antenna impedance and air characteristic impedance

as we know that air characteristic impedance is about 377 ohm. why we do not need to match antenna impedance to this 377ohm?

For an antenna to radiate well, the imaginary part of the impedance needs to be near zero. This is a natural current resonance due to the materials and the geometry of the antenna. To feed an antenna, one designs the antenna ideally for the real part to be matched to the load of the feed and the imaginary part of impedance to zero (not true in real life...but similar)

What you are talking about is true for aperture antennas, think horns. In a horn antenna, you feed the antenna with a waveguide, then you slowly taper the waveguide to a larger cross section until the characteristic impedance of the tapered cross section corresponds to a waveguide with a 377 Ohm impedance. Then it is basically an impedance transformer rather than an antenna per se.

Have Fun!

With all the years I have used horn antennas I had never thought of it like that thanks for a different insight :)

Dave

A ideal halfwave dipole have a characteristic impedance of 377 Ohm, seen from the space side.
Impedance at feeding point depends on were on the antenna feeding-point is placed.
Antenna resonance is related to where in frequency range coupling to space is most power effective.
Current and voltage are required factors but a small loop can have uniform current along whole antenna length and still be in good resonance.
A open-ended waveguide can also be designed for low return loss, no need for a specific horn.

Hi tallface,
Thanks for your reply! What is a feed? the impedance of a feed is real?

---------- Post added at 03:11 ---------- Previous post was at 03:10 ----------

Hi Kafeman,
thanks. Why ideal dipole have characteristic impedance of 377 ohm? Do you have any reference material or weblink you can suggest me?

These are two different things:

377 ohms (120*pi ohms) is the ratio E/H for a plane wave in free space. It is independent of the antenna that originates that plane wave.
The impedance of an antenna is the ratio V/I at the feeding point.

Regards

Z

Thanks for your reply! they are different but how should i relate these two concepts?

Any antenna seen from far field, even poor matched antenna, the wave they emit do always have an impedance 377 Ohm.

About ideal dipole:
A ideal dipole have 100% efficiency in free space.
If an antenna have 100% efficiency does it makes impedance mismatch (reflective losses) impossible.
Free space impedance is assumed to be ~377 Ohm.
A ideal dipole antenna impedance must then be?

A dipole impedance is of course possible to measure, but to get whole picture, measure along whole antenna, not only in the middle.

They are not so directly related.
Intrinsic impedance depends only of permittivity and permeability of the medium. For vacuum it is 120*pi=377 ohms at all frequencies.
Impedance of the antenna depends of the type of antenna and frequency (also effects of its near environment, etc.). It can be measured at its terminals.
Regards

Z