Author Topic: Why do we need antennas?  (Read 1862 times)

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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.
 

Offline 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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Online 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?
 

Offline tautech

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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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Online 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?
 

Offline 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).
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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.
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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 »
 


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