Electronics > Beginners
Why do we need antennas?
tautech:
--- Quote from: fonograph on August 14, 2018, 10:36:23 pm ---But why gain? Antennas are passive devices,not amplifiers,they cant output more power that is put into them.If the transmission line and air are both 377 ohm,100% of energy from the coax should go into air,right? ( probably not right,I know I know... )
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You spurred me to write this:
https://www.eevblog.com/forum/rf-microwave/antenna-project-log/
Damianos:
--- Quote from: bsfeechannel on August 16, 2018, 07:59:38 pm ---
--- Quote from: Damianos on August 16, 2018, 06:36:16 pm ---All the above are transducers/converters between electricity and something else!
Similarly an antenna is converting electricity to radio waves and vise versa. It is a radiator and receptor of radio waves.
The simplest form of it is the dipole...
The transmission line is another story... We need it when we have to transfer the RF power from one place to another. If we connect our antenna directly to our generator, we don't need it...
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Antennas are not transducers not transformers. The principles that govern antennas are the same for transmission lines.
What antennas and transmission lines basically do is to shape the boundary conditions for the propagation of electromagnetic waves.
These conditions are cleverly chosen to make the waves behave as intended.
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It seems that what is missing of the discussion is the definition of what is an antenna.
Some examples:
http://www.globalsecurity.org/military/library/policy/navy/nrtc/14092.pdf on page 25.
https://www.cv.nrao.edu/course/astr534/AntennaTheory.html
https://www.cv.nrao.edu/course/astr534/PDFnewfiles/AntennaTheory.pdf
http://www.ece.mcmaster.ca/faculty/nikolova/antenna_dload/current_lectures/L01_Intro.pdf
An antenna converts voltage and current to electric and magnetic fields and vice versa.
On a transmission line are traveling electric waves, while in the space electromagnetic, the antenna is the "interface" between them.
By the way, a random wire antenna can be very effective, if we move the standing waves (electric) to a "proper" position, without any matching of impedances...
bsfeechannel:
--- Quote from: Damianos on August 18, 2018, 10:45:49 am ---It seems that what is missing of the discussion is the definition of what is an antenna.
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Indeed.
Antennas do not convert currents and voltages to electric or magnetic fields. This is an oversimplification that leads to all kinds of misconceptions.
That's why I said in my first post on this thread that answering noob questions is a challenge.
You cannot have voltages and currents without the respective associated electric and magnetic fields.
And propagation is a property of space, not the antenna.
If you really want to understand how antennas work you need to study Maxwell's equations. There's no way around it.
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