antenna-theory.com

[asterysk]

sorry if this is a stupid question. I did search the forum and look at quite a few postings, however, I still don't understand :

How does a magnetic loop antenna only "see" magnetic waves, what is different about it compared to an electric wave antenna and how does that difference allow it to discriminate between magnetic and electric waves

[admin]

Far from sources, magnetic and electric fields are almost interchangeable: EM waves are made up of E- and H-, but if you know E you also know H. This situation does not apply near EM sources (such as antennas that radiate) or reflectors (such as conductors).

A small loop (by small we always mean the diameter of the loop relative to the wavelength of interest, so a 1 meter loop is very small at 10 kHz but very large for 10GHz) produces only magnetic fields. A small loop is basically an inductor; it has low impedance so charges cannot be separated. Magnetic fields flow around current; a loop is current and hence is circulated by magnetic field. The loop does not allow for charge separation (because it a short circuit); so very little electric fields are produced.

Contrast that with a short dipole antenna. Very little current flows, so very little magnetic fields are produced. The voltage across the dipole though can be large, since it is not shorted out. This means there is charge separation - which is what produces electric fields.

So there aren't magnetic and electric waves, but fields, that certain structures are much more sensitive too.

[asterysk]

[admin]

1) Voltage is the definition of charge separation
2) No
3) yes
4) Short circuit means no voltage = no separation. For large loops, you still have a dc short but not an rf short, so you can have radiation

[R. Fry]

However it should be recognized that in its far field, the e-m radiation of a loop operating as a transmit antenna is indistinguishable from the far-field e-m radiation of all antenna configurations.

An e-m wave necessarily includes both the E- and the H- fields, as neither one can exist without the other.

Those fields propagate at right angles to each other, and are related to each other by the characteristic impedance of space (about 377 ohms).

[phamlindan]

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