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Why do we need antennas?

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fonograph:
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.

tautech:
GAIN ! ! !

rfeecs:

--- Quote from: fonograph on August 14, 2018, 10:21:40 pm ---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.

--- End quote ---

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.

fonograph:
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... )

fonograph:

--- Quote from: rfeecs on August 14, 2018, 10:34:58 pm ---
--- Quote from: fonograph on August 14, 2018, 10:21:40 pm ---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.

--- End quote ---

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.

--- End quote ---

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.

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