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!
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!
Hi Kafeman,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.
Thanks for your reply! they are different but how should i relate these two concepts?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
They are not so directly related.Thanks for your reply! they are different but how should i relate these two concepts?