Light from antena?

Around the antena? By the way, has this ever been tried before?

I believe it is called a light bulb!

snipped-for-privacy@hotmail.com wrote:

i would like to see the set up capable of emitting that spectrum! :) but to answer you question, you would see light but only at the initial take off point and normally with in the first wave length or half wave length.. i am taking this from memory while in school many years ago and back then it was just more theory on some parts of the instructors since the thought of generating RF at the wave length was far fetched! don't remember all of the physics behind it but i seem to remember that you should see only a surface glow effect due lost and wave length space.

"John Fields"

I recommend using a "generic" isotropic point radiator for best results. ;-)

You would need a sinewave oscillator of frequency in the 428-750 terahertz range. A halfwave dipole would have a length of .2 to .35 micrometer. If you want to make an oscillator along the line of a Hartley or a Colpitts or an Armstrong, you need an inductor wound with a wire a fraction of a wavelength long (less than .2-.35 micrometer), and all other parts much smaller still - submicroscopic.

As for semiconductors that oscillate at such frequecies nowadays: We have laser diodes.

A halfwave dipole antenna in a laser beam will produce a sinewave electrical signal - but have fun finding a way to detect AC at frequencies that high, or building an antenna too small to see with any ordinary microscope!

- Don Klipstein ( snipped-for-privacy@misty.com)

Why antennas made that are 1/4 or 1/2 the wave length? Is it because the energy emitted is a sine wave? Or am I totally off base. I'm just curious and have little electrical knowledge. Eric

Actually, it's not. It's a different mechanism.

What you really need is an atom-sized klystron! ;-)

Cheers! Rich

The basic idea is sound, but consider that FM radio has a wavelength of several meters, long wave radio of several km. That makes building antennas (which have 1/4 or 1/2 of that length) relatively straightforward. Wavelength for visible light is 400--700 nm, light antennas are therefore a subject of nano-technology.

Physical size/convenience and radiating properties.

A quarter or half-wave antenna can send/receive the tuned frequency just fine, in literally half or a quarter the space of a full-wave antenna for that frequency. Which, in the case of low frequencies, might literally mean hundreds of miles. (See also "Project Seafarer", later renamed "Project ELF" (for "Extremely Low Frequency") and the plans they had for turning a strip of Michigan's upper penninsula into an ultra-jumbo antenna for talking to the nuke subs back in the 70s.)

I do recommend getting from your library and reading relevant sections of the "ARRL Handbook" and the "ARRL Antenna Book". ARRL stands for "Amateur Radio Relay League", and editions of these books 50 or probably more years old remain valid in this area. Antenna theory known as of WWII covers this.

- Don Klipstein ( snipped-for-privacy@misty.com)

No, it doesn't. ARRL stands for "American Radio Relay League". I have a copy right here by my desk.

A short-form, somewhat-simplified explanation is as follows:

Consider what happens on a quarter-wave long length of transmission line, open at the far end. Or better, at multiples of a quarter-wavelength back from the open end on such a line. The "standing wave" pattern on such a line, resulting from the total reflection of the forward signal by the "open," has a voltage maximum (and obviously, a current minimum) at the open end, and conversely a voltage minimum (and current maximum) at a quarter-wavelength back (with this pattern repeating from there).

Now, think about a half-wavelength antenna - a simple "dipole"

- as being nothing more than such a quarter-wave section of line, "opened up" such that one conductor points this-a-way and the other conductor points that-a-way (in the opposite direction). Our first guess might be that the standing-wave situation described above remains in effect - the tips of this "opened-up" structure are points of high voltage (and opposite polarity, with respect to one another), while the center is a low-voltage/high-current (read: low impedance) point. This is precisely what you want for making EM radiation - an electric field created "across" the structure (i.e., between the high-potential ends), plus a magnetic field created "around" it and in the correct phase relationship. So what we've just described ought to be just fine for launching EM radiation, while providing a reasonably decent impedance at the point we wish to drive (the center of the structure). So, with at least very minimal math, this is a workable model or visualization as to why half-wave dipoles work well for transmitting radio waves. (And fortunately, what works well for sending such signals works just as well for receiving them.)

A quarter-wave antenna results from noting that the two pieces of the above structure are really just quarter-wave-long mirror images of one another, and either of them will work in the same manner if the other is replaced by a "ground plane" normal to the remaining element and passing through the center point of the original dipole structure. This plane, from the standpoint of determining the fields produced by the remaining element, really DOES act like a "mirror," although obviously nothing is really emitted by the "mirror image" viewed as being on the other side of that plane.

In reality, various factors make for neither the half-wave dipole or the quarter-wave derived from it exactly a half- or quarter-wave long, relative to the wavelength of the signal in question in free space. They're generally going to turn out being a bit shorter, to get the proper "resonance" and a decent (resistive only, in the ideal case) feedpoint impedance. But they're close enough that these names work just fine.

Bob M.

It's a natural length that works best without extra parts. Shorter antennas less than 1/4 wavelength are sometimes used where the antenna needs to operate at different frequencies. But they need to be tuned with extra inductors and capacitors to keep the whole arrangement resonant at whatever frequency is used.

The basic idea is to make the antenna look like a resistor to the transmitter, and a 1/4 wavelength looks pretty close to 50 ohms without extra parts.

Think about it. If the transmitter is connected to a resistor, all the energy is converted to heat. Now if you connect the transmitter to an antenna that acts the same as a resistor, and the antenna doesn't get hot, the energy must have gone out into space, which is what you want to do.

But, if the antenna is not the right length, and looks like a capacitor or inductor, the energy will flow back a forth between the transmitter and antenna and probably make the transmitter hot, and not radiate much energy, other than heat.

-Bill

Thanks Don, I'm going on vacation and will try to get those books before I leave. Nice, relaxing, reading. Cheers, Eric