Antennas

Hi all, I have an idle question. How do receiving antennas work. I have looked at Wikipedia and Google but I cannot find an answer. So what is actually picked up by the antenna? and how? Thanks

-Kit

Reply to
Kit
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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.

Reply to
BobG

Did you find this page on Wikipedia:

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

Tim

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Reply to
Tim Auton

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

Reply to
Dominic-Luc Webb

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?

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Reply to
Don Bruder

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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

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Reply to
Tim Williams

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!

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Reply to
Don Bruder

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

Reply to
Dominic-Luc Webb

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

Reply to
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

Reply to
Dominic-Luc Webb

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

Reply to
James Thompson

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

Reply to
Rich Grise

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

Reply to
Rich Grise

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

Reply to
Bill Bowden

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

Reply to
BobG

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.)

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One way to understand antennas is to look only at the
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Reply to
billb

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

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Reply to
Tim Williams

[1] "But" because you present your statement in the inverse direction. That I present my case opposite is just semantics, of course, and only serves to further prove the reciprocity of the case. ;-)

Tim

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Deep Fryer: a very philosophical monk.
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Reply to
Tim Williams

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

Reply to
Rich Grise

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

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

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Reply to
billb

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