Normally an antenna uses a sinusoidal wave whose positive half-period corresponds with positive direction of the current and vice versa. A pulsating signal/current is a signal in which the direction of the current may not reverse. Is such a signal still able to radiate from a dipole or other antennas?
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2\$\begingroup\$ Well, I am having trouble seeing where the charges would go. They can't just pile up in the antenna. So I think they will have to stop and reverse. \$\endgroup\$– user57037Mar 7, 2022 at 5:15
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\$\begingroup\$ But I also think that any time an antenna is driven with time varying voltage it can radiate. \$\endgroup\$– user57037Mar 7, 2022 at 5:50
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\$\begingroup\$ Then again there are some antennas, such as folded dipoles, which would pass DC current readily (very low impedance at DC). So maybe the question does make more sense than I initially gave it credit for. \$\endgroup\$– user57037Mar 7, 2022 at 17:42
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3 Answers
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A mundane example is DC with AC ripple superimposed on it like that in virtually every closed loop DC circuit that exists. Current never reverses but EMI is radiated nonetheless. The current changes (increases and decreases) but never reverses direction.
What you need are changes in charge flow, not bidirectionality but if you have an antenna that is open-ended and thus a "dead-end" for charge flow then you need bidirectionality unless you want to forever increase the voltage so charge continues to flow into and accumulate in the antenna (until a giant arc is produced*).
EDIT: *@Lorenzo Donati -- Codidact.com says
Not necessarily. If the charge buildup is sufficiently slow some charge could escape the antenna by other means than arcing (corona discharge, for example). So if the antenna is pulsed at a slow rate an equilibrium could still be reached without arcing. –
4 hours ago
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3\$\begingroup\$ "until a giant arc is produced" Not necessarily. If the charge buildup is sufficiently slow some charge could escape the antenna by other means than arcing (corona discharge, for example). So if the antenna is pulsed at a slow rate an equilibrium could still be reached without arcing. \$\endgroup\$ Mar 7, 2022 at 9:53
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\$\begingroup\$ Some antennas such as folded dipoles and also slot antennas have low impedance at DC. \$\endgroup\$ Mar 7, 2022 at 18:46
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\$\begingroup\$ Isn't there some superposition principle that could be applied as well? \$\endgroup\$ Mar 7, 2022 at 22:59
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Start with a charged particle, in vacuum at rest in an inertial frame. The electric field lines converge at the particle's position radially. Now, move the particle over, and bring it back to rest. The electric field lines converge at its new position. However, far away, the electric field lines still point toward the old position. The boundary between the old field configuration and the new configuration expands at the speed of light. On the boundary, the electric field lines are kinked to match the old configuration with the new. That's an electromagnetic impulse.
Now, you may move the particle again, in whatever direction, creating a new kink. The new direction may be the same as the old, opposite, perpendicular, whatever. The details of the kinks will depend on the direction, but the charge will radiate regardless. The direction of the current need not reverse. However, this process requires acceleration of the radiating charge, so even if the direction of the current doesn't change, its magnitude must change.
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\$\begingroup\$ So an electron flying straight through space at constant speed generates waves as it enters new space and one in a closed loop generates them too because to change directions to close the loop requires acceleration? \$\endgroup\$– DKNguyenMar 7, 2022 at 16:15
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\$\begingroup\$ @DKNguyen An electron flying at constant velocity generates no waves. Acceleration, either change of speed or direction, is necessary. \$\endgroup\$ Mar 7, 2022 at 16:27
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\$\begingroup\$ Isn't it still reaching areas of space where the electric field has not yet reoriented itself to the electron? \$\endgroup\$– DKNguyenMar 7, 2022 at 16:27
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1\$\begingroup\$ @DKNguyen with no acceleration, there is a frame in which the electron doesn't move. If there are no waves in some frame, there are no waves in any frame. If you follow this out, as Einstein did, it leads to special relativity. \$\endgroup\$ Mar 7, 2022 at 16:34
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\$\begingroup\$ So not quite as straightforward as the field lines in space reorienting themselves to the electron has it passes by. \$\endgroup\$– DKNguyenMar 7, 2022 at 16:40
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"Pulsating" implies "alternating current" (alternating between "on" and "off" is alternating current. Think "square wave". or "rectangular wave" which is a more accurate term. Although each is made of a fundamental sine wave and its harmonics.)
Then think about the law of Conservation of Energy. Energy can neither be created or destroyed. Only converted to other forms of energy.
Then think about Faraday's laws of magnetics. How does an antenna work? What happens when current flows in a conductor? (Hint: the energy is converted into a magnetic field expanding outward from the wire at the velocity of light which is abbreviated as "c".)
What happens when the energizing current ceases to flow in the conductor? (Hint: The magnetic field is converted into electric current when it collapses back into the conductor at c, and "induces" a reverse current in the conductor.)
And don't forget c! It takes time for a pulse to travel from the feedpoint to the end of the conductor, and back when the pulse ends. The length of the conductor determines the length of time, which in turn determines the resonant frequency of the conductor. That's why antennas for different frequencies are made in different lengths.
You original question is a very good one, and you will have a great experience duplicating the thought processes of the pioneers such as Maxwell, Lorenz, Faraday, Einstein and many more. Work to master the math and you will become enlightened with a whole new understanding of how the universe actually works!
