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Antennas

It's no different than with any conductor.

If your stereo speakers are playing several notes at the same time, this means that the voltage across the terminals and the charges inside the wires are *not* vibrating back and forth in a sine wave. Instead several sine-wave motions are being added together.

If you connect several different signal generator outputs to your oscilloscope, you can see what such a composite signal would look like.

On an antenna we can either imagine that there are several separate signals having different frequencies. Or we can imagine that there is just one current/voltage signal, but it's a sum of many sinewaves of different frequencies.

((((((((((((((((((( ( ( (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/

Reply to
billb
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Another way to look at antennas is to realize that conductors do not carry AC energy inside them, they simply guide photons that are essentially attached to them. For instance, consider a parallel pair transmission line carrying an RF signal. The wires do not really contain the energy that is passing along the transmission line. An EM wave is simple guided by the pair and resides mostly between the two conductors, but some energy also spreads out in the volume around the pair. Now, if you gradually fan the two conductors apart, and also gradually increase their diameters, so that the transmission line impedance remains fixed, the EM wave spreads out in the space between the conductors, till they get about a wavelength apart, and then the wave ceases to be guided by the conductors, and just launches off into free space and continues without any guidance. There is no particular point where the conductors cease to be a waveguide and become an antenna, since this process is smeared out over a wavelength or two.

Of course, the process works the other directions, with a free traveling EM wave becoming attached to the conductors and following them efficiently, regardless of twists and turns, once the attachment becomes effective.

An antenna is a mechanism that performs the transition between a free traveling wave to an attached or guided one. Some antennas include a resonance process to improve the transition over a narrow range of frequencies, but some involve no resonance and perform the transition over a considerable bandwidth.

Reply to
John Popelish

One of the weirdest demos of this that I've ever seen was when some guy had a disk of some superconducting material lying on the bottom of a beaker, and he dropped a little magnet in on top of the SC disk. It lay right there, because at room temp, it wasn't a SC. So, he poured in some LN2 or LHe, and when the SC got to its right temperature, the magnet rose, seemingly of its own accord.

Speaking of maglev, is anybody still trying to come up with a room-temp superconductor?

(sometimes, when I'm having a really vivid dream, and I'm not sure whether it's a dream or reality, I try to levitate. If I can, I know I'm dreaming, and start molesting hot babes. ;-) )

Cheers! Rich

Reply to
Rich Grise

Good point!

If a student wrongly believes that electrical energy travels *inside* wires; believing that the energy is somehow contained within each flowing electron ...then its very hard to explain antennas.

((((((((((((((((((((((( ( ( (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/

Reply to
billb

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