Electronics > Beginners
Why do we need antennas?
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fonograph:

--- Quote from: IanB on August 17, 2018, 12:03:35 am ---
--- Quote from: fonograph on August 16, 2018, 11:58:07 pm ---I never thought that antenna size doesnt matter.

--- End quote ---

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.

--- End quote ---

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.
bsfeechannel:
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.
Teledog:
We need antennae to communicate with our imperious leaders  8)
LukeW:
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.)
fonograph:
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?

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