Author Topic: Why do we need antennas?  (Read 6887 times)

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Offline rfeecs

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Re: Why do we need antennas?
« Reply #50 on: August 17, 2018, 04:59:11 pm »
An antenna is a transformer which transforms the wave impedance of the transmission line to match the wave impedance of free space.

No.

The definition of "the wave impedance of free space" is the ratio of the magnitude of the E-field to the magnitude of the H-field of a plane wave in free space.  A plane wave is a transverse electromagnetic wave (TEM wave).

Take a transmission line like a coax line with an air dielectric.  The wave that propagates down this line is a TEM wave.  The wave impedance of this line is 377 ohms.

So the transmission line with an air dielectric already has the same wave impedance as air.  But it doesn't radiate.

It doesn't radiate because the currents in the two conductors of the line are equal and opposite.  So outside of the line, the fields cancel.

To radiate efficiently, you need an antenna to arrange the direction of the currents so that their fields reinforce each other instead of cancel.

Of course there are other requirements as well for good radiation efficiency, but just matching wave impedance isn't why you need antennas.



 
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Offline bsfeechannel

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Re: Why do we need antennas?
« Reply #51 on: August 18, 2018, 12:00:15 am »
Quote
Take a transmission line like a coax line with an air dielectric.  The wave that propagates down this line is a TEM wave.  The wave impedance of this line is 377 ohms.

If you're implying that because the dielectric is air the impedance is 377 ohms, that's not true, I'm afraid.

The impedance of a coax cable is given by:


Source: Wikipedia.

Where D is the internal diameter of the shield, d is the diameter of the internal conductor and εr is the relative permittivity of the dielectric (you can consider it 1 in the case of air as a good approximation).

You can obviously have a coax with Z0 = 377 Ω, but you'd have to choose D/d to be around 540.

Commercially, coax cables are 50 or 75 ohms. Depending on the dielectric you use, that relation would be in the range of 30 or thereabouts.

Otherwise your explanation is very good. I'd like to have that power of synthesis.
 
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Offline fonographTopic starter

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Re: Why do we need antennas?
« Reply #52 on: August 18, 2018, 01:06:46 am »
bsfeechannel  Are you sure? The thing is,rfeecs was writting about wave impedance,not electric impedance.Maybe what you wrote applies to electric impedance but not to radiation impedance.
 

Offline rfeecs

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Re: Why do we need antennas?
« Reply #53 on: August 18, 2018, 01:19:47 am »
Quote
Take a transmission line like a coax line with an air dielectric.  The wave that propagates down this line is a TEM wave.  The wave impedance of this line is 377 ohms.

If you're implying that because the dielectric is air the impedance is 377 ohms, that's not true, I'm afraid.

The impedance of a coax cable is given by:


Source: Wikipedia.

Where D is the internal diameter of the shield, d is the diameter of the internal conductor and εr is the relative permittivity of the dielectric (you can consider it 1 in the case of air as a good approximation).

You can obviously have a coax with Z0 = 377 Ω, but you'd have to choose D/d to be around 540.

Commercially, coax cables are 50 or 75 ohms. Depending on the dielectric you use, that relation would be in the range of 30 or thereabouts.

Otherwise your explanation is very good. I'd like to have that power of synthesis.

I said wave impedance.  Not electrical impedance.
For a TEM transmission line, the wave impedance (E-field magnitude / H-field magnitude) is determined by the dielectric.  For air, it's 377 ohms, independent of the geometry of the conductors.

As you point out, the characteristic impedance (V/I) is determined by the geometry of conductors.

I've been trying to point out that these two types of impedance are constantly being confused.

 
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Offline bsfeechannel

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Re: Why do we need antennas?
« Reply #54 on: August 18, 2018, 01:47:37 am »
Of course. Silly me.
 

Offline tautech

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Re: Why do we need antennas?
« Reply #55 on: August 18, 2018, 04:33:54 am »
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... )
You spurred me to write this:
https://www.eevblog.com/forum/rf-microwave/antenna-project-log/
Avid Rabid Hobbyist
Siglent Youtube channel: https://www.youtube.com/@SiglentVideo/videos
 

Offline Damianos

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Re: Why do we need antennas?
« Reply #56 on: August 18, 2018, 10:45:49 am »
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...

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.

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...
 

Offline bsfeechannel

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Re: Why do we need antennas?
« Reply #57 on: August 18, 2018, 05:28:24 pm »
It seems that what is missing of the discussion is the definition of what is an antenna.

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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