas. I've yet to encounter similar things elsewhere, so I can't compare it against textbooks for accuracy. (The textbooks go about things differently.)

---

One way to understand antennas is to look only at the EM fields and waves surrounding them.

Suppose we could *see* EM waves. If we illuminate an antenna with a parallel beam of radio waves, so the antenna is encountering a pattern of plane waves, what would this look like?

Well, first we'd notice that EM waves behave much like light, and an antenna would both reflect the waves and also cast a shadow. Even if the antenna only scatters the incoming waves without absorbing any, there'll still be a region of shadow behind the antenna. We might say that the antenna "punches a hole" in the planewave pattern, leaving a long fuzzy slot in the waves moving past the antenna.

Physics has a simple description for such a process, and it applies both to light and radio. Whenever a small opaque object casts a shadow, we can describe the object as being a wave-emitter ...where the emitted waves are out of phase with the incoming waves. The small object scatters waves in all directions, like a concentric bullseye pattern. During the wave-scattering process, the object absorbs incoming waves and then re-emits them in a spherical pattern. And downstream from the object, the two waves partly cancel, forming a shadow region. In this shadow the sphere-pattern of scattered waves is being subtracted from the incoming planewaves. Or in other words, there is an interference pattern in the waves surrounding the object, with one of the minima forming the shadow in the wave pattern behind the object.

The above explanation contains a central concept for understanding receiving antennas:

In order to absorb waves, a receiving antenna *must* emit waves.

This might sound impossible. But as long as no energy comes magically from nowhere ...as long as the total energy passing out of a closed surface surrounding the antenna is zero or negative, we're not breaking the rules.

In order to receive, antennas must transmit.

See what's coming next?

Because a receiving antenna interacts with incoming EM waves via an emission process, therefore the physics of transmitting antennas is a subset of the physics of receiving antennas. If we can figure out how an antenna can emit waves, then we'll know how the same antenna can act as a wave-absorber.

The explanation isn't complete yet, since our antenna could very well be a perfect conductor which only redirects waves without absorbing any. A superconducting antenna would still radiate a sphere-wave pattern and still cast a shadow, but it could only send out as much energy as it absorbed, so it wouldn't receive any EM signal on average.

The above explanation makes lots more sense if you can *see* what's going on. Get a couple of Moire transparencies, one with fine dark parallel lines, and another with a bullseye pattern of concentric circles with the same spacing as the parallel lines. Overlap them to produce the interference pattern, then slide them a bit so you get a minimum or "shadow" which extends behind the center. This shows how an opaque object blocks waves by (re)emitting a wave pattern which produces a shadow by wave cancellation.

In the shadow, where did the missing waves go? Clearly they're part of the maxima lobes going off in other directions. But if our antenna was actually absorbing energy on average, the shadow would be a little bit bigger, or the forward-scattered waves a little bit smaller. The missing energy would end up inside the radio receiver sitting at the center of our antenna.

((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))) William J. Beaty Research Engineer snipped-for-privacy@chem.washington.edu UW Chem Dept, Bagley Hall RM74 snipped-for-privacy@eskimo.com Box 351700, Seattle, WA 98195-1700 ph425-222-5066 http//staff.washington.edu/wbeaty/

Indeed! But[1] the case can also be made that a transmitting antenna can also recieve: consider that external waves cause a voltage/current, well energy on the resonant antenna element(s). If there is no loss, then the energy will be in equilibrium with the incoming waves. But since it's a transmitting antenna, this energy is also being radiated. In effect, we have a diamagnetic system, reflecting the incident signal, like a magnet floating on a superconductor.

This isn't that bad an analogy, since magnetic fields do play a role, and the conductors in an antenna are effectively diamagnetic to AC signals (Lenz's law).

Tim

Tim

Think of the two antennas as the primary and secondary of a HUGE air-core transformer, with a very small mutual inductance. Electrically, it doesn't matter which is the "primary" and which is the "secondary" - the EM field can go either way.

Hope This Helps! Rich

Diamagnetic levitation is also great for illustrating another part of antenna theory.

We might *say* that an antenna absorbs EM waves and then re- radiates them. But in fact the absorption and radiation processes are simultaneous. In diamagnetic levitation, whenever a magnet approaches a conductor, the conductor essentially responds instantly: as the magnet approaches, the current in the conductor rises. And as the current rises, the conductor creates its own b-field which repels the magnet. So whenever an externally- produced magnetic field hits a conductor, the conductor's own field and current appears at the same time.

The same applies with voltage, charge, and metal mirrors: when an e-field impinges on a perfectly conductive metal plate, the movable charges within that plate will smoothly change their position to produce an exactly opposite e-field, in order to "short out" the part of the incoming external e-field that's parallel to the metal surface. The charges move in such a way that they zero out any voltage measured parallel to the metal plate, so as the incoming e-field changes, the charges move at the same time, keeping the voltage always zero.

If all of this obeys the conservation of energy, then the EM waves emitted by the metal plate must be out of phase with the incoming waves, so they subtract from the incoming waves to create a shadow behind the plate. In other regions they produce what looks like "reflected" or "scattered" waves.

When we think of mirrors, we think of reflected waves. But to be accurate, we should be thinking of "simultaneously-re-emitted waves." The mirror emits radiation that magically creates a shadow behind the mirror, and also creates something *resembling* reflected/scattered waves ...but which actually is some waves emitted by the mirror.

So to understand receiving antennas, we have to see them as emitters, emitters which try to match the fields of the incoming waves.

Here's another piece of the puzzle. If the antenna only emits part of the energy it receives, and keeps the rest inside itself, then it's acting as an absorber. And if it swallows up *exactly half* of the incoming energy and radiates the rest, then in that case it absorbs the maximum possible EM energy.

See equation 989 on this page:

Antenna directivity and Effective Area

So any simple dipole receiving antenna must, at best, throw away half the incoming EM energy in the form of "scattered waves."

((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))) William J. Beaty Research Engineer snipped-for-privacy@chem.washington.edu UW Chem Dept, Bagley Hall RM74 snipped-for-privacy@eskimo.com Box 351700, Seattle, WA 98195-1700 ph425-222-5066 http//staff.washington.edu/wbeaty/