Electronics > Beginners

Why do we need antennas?

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rfeecs:

--- Quote from: LukeW on August 17, 2018, 10:06:45 am ---An antenna is a transformer which transforms the wave impedance of the transmission line to match the wave impedance of free space.

--- End quote ---

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.



bsfeechannel:

--- 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.
--- End quote ---

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.

fonograph:
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.

rfeecs:

--- Quote from: bsfeechannel on August 18, 2018, 12:00:15 am ---
--- Quote ---Take a transmission line like a coax line with an air dielectric.  The wave that propagates down this line is a TEM wave.  The wave impedance of this line is 377 ohms.
--- End quote ---

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.

--- End quote ---

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.

bsfeechannel:
Of course. Silly me.

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