[mathew_orman (OP)]

Is there a magnetic field around ray of electrons in vacuum just like around the CRT beam?

[lightarrow]

Is there a magnetic field around ray of electrons in vacuum just like around the CRT beam?

Why there should be a difference? Isn't a CRT beam made of electrons in the void?

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[mathew_orman (OP)]

The question was about field and not about differences in description of ray of electrons...

[alancalverd]

Yes

[mathew_orman (OP)]

So the ray has self inductance and can produce electromagnetic waves?
Two rays in parallel would have mutual inductance?

[chiralSPO]

Any net motion of charge will have an associated magnetic field. A cathode ray is no different from a wire with current in this regard.

[PmbPhy]

... can produce electromagnetic waves?

Only when the current is time varying so as to create an electromagnetic wave.

[mathew_orman (OP)]

It is easy to modulate the ray using gate electrode but would it radiate and act as a dipole antenna?

[alancalverd]

If you are careful and diligent, you could end up designing a betatron.

[lightarrow]

The question was about field and not about differences in description of ray of electrons...

If we knew that all dogs barks, you say that dog A barks and ask me if dog B barks too and I reply that it's still a dog, why do you ask me that the question was about barking?  []

I ask you again: why should electrons moving in the void of a CRT should behave differently from electrons moving in another void region of space? If they generate a magnetic field in the first case, why shouldn't they do in the second? It's the fact that a charge moves, that makes it generate a magnetic field, nothing else than this.

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[mathew_orman (OP)]

There are many differences  like resistance skin effect and etc.

[evan_au]

If you want to efficiently generate an electromagnetic wave in space, a linear vacuum tube (like an old-fashioned TV CRT) is not necessarily the best way to do it.

A vacuum is basically a pretty good insulator, so you need very high voltages to produce a low current. You can help it along by heating the cathode, but you end up with a very high impedance AC source. This is not a good impedance match for free space, so you need to transform this high impedance source to match the low impedance of free-space radiation. Traditional vacuum tune transmitters used transformers to match the high source impedance to the low impedance of a conductive copper wire, which is matched to the impedance of free space at the intended operating frequency.

Mobile phones use semiconductor amplifiers, which are fairly low impedance devices, and a reasonable match to the antenna impedance. But they can only deal with fairly modest power levels.

For really high power transmitters (like TV transmitters), vacuum tube + transformer + copper antenna is still the best way to go.

One area where vacuum tubes are still used in residential applications is the cavity magnetron in microwave ovens. The magnetic field in the magnetron means that the electrons do not propagate in straight lines, but in helices. At GHz frequencies and higher, high-powered electromagnetic signals are not carried on wires, but more in "plumbing" - they propagate down pipes. In this application as an oscillator, the low efficiency of coupling to radiation is an advantage, as it keeps the oscillator running.

So I guess the next questions are:

• What range of frequencies would you like to transmit?
  
• At what power levels? and:
  
• Is efficiency important in your application? 

[mathew_orman (OP)]

The only property of empty space or vacuum is containment of matter...
And there is no such property as space resistance, impedance and etc...
Antenna theory creates many abstract quantities to satisfy the wave equations for empty space...
Examples of such are permittivity and permeability of vacuum, clear fudge number derived from speed of light...
The absence of influence is not a property of any kind...
The property of space can only be defined in presence of matter enclosed by that space or another words space between...

[PmbPhy]

The only property of empty space or vacuum is containment of matter...

There are other properties of empty space such as the geometry of the space itself. Sometimes space is Euclidean and sometimes its not. Euclidean space can be altered by matter to create Non-Euclidean space.

[mathew_orman (OP)]

So there is a way to measure empty space?

[lightarrow]

There are many differences  like resistance skin effect and etc.

But you were interested in "magnetic field".
I repeat: a beam of electrons (that is, many charges moving all together in the same direction with the same speed) generates a magnetic field around them. Period.
So, why should there be a difference if this beam is in the void between two galaxies or inside a CRT monitor? It's not a critic, it's a question, to understand what you have in mind.

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[evan_au]

Antenna theory creates many abstract quantities to satisfy the wave equations for empty space... Examples of such are permittivity and permeability of vacuum, clear fudge number derived from speed of light...

I disagree that permittivity and permeability are purely abstract*.

Both parameters can be fairly readily measured in the lab. The large number of people wearing glasses or contact lenses would find that they wouldn't work if permittivity was purely abstract.

Optical fibres are very concrete (figuratively speaking), and the speed of light in this medium is determined by its permittivity and permeability. The speed of optical communications has real economic impacts.

The speed of light in a vacuum is just a special case of the speed of light in any transparent medium (including air, spectacles and the lens on the digital camera in your pocket).

*I recognise that permittivity and permeability may be just micrometer-scale averages of parameters that have more complex properties at nanometer scales.

[mathew_orman (OP)]

What you are measuring is dielectric and magnetic property of atomic structures but vacuum has no such property...

[lightarrow]

What you are measuring is dielectric and magnetic property of atomic structures but vacuum has no such property...

Not only vacuum does have those properties, it has by definition!

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[mathew_orman (OP)]

What definition is that?