Author Topic: Why do we need antennas?  (Read 1855 times)

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Offline fonograph

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Why do we need antennas?
« on: August 15, 2018, 08:21:40 am »
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.
« Last Edit: August 15, 2018, 08:24:11 am by fonograph »
 

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Re: Why do we need antennas?
« Reply #1 on: August 15, 2018, 08:30:48 am »
GAIN ! ! !
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Offline rfeecs

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Re: Why do we need antennas?
« Reply #2 on: August 15, 2018, 08:34:58 am »
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.
 
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Offline fonograph

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Re: Why do we need antennas?
« Reply #3 on: August 15, 2018, 08:36:23 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... )
 

Offline fonograph

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Re: Why do we need antennas?
« Reply #4 on: August 15, 2018, 08:40:50 am »
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.

« Last Edit: August 15, 2018, 08:45:22 am by fonograph »
 

Offline rfeecs

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Re: Why do we need antennas?
« Reply #5 on: August 15, 2018, 08:49:38 am »
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
 
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Offline CatalinaWOW

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Re: Why do we need antennas?
« Reply #6 on: August 15, 2018, 08:55:24 am »
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.
 

Offline jMrL

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Re: Why do we need antennas?
« Reply #7 on: August 15, 2018, 09:10:39 am »
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.

 |O

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

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Offline rfeecs

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Re: Why do we need antennas?
« Reply #8 on: August 15, 2018, 09:21:02 am »
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/
 

Offline xrunner

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Re: Why do we need antennas?
« Reply #9 on: August 15, 2018, 09:21:56 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... )

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.
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Offline JS

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Re: Why do we need antennas?
« Reply #10 on: August 15, 2018, 10:25:07 am »
I'd like to see a 1MHz wave guide for an AM radio ransmitter...

JS

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Offline GregDunn

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Re: Why do we need antennas?
« Reply #11 on: August 15, 2018, 03:10:18 pm »
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.
 
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Online tautech

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Re: Why do we need antennas?
« Reply #12 on: August 15, 2018, 03:19:15 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... )
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 !  :o

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Offline fonograph

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Re: Why do we need antennas?
« Reply #13 on: August 15, 2018, 08:15:48 pm »
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.
 

Offline fonograph

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Re: Why do we need antennas?
« Reply #14 on: August 15, 2018, 08:28:41 pm »
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?
 

Offline fonograph

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Re: Why do we need antennas?
« Reply #15 on: August 15, 2018, 08:37:21 pm »
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?
 

Offline exe

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Re: Why do we need antennas?
« Reply #16 on: August 16, 2018, 01:55:26 am »
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 :(
 

Offline fonograph

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Re: Why do we need antennas?
« Reply #17 on: August 16, 2018, 03:13:05 am »
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?

 

Offline rfeecs

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Re: Why do we need antennas?
« Reply #18 on: August 16, 2018, 03:44:32 am »
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:

 

Offline In Vacuo Veritas

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Re: Why do we need antennas?
« Reply #19 on: August 16, 2018, 04:11:43 am »
Why do we need microphones?
 
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Offline metrologist

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Re: Why do we need antennas?
« Reply #20 on: August 16, 2018, 04:21:08 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... )

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.
 
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Offline IanB

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Re: Why do we need antennas?
« Reply #21 on: August 16, 2018, 04:40:49 am »
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?
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Offline PhilipPeake

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Re: Why do we need antennas?
« Reply #22 on: August 16, 2018, 04:55:12 am »
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.
 

Offline IanB

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Re: Why do we need antennas?
« Reply #23 on: August 16, 2018, 05:22:35 am »
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.

Quote
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.
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Offline PhilipPeake

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Re: Why do we need antennas?
« Reply #24 on: August 16, 2018, 05:40:37 am »
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.

Quote
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.
 

Offline fonograph

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Re: Why do we need antennas?
« Reply #25 on: August 16, 2018, 05:43:43 am »
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.

How can you possibly match electrical impedance to wave impedance? If I understand it correctly these two impedance types are completly different things hence why  377 ohm cable cut open at one end isnt going to radiate away 100% energy into air.

Isnt wave and electric impedance kind of a apples and oranges situation? So far I am understanding that they are two separate independent things.So when you write that antenna matches electric impedance to the impedance of free space,to my brain it appears as if you wrote about matching student debt pressure  to water pressure inside pressurized tank,seems completly unrelated.
 

Online tautech

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Re: Why do we need antennas?
« Reply #26 on: August 16, 2018, 06:23:28 am »
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.

How can you possibly match electrical impedance to wave impedance? If I understand it correctly these two impedance types are completly different things hence why  377 ohm cable cut open at one end isnt going to radiate away 100% energy into air.

Isnt wave and electric impedance kind of a apples and oranges situation? So far I am understanding that they are two separate independent things.So when you write that antenna matches electric impedance to the impedance of free space,to my brain it appears as if you wrote about matching student debt pressure  to water pressure inside pressurized tank,seems completly unrelated.
There's two main things an antenna needs to provide and because of antenna reciprocity we design and prove transmission characteristics and it's easy to do with a VNA.
First the antenna needs be emissive at the chosen frequency and then how it's fed determines it's match to the feed line/system.
If you look at the last few pics I've posted in the SVA1015X thread you can see emissive properties in a Log Magnitude screenshot and it's match to the feeder in an SWR screenshot.
Just getting those 2 things close to optimum had the antenna performing close to ideal.
You could work in the reverse and tweak a receiving antenna with just a SA and an existing external transmission but good results would be more by chance.
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Offline RoGeorge

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Re: Why do we need antennas?
« Reply #27 on: August 16, 2018, 07:23:38 am »
"Why do we need antennas" is a very good question, and very deep, IMO.

Yet, here seems to be more a problem about words. As it was pointed out before, the "impedance of free space" and the "electrical impedance" are not the same type of beast.

The concept of impedance has a much broader sense than just the electrical impedance: The concept of impedance, by definition, means the ratio between cause and effect.. (Now, who's the cause and who's the effect, especially in electromagnetism, is a can of worms, and whoever dare to open it, will eventually end up eaten by those worms.  ^-^)
 
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Offline fonograph

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Re: Why do we need antennas?
« Reply #28 on: August 16, 2018, 07:28:33 am »
If wave impedance and electric impedance are not related.Can you two antennas,one low electric impedance high current,second high electric impedance high voltage one and if their wave impedance is same and they are both fed samw amount of power,then they will radiate the same?
 

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Re: Why do we need antennas?
« Reply #29 on: August 16, 2018, 07:34:51 am »
If wave impedance and electric impedance are not related.Can you two antennas,one low electric impedance high current,second high electric impedance high voltage one and if their wave impedance is same and they are both fed samw amount of power,then they will radiate the same?
No.
Poor SWR will rob the high impedance version of performance.
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Offline IanB

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Re: Why do we need antennas?
« Reply #30 on: August 16, 2018, 07:36:35 am »
If wave impedance and electric impedance are not related.Can you two antennas,one low electric impedance high current,second high electric impedance high voltage one and if their wave impedance is same and they are both fed samw amount of power,then they will radiate the same?

Think about electricity. You can transmit the same amount of power using high voltage and low current (requiring thinner cables), or using lower voltage and higher current (requiring thicker cables).

With electromagnetic radiation something similar happens with frequency. At higher frequencies and shorter wavelengths photons have more energy, so you can transmit a certain amount of power using smaller antennas. At lower frequencies and longer wavelengths photons have less energy, so you need bigger antennas to transmit the same amount of power.

Once again, size comes into it.
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Offline bsfeechannel

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Re: Why do we need antennas?
« Reply #31 on: August 16, 2018, 07:57:53 am »
I love these noob questions. They're disconcerting and embarrassing. Like, where do babies come from? The simple answer will let your inquirer confused. The complete version with the "sordid" details will let them horrified.

According to this article, we do not use coax with the exact impedance of the air because that is not convenient from the standpoint of power and voltage. Waveguides have their restrictions too.

If you leave a transmission line open, you will have some propagation, but the impedance mismatch will make part of the radio wave be reflected, causing all kinds of nasty problems, some of them catastrophic in the case of power signals.

Antennas match the line with the air by simply propagating all or almost all the signal that is fed to them. So no signal is reflected. To do this several tricks are employed. In resonant dipoles, the standing wave will induce currents and voltages that will match that of the cable.

Feeders of parabolic antennas behave much like a speaker horn, by gradually changing the impedance of the guide.

The design of an antenna defines its bandwidth, directivity, polarization, and many other parameters, some of them legally required.

As for the antenna gain, the reference is an ideal isotropic antenna. It radiates power uniformly in all directions. If your antenna concentrates power in a specific direction, you will have a "gain" in that direction.

So parabolic antennas have high gain, but they are very directive. Dipoles have low gain but they propagate in all directions perpendicular to the axis of the dipole.
« Last Edit: August 16, 2018, 09:18:27 am by bsfeechannel »
 

Offline PhilipPeake

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Re: Why do we need antennas?
« Reply #32 on: August 16, 2018, 07:58:46 am »
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.

How can you possibly match electrical impedance to wave impedance? If I understand it correctly these two impedance types are completly different things hence why  377 ohm cable cut open at one end isnt going to radiate away 100% energy into air.

Isnt wave and electric impedance kind of a apples and oranges situation? So far I am understanding that they are two separate independent things.So when you write that antenna matches electric impedance to the impedance of free space,to my brain it appears as if you wrote about matching student debt pressure  to water pressure inside pressurized tank,seems completly unrelated.

But they are related. If it helps, look up single wire transmission lines:

https://en.wikipedia.org/wiki/Single-wire_transmission_line

That maybe helps, because there there isn't the same return path that you get wirh twin wire or co-ax transmission lines, its more pure EM wave. All you have to do is couple that into free space to allow the RF to flow outwards. Without the matching doo-dad at the end, you still get reflections, standing waves and dissipation as heat.

Put a terminator on the end, and all that vanishes and the power dissipates in the terminator. Put an antenna in place of the terminator and the energy still flows cleanly, but radiates outwards instead of being dissipated as heat.

The antenna is coupling the energy into free space impedance - its matching your transmission line to free space.
 

Offline xrunner

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Re: Why do we need antennas?
« Reply #33 on: August 16, 2018, 09:29:35 am »
Just as a side note -

Why do we use 50 Ohm Coax?
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Offline RoGeorge

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Re: Why do we need antennas?
« Reply #34 on: August 16, 2018, 05:48:30 pm »
Just as a side note -

Why do we use 50 Ohm Coax?

This is the only paper I could find about 1929, Lloyd Espenschied and Herman A. Affel: The U.S. Patent 1,835,031 for "Concentric Conducting System" https://patentimages.storage.googleapis.com/04/dd/87/9c9d8a899ef3f1/US1835031.pdf

Offline Damianos

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Re: Why do we need antennas?
« Reply #35 on: August 17, 2018, 04:36:16 am »
The question is: Why do we need antennas?

Some similar questions may be:
Why do we need microphones and speakers?
Why do we need light bulbs?
Why do we need electric fans?
... electric water pumps?
... ...
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...
 

Offline bsfeechannel

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Re: Why do we need antennas?
« Reply #36 on: August 17, 2018, 05:59:38 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.
 

Offline fonograph

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Re: Why do we need antennas?
« Reply #37 on: August 17, 2018, 07:50:09 am »
If wave impedance and electric impedance are not related.Can you two antennas,one low electric impedance high current,second high electric impedance high voltage one and if their wave impedance is same and they are both fed samw amount of power,then they will radiate the same?
No.
Poor SWR will rob the high impedance version of performance.

Why would there be SWR problem?
 

Offline fonograph

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Re: Why do we need antennas?
« Reply #38 on: August 17, 2018, 07:53:00 am »
If wave impedance and electric impedance are not related.Can you two antennas,one low electric impedance high current,second high electric impedance high voltage one and if their wave impedance is same and they are both fed samw amount of power,then they will radiate the same?

Think about electricity. You can transmit the same amount of power using high voltage and low current (requiring thinner cables), or using lower voltage and higher current (requiring thicker cables).

With electromagnetic radiation something similar happens with frequency. At higher frequencies and shorter wavelengths photons have more energy, so you can transmit a certain amount of power using smaller antennas. At lower frequencies and longer wavelengths photons have less energy, so you need bigger antennas to transmit the same amount of power.

Once again, size comes into it.

While I am thankful for  your informative post it was information completly unrelated to my question.At no point did I mention antenna size.I know lower frequencies need bigger antenna,my question was about antenna electric impedance.
 

Offline fonograph

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Re: Why do we need antennas?
« Reply #39 on: August 17, 2018, 08:01:32 am »
But they are related.

How are they related? First success of this thread was informing me there are two kinds of impedances,electric and wave.Now you claim they arent indepedent and unrelated as I expected,then in what way are they related? Correct me if I am wrong,but you want to have 377 ohm wave impedance antenna so it can theoretically radiate 100% energy.

Can this 377 ohm wave impedance be achieved no matter what electric impedance the antenna have? Can there be high electric impedance antennas like 1 M ohm,high voltage and still have 377 ohm wave impedance and as result be efficient?
 

Online tautech

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Re: Why do we need antennas?
« Reply #40 on: August 17, 2018, 08:03:43 am »
If wave impedance and electric impedance are not related.Can you two antennas,one low electric impedance high current,second high electric impedance high voltage one and if their wave impedance is same and they are both fed samw amount of power,then they will radiate the same?
No.
Poor SWR will rob the high impedance version of performance.
Why would there be SWR problem?
Unless the signal is absorbed into the antenna and emitted, the result is poor SWR and poor antenna performance.
Two issues are relevant here, feedline matching and antenna resonant frequency.

Yes it is black magic until you get your head around a few simple principles however without the right tools getting an antenna to perform is mostly trial and error. Sure we can just feed KW into it to overcome poor design but the result would swamp out others attempting to use RF bands too and that's precisely why there's country specific regs on which bands are available and how much EIRP is permitted.
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Offline IanB

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Re: Why do we need antennas?
« Reply #41 on: August 17, 2018, 08:42:27 am »
While I am thankful for  your informative post it was information completly unrelated to my question.At no point did I mention antenna size.I know lower frequencies need bigger antenna,my question was about antenna electric impedance.

But you did mention antenna size. You clearly implied the size of the antenna would be the size of the open end of your coaxial cable floating in the air. For various reasons (size and geometry among them), the cut end of a coaxial cable is not able to act as an efficient antenna (but it can of course act as a very inefficient antenna with low power handling capability).
I'm not an EE--what am I doing here?
 

Offline xrunner

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Re: Why do we need antennas?
« Reply #42 on: August 17, 2018, 09:24:11 am »

Can this 377 ohm wave impedance be achieved no matter what electric impedance the antenna have? Can there be high electric impedance antennas like 1 M ohm,high voltage and still have 377 ohm wave impedance and as result be efficient?

Well I didn't specialize in antenna design theory as a career, but if you have a perfect dipole cut for 10 MHz and a 50 ohm transmission line, and connect it to a dummy load of 50 ohms, then all the energy will be absorbed into the load. So far so good.

Now if you have the same system and put a perfectly cut dipole for 10 MHz on the end, the energy will be radiated away into free space. Free space has that impedance of 377 ohms as a consequence of what? The system designer? Well, because it's one of those fundamental physical things that is what it is. There is no matching required because once its radiating into free space it has been transformed. It has left one system and entered another. The act of radiating is the transformation. A perfectly radiating dipole radiates it's energy into free space, which happens to have an impedance of 377 ohms. It's not "matched" as you seem to think it needs to be, it simply goes into the only medium it can go into.

The only matching has to be impedance of the coax and then to have a perfect dipole at the end which radiates all energy into space. It goes the only place it can go, into free space.

I'm sure others will correct me as needed of course.  :popcorn:
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Offline fonograph

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Re: Why do we need antennas?
« Reply #43 on: August 17, 2018, 09:58:07 am »
While I am thankful for  your informative post it was information completly unrelated to my question.At no point did I mention antenna size.I know lower frequencies need bigger antenna,my question was about antenna electric impedance.

But you did mention antenna size. You clearly implied the size of the antenna would be the size of the open end of your coaxial cable floating in the air. For various reasons (size and geometry among them), the cut end of a coaxial cable is not able to act as an efficient antenna (but it can of course act as a very inefficient antenna with low power handling capability).

Thats interesting take on my post,I didnt think about it that way.It was more of a irrational fantasy scenario to serve as bread that I can spread my buttery question on,I guess you native English speakers call it painting the picture,I did not believe in it nor did I expect anyone to view it from a angle that would make it seem like I am implying size doesnt matter.I never thought that antenna size doesnt matter.


 

Offline IanB

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Re: Why do we need antennas?
« Reply #44 on: August 17, 2018, 10:03:35 am »
I never thought that antenna size doesnt matter.

That's good, as that is one obstacle out of the way.

The next step is to believe that antenna shape matters.

Once you believe that antenna size and antenna shape both matter, then it becomes clear why we need antennas.
I'm not an EE--what am I doing here?
 

Offline fonograph

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Re: Why do we need antennas?
« Reply #45 on: August 17, 2018, 10:10:13 am »
I never thought that antenna size doesnt matter.

That's good, as that is one obstacle out of the way.

The next step is to believe that antenna shape matters.

Once you believe that antenna size and antenna shape both matter, then it becomes clear why we need antennas.

I cant believe you believe,especially after my last post that I believe antenna shape doesnt matter.Do I really have to explicitly explain it again? It was just irrational,hypothetical,completly unimportant,believed by no one,fantasy with sole purpose to deliver my question about IMPEDANCE,not size,not shape,not whatever,just impedance.
 

Offline bsfeechannel

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Re: Why do we need antennas?
« Reply #46 on: August 17, 2018, 11:44:47 am »
Mr. fonograph, you have the perfect example of an antenna in your avatar picture. That horn is an acoustic antenna. It matches the impedance of the air with the impedance of the transducer at the base of the horn that is connected to the needle. The transducer produces high pressure with low displacement. This means high impedance. Since the impedance of the air is lower, it will not be capable of producing the necessary displacement to be heard. It will not be capable of transmitting the maximum power to the air.

How the horn does that? It is itself a boundary. A limit at which the sound wave can propagate. At its throat it has the same area as the transducer, so the pressure will be the same. Without the horn, the pressure will drop dramatically some fractions of millimeters from the transducer, because the area will be that of the free space.

As the wave progresses to the mouth, the area gradually gets larger. The pressure gradually reduces until the relation between the pressure and displacement matches that of the free air.

That way the transducer is able to transfer all its power to the air. Without the horn, most of the acoustic power will be dissipated as heat.

Antennas do exactly the same: they place electromagnetic boundaries to the free space. For example, electric fields are always perpendicular to ideal conductors. So they limit the way electromagnetic waves can propagate to the advantage of the intended purpose.

The wave impedance and the electrical impedance are totally related. Voltages and currents are related to electromagnetic fields by the Maxwell equations.
 
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Offline Teledog

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Re: Why do we need antennas?
« Reply #47 on: August 17, 2018, 02:59:22 pm »
We need antennae to communicate with our imperious leaders  8)
 
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Offline LukeW

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Re: Why do we need antennas?
« Reply #48 on: August 17, 2018, 08:06:45 pm »
An antenna is a transformer which transforms the wave impedance of the transmission line to match the wave impedance of free space.
It may also have a radiation pattern which is anisotropic, providing antenna "gain" in a desired pattern.

If there isn't a good match, reflection of transmitted power will occur and the antenna won't work efficiently. This can be quantified by measuring S11, or measuring VSWR (VSWR is really just a different way of expressing S11, you're looking at the same thing.)
 
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Offline fonograph

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Re: Why do we need antennas?
« Reply #49 on: August 17, 2018, 10:29:17 pm »
So... the conductor,for example coax cable have both electric impedance and wave impedance and antenna is device that matches the wave impedance of coax to wave impedance of free space.Also,while electric impedance and wave impedance are two different things,they are connected,they affect each other....  is this about right?

About the VSWR... does that mean VSWR can not only be affected by your typical electric impedance,but in antenna,also the wave impedance? In 50 ohm transmission line system,does ideal antenna have 50 ohm electric impedance and 377 ohm wave impedance?

« Last Edit: August 17, 2018, 10:33:26 pm by fonograph »
 

Offline rfeecs

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Re: Why do we need antennas?
« Reply #50 on: August 18, 2018, 02:59:11 am »
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, 10:00:15 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 fonograph

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Re: Why do we need antennas?
« Reply #52 on: August 18, 2018, 11:06:46 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, 11:19:47 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, 11:47:37 am »
Of course. Silly me.
 

Online tautech

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Re: Why do we need antennas?
« Reply #55 on: August 18, 2018, 02:33:54 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... )
You spurred me to write this:
https://www.eevblog.com/forum/rf-microwave/antenna-project-log/
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Offline Damianos

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Re: Why do we need antennas?
« Reply #56 on: August 18, 2018, 08:45:49 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...

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 19, 2018, 03:28:24 am »
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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