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Transmitting portion of a transmitter works

by Aethaeon
Tags: portion, transmitter, transmitting
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Aethaeon
#1
Jul13-09, 04:57 PM
P: 4
(Update -- please read the third post -- I guess I'm wondering how an antenna works in particular).


Hey,

I can't seem to find much information on how the actually transmitting portion of a transmitter works -- how does one efficiently generate EM waves?

Thanks!


(I'm aware that the rough idea is to accelerate charges back and forth at a particular frequency, what I want to know is how one actually goes about doing that)
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TurtleMeister
#2
Jul13-09, 06:36 PM
TurtleMeister's Avatar
P: 755
Welcome to PF. Briefly, EM waves in the radio frequency band are generated using crystal oscillators or VFOs. The signal is then amplified to the desired power level and fed to the antenna through a matching network which matches the impedance of the amplifier to the impedance of the transmission line. Modulation of the signal depends on the mode (AM, FM, SSB etc...) You may find one or more of these books of interest:
http://www.arrl.org/catalog/index.ph...+for+Beginners
Aethaeon
#3
Jul14-09, 10:59 AM
P: 4
Thanks for the reply, TurtleMeister.

I should clarify exactly what I was wondering --

Suppose I already have a signal that I want to transmit, and its already amplified, etc. How do I actually turn that signal into EM waves?

I guess, basically, I don't understand how to analyze how an antenna works. Since an antenna is conductive, so there's no potential across any parts of it -- why do electrons in the antenna oscillate?

If I hack apart a power cord to get a plug and two bare wires, I ground one and leave the other one free, and then plug it in, will I get strong EM waves at 60 Hz (outlet frequency)?

TurtleMeister
#4
Jul14-09, 04:14 PM
TurtleMeister's Avatar
P: 755
Transmitting portion of a transmitter works

The electrons oscillate in an antenna for the same reason they oscillate in an electric light bulb. They are forced to do so by the power source. However, that is where the similarity ends. The difference is in the frequency of oscillation.

The antenna may seem like an open circuit but it's not at rf frequencies. The reason has to do with the frequency and the length of the antenna. When a dipole antenna is excited with RF, electrons will flow back and forth at nearly the speed of light. When the antenna length matches the wavelength of the RF then the antenna will radiate with maximum efficiency. The length of the dipole can be determined by: l = 468,000,000 / f where l is the antenna length in feet and f is the frequency in hertz. The antenna is not an open circuit because the electrons never have time to travel farther than the wavelength of the frequency.

Quote Quote by Aethaeon
If I hack apart a power cord to get a plug and two bare wires, I ground one and leave the other one free, and then plug it in, will I get strong EM waves at 60 Hz (outlet frequency)?
No. If you use the formula you will see that your power cord would need to be 7,800,000 feet long.

Edit: Actually, since you're grounding one side of the power cord it would be 3,900,000 feet for a quarter wave.
rosetaylor01
#5
Jul16-09, 04:25 AM
P: 3
As a means of monitoring activity in a number of manufacturing facilities, marine research, and general production control, a pressure transmitter will not only help to ensure positive results; it also can be a great way of maintaining acceptable levels of safety.

1. a. An electronic device that generates and amplifies a carrier wave, modulates it with a meaningful signal derived from speech or other sources, and radiates the resulting signal from an antenna. b. The portion of a telephone that converts the incident sounds into electrical impulses that are conveyed to a remote receiver. c. A telegraphic sending instrument. 2. A neurotransmitter.


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