| Electronics > Beginners |
| Why do we need antennas? |
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| fonograph:
Ok,so... the answer to my original question is that you cant just connect 377 ohm cable to air and expect to radiate 100% energy becose there are two kinds of impedances.First,there is electronic impedance and then there is wave impedance,just becose some cable have 377 ohm electric impedance doesnt mean it have 377 ohm wave impedance. More questions : Is the electric impedance a nearfield impedance and the wave impedance a farfield/photon impedance? Is this electric/wave impedance in some way synonym of near-field/far-field? |
| exe:
I don't know much about antennas, but I think the signal won't be radiated from the open end of the cable. It will bounce back. Depending on wavelength and cable length, it may appear as open circuit, or, if is 1/2 of the wavelength, it will be a short that can potentially fry the circuit. Some tutorials on transmission lines: There are other good videos, but I can't find links. The one I particuraly like shows standing waves, reflection, termination, etc using ropes and weights. It's very interesting to see things. But can't find the link :( |
| fonograph:
I made thread about week ago asking if making the antenna have 377 ohm impedance improves its radiation performance and the answer I got is yes.But now,I must re-ask that question becose previously I had no idea there are two kinds of impedances.When I originaly asked that question,I was really thinking about electric impedance,not radiation impedance. Does the electric impedance matters in antenna? Should we make antennas have 377 ohm electric impedance,or does electric impedance not matter and we only need to make the antenna have 377 ohm wave impedance? |
| rfeecs:
--- Quote from: GregDunn on August 15, 2018, 05:10:18 am ---You could take a coaxial cable and stretch the shield and conductor out in opposite directions, and you will have an antenna, in fact a dipole. It will have more RF signal gain in the directions at right angles to the conductors, and much less gain in the direction along the conductors (the overall energy delivered in all cases is of course less than 100%). It will in fact have a wave impedance of about 72 ohms, which means it won't transfer power to the E and B fields as efficiently as something like a folded dipole, even if you can deliver 100% of the power to the element itself by matching its electrical impedance. --- End quote --- Doesn't sound quite right. 72 ohms is approximately the radiation resistance, not the wave impedance. A folded dipole has approximately the same radiation pattern as a single wire dipole. The folded dipole just gives you 4 x the feedpoint impedance, so about 292 ohms, and a better match to 300 ohm balanced twin feed ribbon cable. Once again there is confusion between electrical impedance and wave impedance. In general it might be better for ham radio people and electronic hobbyists to forget about wave impedance. It really only applies to EM theory. One place where wave impedance is actually useful is discussed here, it allows you to treat problems of dielectric layers, like dielectric mirrors, just like transmission lines: |
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