4
\$\begingroup\$

An antenna is an electrical device which converts electric power into radio waves and vice versa we all know this.

In transmission, a radio transmitter supplies an electric current oscillating at radio frequency (i.e. a high frequency alternating current (AC)) to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). The electromagnetic wave has an E and B field which are perpendicular to each other; but in antenna reception we only use the E field of an electromagnetic wave in order to produce a tiny voltage at its terminals, that is applied to a receiver to be amplified.

Why do we not use both E and B fields for reception? Can we produce more voltage by using both E and B fields?

\$\endgroup\$
  • 2
    \$\begingroup\$ Antennas that use the E field are almost always a very different design than antennas (usually Loop antennas) that use the B field. Therefore, designing an antenna for both E field and B field is at cross purposes as your design will either favor E field or the B field. But, antennas that are designed for the E field only, or B field only are common and in use. \$\endgroup\$ – K7PEH Aug 18 '15 at 16:05
  • \$\begingroup\$ On your last question about producing more voltage. The voltage produced by the E-field and B-field do not add to create higher voltage levels. They are out of phase with each other. You use one or the other but using both is definitely not typical. \$\endgroup\$ – K7PEH Aug 18 '15 at 17:12
  • \$\begingroup\$ @K7PEH: They are actually in phase only in the near field they are out of phase (90°). But later because of the maxwell equations they get in phase (stable state) \$\endgroup\$ – Sider Aug 18 '15 at 19:43
4
\$\begingroup\$

The premise of your question is incorrect. Receiving antennas "use" both the E field and the B field to develop the voltage at their terminals. Antennas constructed from straight conductors are most sensitive to signals whose E field is parallel to the conductor (inducing a voltage) and B field is perpendicular to the conductor ("cutting across", which also induces voltage).

Loop antennas are a special case. They depend almost entirely on the B field alone, and are not very sensitive to the E field at all.

\$\endgroup\$
  • 1
    \$\begingroup\$ I don't think the premise of the OP's question is wrong at all. Actual receiving antennas are designed either to use the E field or the B field, rarely both. In fact, without doing some research, I can't think of an antenna design that uses both. Your point seems to be mostly saying that the E and B fields are both present, which is true, but they are not equally used in generating the signals processed by the receiver. For receiving, by a large majority, most antennas are Loop (built in loop-stick) antennas based on B field. \$\endgroup\$ – K7PEH Aug 18 '15 at 17:11
  • \$\begingroup\$ Note: my comments above are primarily directed towards broadcast, HF, and VHF antennas. There are other antennas used for Radar, cellular phones, radio-astronomy based on other methods that I am not including in my comments. \$\endgroup\$ – K7PEH Aug 18 '15 at 17:15
  • \$\begingroup\$ @K7PEH: Your username suggests that you're a ham, so you really should know better. Loop antennas are only commonly used for MW signals and below (roughly, anything less than 1 MHz). "Straight conductor" antennas are used for pretty much everything else. And I have no idea why you think the B field is irrelevant to straight-conductor antennas. Consider the half-wave dipole, and how the impedance varies along its length: The ends are high-impedance and sensitive to the E field, while the center is low-impedance, which allows the B field to induce a current. \$\endgroup\$ – Dave Tweed Aug 18 '15 at 17:49
  • \$\begingroup\$ Yes, I am a ham operator, also degrees in Physics (graduate work Mathematical Physics). I have made a Loop antenna that I used on the 40-meter band (HF) so loop antennas can be used for many different frequencies although I agree that the majority in use are the broadcast band. Loop antennas are also commonly used for RDF and their size and matching circuits different only due to frequency (for most). Your comment about the dipole is not correct for the main reason of polarization. A dipole is most sensitive as an antenna when it is polarized the same as the E-field, not B. \$\endgroup\$ – K7PEH Aug 18 '15 at 17:54
  • 1
    \$\begingroup\$ Yes, I regularly use NEC4 (the licensed version) software along with other additions I do in Mathematica. I have also written my own analysis software (in Mathematica) using both MOM and FTDM. I am thinking of doing a finite-element analysis program but have not gotten around to it. The B-field in a dipole is irrelevant as far as almost everyone who uses dipole antennas -- this is because of coupling. The coupling between an E-field (which then accelerates charges in the conductor) is much stronger than any B-field coupling to the Dipole. If you use a magnetic loop, it is the other way. \$\endgroup\$ – K7PEH Aug 18 '15 at 18:18
0
\$\begingroup\$

The energy from both fields is too small to convert in other forms of enjoyable energy (electro chemical, kinetic, etc), only to activate some small electronic pieces. Remembering a formula B=E/c, c speed of light and E and B are the respective fields, for example, E field of 1000 V/m, a very strong field, produce a magnetic field with a small density of 0.000003333 Tesla or 0.03333 Gauss. Imagine the difficulty to install a device that could absorb enough energy to light a bulb or turn a shaft. At the same way, the E field is very weak to be used at practical level. And how to use the changing E field? A capacitor? Could be, but with hundred or thousands of capacitors with very low capacitance. I think Nikola Tesla tried something based on transmit EMG waves and the houses receiving the energy, but forgot the project.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.