Why do we need antennas?

[fonograph]

Since air have 377 ohm impedance,why not just make some transmission line with  377 ohm impedance and then cut the cable and let it hang it air.Normaly cable that is cut and hanging in air is open circuit,but since this will be transmission line cable like coax with same impedance as air,then the electric wave should not reflect at the end,instead it would begin  moving in air...  this is probably completly wrong but I cant tell why.

I read and heard that its all about impedance matching,so I dont see any reason why electromagnetic field should not smoothly transition from any kind of 377 ohm transmission line like microstrip or coax into air.And if that would work,then I dont see the reason to make antennas.But since antennas are made despite big expense I am sure my ideas are ultra wrong ,I just cant see why it cant be done.Please explain why we need antennas and cant just let 377ohm coax cables hang in  air and call it a day.

[tautech]

GAIN ! ! !

[rfeecs]

Since air have 377 ohm impedance...why we need antennas and cant just let 377ohm coax cables hang in  air and call it a day.

Air has approx 377 ohm wave impedance (The ratio of electric field to magnetic field).

Coaxial cable has electrical impedance (The ratio of voltage to current).

They are not the same thing.

[fonograph]

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... )

[fonograph]

Since air have 377 ohm impedance...why we need antennas and cant just let 377ohm coax cables hang in  air and call it a day.

Air has approx 377 ohm wave impedance (The ratio of electric field to magnetic field).

Coaxial cable has electrical impedance (The ratio of voltage to current).

They are not the same thing.

Conductor = electron impedance.      Air = photon impedance?     
So that means I cant do it with cable,but waveguide would couple 100% energy into air if it was 377 ohm waveguide?

Edit: There are 2 kinds of impedance,wave and electric,and the antenna then is like impedance matching device that changes the wave impedance of conductor to the same wave impedance as air? So the antenna is sort of like transformer or buffer.

[rfeecs]

Since air have 377 ohm impedance...why we need antennas and cant just let 377ohm coax cables hang in  air and call it a day.

Air has approx 377 ohm wave impedance (The ratio of electric field to magnetic field).

Coaxial cable has electrical impedance (The ratio of voltage to current).

They are not the same thing.

Conductor = electron impedance.      Air = photon impedance?     
So that means I cant do it with cable,but waveguide would couple 100% energy into air if it was 377 ohm waveguide?

Edit: There are 2 kinds of impedance,wave and electric,and the antenna then is like impedance matching device that changes the wave impedance of conductor to the same wave impedance as air? So the antenna is sort of like transformer or buffer.

For waveguides, we do use wave impedance instead of electrical impedance.  An open ended waveguide can act as an antenna:
http://www.w1ghz.org/antbook/app-6c.pdf

[CatalinaWOW]

The energy radiated be a transmitter expands roughly as a sphere (ignoring antenna directionality and all of that).  So the energy density drops off with distance (you are spreading the same amount of energy over a larger and larger area).

Now on the receiving end, the most energy you can collect is the area that is intercepted by your antenna.  The bigger the antenna the more potential energy collected.  But there are many ways to get less than the amount implied by intercepted area.   Impedance matching is one of the things that allows you to get close to that maximum possible amount.

[jMrL]

Antennas are required by any radio receiver or transmitter for the electromagnetic field. Radio waves are electromagnetic waves which carry signals through the air (or through space) at the speed of light with almost no transmission loss.

 

They come in all sizes now. You can always find some that are barely extruding from the surface or even completely invisible.

GL

[rfeecs]

There are 2 kinds of impedance,wave and electric,and the antenna then is like impedance matching device that changes the wave impedance of conductor to the same wave impedance as air? So the antenna is sort of like transformer or buffer.

The antenna takes the current and voltage at the two terminals of the transmission line and creates an electric and a magnetic field that is oriented to match a propagating wave, like a transverse EM wave for example:
https://courses.lumenlearning.com/physics/chapter/24-2-production-of-electromagnetic-waves/

[xrunner]

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... )

They are passive yes, so what we mean by gain is the comparison of one type of antenna with respect to another reference antenna. For example the gain of a half wave dipole is 2.15 dBi (directive gain relative to an isotropic radiator). Compared to other types of antennas, the half wave dipole might have less gain. It's all relative.

[JS]

I'd like to see a 1MHz wave guide for an AM radio ransmitter...

JS

[GregDunn]

They are passive yes, so what we mean by gain is the comparison of one type of antenna with respect to another reference antenna. For example the gain of a half wave dipole is 2.15 dBi (directive gain relative to an isotropic radiator). Compared to other types of antennas, the half wave dipole might have less gain. It's all relative.

This.  You could take a coaxial cable and stretch the shield and conductor out in opposite directions, and you will have an antenna, in fact a dipole.  It will have more RF signal gain in the directions at right angles to the conductors, and much less gain in the direction along the conductors (the overall energy delivered in all cases is of course less than 100%).  It will in fact have a wave impedance of about 72 ohms, which means it won't transfer power to the E and B fields as efficiently as something like a folded dipole, even if you can deliver 100% of the power to the element itself by matching its electrical impedance.  In practice matching the signal impedance is more of a concern than matching the free space impedance, though both will improve the antenna performance.  Optimization of the antenna design for one over the other depends on many competing factors.

The total power radiated (or received) by a certain type of element can also be "forced" along a certain direction more efficiently than others by shaping or changing the configuration of the conductor(s).  A parabolic reflector behind the active elements, for example, can make the effective signal many dB higher in that one direction (I've worked with radar antennas having as much as 30 dB higher pattern efficiency along one axis).  Passive elements in front of and behind a dipole can change the radiation pattern in interesting and very useful ways too.  This changes the free space impedance, but the improvement from directing the energy often overwhelms any potential losses from that.

Also, just for completeness, the impedance of the medium you're radiating into is actually proportional to (square root of) the ratio of the magnetic permeability to the electric permittivity of the medium.  For free space, that works out to 377 ohms.  If you take the product of those two numbers instead, find the square root and then invert it, you will get a number equal to the speed of light in that same medium (or the speed of radio propagation).  This was Maxwell's contribution to the laws of physics, wherein we understand that light and radio waves are the same thing.

[tautech]

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... )

Have a little study up on Yagi's.....not 5 minutes worth, an hour or two.
Then a little research into when Yagi did all this and back then probably with an abacus ! 

I don't know shite about RF but hell is it a fascinating field. (pun intended  )

[fonograph]

I'd like to see a 1MHz wave guide for an AM radio ransmitter...

JS

It would 150 meters thick! 1 MHz wavelenght is 300 meters and waveguide needs to be atleast half wavelenght thick.

[fonograph]

Also, just for completeness, the impedance of the medium you're radiating into is actually proportional to (square root of) the ratio of the magnetic permeability to the electric permittivity of the medium.  For free space, that works out to 377 ohms.  If you take the product of those two numbers instead, find the square root and then invert it, you will get a number equal to the speed of light in that same medium (or the speed of radio propagation).  This was Maxwell's contribution to the laws of physics, wherein we understand that light and radio waves are the same thing.

That was great post,thank you 

I always wondered why the impedance of free space is 377 ohm,is that "recipe" for calculating it you provided one of those Maxwell equations? 

I have trouble calculating it,I read that both permitivity and permeability if vacuum is 1...  ratio of 1:1 is 1,not 377,what I am missing here?

[fonograph]

Ok,so... the answer to my original question is that you cant just connect 377 ohm cable to air and expect to radiate 100% energy becose there are two kinds of impedances.First,there is electronic impedance and then there is wave impedance,just becose some cable have 377 ohm electric impedance doesnt mean it have 377 ohm wave impedance.

More questions : Is the electric impedance a nearfield impedance and the wave impedance a farfield/photon impedance?
Is this electric/wave impedance in some way synonym of near-field/far-field?

[exe]

I don't know much about antennas, but I think the signal won't be radiated from the open end of the cable. It will bounce back. Depending on wavelength and cable length, it may appear as open circuit, or, if is 1/2 of the wavelength, it will be a short that can potentially fry the circuit.

Some tutorials on transmission lines:



There are other good videos, but I can't find links. The one I particuraly like shows  standing waves, reflection, termination, etc using ropes and weights. It's very interesting to see things. But can't find the link

[fonograph]

I made thread about week ago asking if making the antenna have 377 ohm impedance improves its radiation performance and the answer I got is yes.But now,I must re-ask that question becose previously I had no idea there are two kinds of impedances.When I originaly asked that question,I was really thinking about electric impedance,not radiation impedance.

Does the electric impedance matters in antenna? Should we make antennas have 377 ohm electric impedance,or does electric impedance not matter and we only need to make the antenna have 377 ohm wave impedance?

[rfeecs]

You could take a coaxial cable and stretch the shield and conductor out in opposite directions, and you will have an antenna, in fact a dipole.  It will have more RF signal gain in the directions at right angles to the conductors, and much less gain in the direction along the conductors (the overall energy delivered in all cases is of course less than 100%).  It will in fact have a wave impedance of about 72 ohms, which means it won't transfer power to the E and B fields as efficiently as something like a folded dipole, even if you can deliver 100% of the power to the element itself by matching its electrical impedance.

Doesn't sound quite right.  72 ohms is approximately the radiation resistance, not the wave impedance.  A folded dipole has approximately the same radiation pattern as a single wire dipole.  The folded dipole just gives you 4 x the feedpoint impedance, so about 292 ohms, and a better match to 300 ohm balanced twin feed ribbon cable.

Once again there is confusion between electrical impedance and wave impedance.  In general it might be better for ham radio people and electronic hobbyists to forget about wave impedance.  It really only applies to EM theory.

One place where wave impedance is actually useful is discussed here, it allows you to treat problems of dielectric layers, like dielectric mirrors, just like transmission lines:

[In Vacuo Veritas]

Why do we need microphones?

[metrologist]

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... )

I was going to explain it by using optical lenses. They too are passive, but can concentrate light or magnify images. Antennas have aperture just as lenses.

A lens and mirror, for example, can take an omnidirectional light and focus it onto a single beam, a flashlight. A Yagi works similarly, with a reflective element and directive elements, that focus the radio energy from/to the driven element.

[IanB]

I read and heard that its all about impedance matching,so I dont see any reason why electromagnetic field should not smoothly transition from any kind of 377 ohm transmission line like microstrip or coax into air.And if that would work,then I dont see the reason to make antennas.

You've got to think about it in terms of the amount of energy transferred. An antenna is an energy harvester or radiator.

For example, solar panels harvest sunlight. Big panels harvest more sunlight and generate more electricity than smaller ones. Why?

Loudspeakers radiate sound energy. Big loudspeakers radiate more sound energy than small earphones. Why?

[PhilipPeake]

If you are looking for a simple answer (behind which things get very complex) the antenna is a device to match electrical impedance to free-space impedance.

With radio (and EM in general) its all about matching.

[IanB]

If you are looking for a simple answer (behind which things get very complex) the antenna is a device to match electrical impedance to free-space impedance.

While partly correct, that is not the whole story. An antenna is also a device for transferring power from one medium to another.

With radio (and EM in general) its all about matching.

It's not all about matching. It's also about power handling. Impedance matching affects efficiency. Size affects power handling.

Consider: a TV broadcast antenna is very large, a home TV receiving antenna is quite small in comparison. Both correctly match impedance, yet one is much bigger than the other? Why might that be?

(And the same can be asked about the transmitting antenna on a cellphone mast and the receiving antenna in your cellphone.)

There is something else that matters here apart from impedance matching.

[PhilipPeake]

If you are looking for a simple answer (behind which things get very complex) the antenna is a device to match electrical impedance to free-space impedance.

While partly correct, that is not the whole story. An antenna is also a device for transferring power from one medium to another.

With radio (and EM in general) its all about matching.

It's not all about matching. It's also about power handling. Impedance matching affects efficiency. Size affects power handling.

Consider: a TV broadcast antenna is very large, a home TV receiving antenna is quite small in comparison. Both correctly match impedance, yet one is much bigger than the other? Why might that be?

(And the same can be asked about the transmitting antenna on a cellphone mast and the receiving antenna in your cellphone.)

There is something else that matters here apart from impedance matching.

Power handling is in the physical size of conductors and antenna elements. The basics stay the same.
The difference on the cell phone tower antenna is that it is designed to direct all its energy (and reciprocally, receive gain) in a particular direction. Each antenna only covers a small segment of the 360 degrees coverage. It doesn't waste energy going up into space, down into the ground, or behind it. That is why it is bigger and more complicated.

The antenna in your phone needs to radiate in 360 degrees. Having it radiate only at, say, 0 to 30 degrees above the horizon would be nice but reality is that people use their phones in all sorts of orientations. The antenna also has to fit in the phone. In general, aesthetics in cell phones counts for much more than any sort of efficiency.

Cell phone antennas mostly are not even matched that well. They can get away with it because of the much better antennas and TX/RX in the cell towers.