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

 

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:

http://en.wikipedia.org/wiki/Radio

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?

 

http://en.wikipedia.org/wiki/Right_hand_rule

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!

 

http://en.wikipedia.org/wiki/Right_hand_rule

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

 

But how can the AC current in the antenna be at say 550 Hz and 570 Hz

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.

http://amasci.com/graphics/antenna2.gif


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
UW Chem Dept, Bagley Hall RM74
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

 

[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

 

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
http://farside.ph.utexas.edu/teaching/jk1/lectures/node83.html

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
UW Chem Dept, Bagley Hall RM74
Box 351700, Seattle, WA 98195-1700
ph425-222-5066 http//staff.washington.edu/wbeaty/