The voltage and current distribution in a dipole is usually shown as ninety degrees out of phase, yet the radiated E and H fields are shown in time phase and space quadrature, how is this possible, how do the waves shift back into phase.
When the dipole radiates, it is at or near resonance, which means the voltage and current are in time phase, which supports the depiction of the E and H fields being in phase.
When anything is in resonance the current and voltage are out of phase. Current and voltage being out of phase is exactly what is expected and required when energy is being stored, which is what resonance is. A small proportion of the current is in phase with the voltage, and this represents the power that is either lost in the resistance of the dipole or radiated. The ratio of the stored power to the power lost (radiated or gone some other way) per radian is the Q of the resonator. A dipole needs to be in resonance to get the current high enough to radiate a reasonable amount. As an illustration, the initial current may only radiate 1/100th of the power it represents. Resonance means that the 99% remaining current keeps coming back 100s of times until it is all gone. If it wasn't resonant the power would be reflected and the 99% would be absorbed back in the generator. Making the dipole fat increases the proportion that can be radiated first time (lowers the Q, increases the bandwidth).
Thanks for taking the time to responding. But when you take an RLC circuit and operate it at resonance the impedance is purely resistive (the reactances cancel). Why is it not the same with an antenna?
In an RLC circuit the voltage and current in the inductor are 90 degrees out of phase, and the voltage and current in the capacitor are 90 degrees out of phase, and the voltage in the capacitor is 90 degrees out of phase with the current in the inductor. Only the current and voltage in the resisitor are in phase, which represents only a small proportion of the current or voltage in the resonator, and results in a small power loss compared to the energy stored in the resonator. E and H, or V and I out of phase represents stored energy = resonance (usually) and E and H, or V and I in phase represents energy going somewhere, such as into heat or travelling in space. Look up Poynting vector, for instance.
But the overall current into the RLC circuit across the terminals are in phase. Yes the individual currents in the cap and inductor are 90 degrees out of phase, and cancel each other to give nett current that is in phase with voltage.
Below resonance and antenna is capacitive, at resonance it is resistive and above resonance it is inductive, same as RLC circuit.
The impedance you are referring to is at the antenna terminals. The current and voltage here are in phase too, at resonance.
The ends of the antenna are similar to a capacitor, with a high electric field between them, and the straight wires like an inductor with the magnetic field lines around them. The circuit is similar to a series RLC circuit. The electric field between the ends of the antenna is maximum when the current has stopped. The current in the arms is maximum when the charge is all gone, so there is no voltage across the capacitor. Only a small voltage remains along the wire, to oppose the resistance and to feed the radiation, and that voltage is in phase with the current. You would have a similar result if you put a small transformer between the inductor and capacitor in a series LCR resonator, and took a small amount of power from it.
There is a small amount of in-phase magnetic and electric field around the antenna - enough to give the radiated power. The rest represents stored energy; energy that goes in and out or appears in different places each half cycle. Look at the equations for the fields around a radiating dipole. Some of them go down as 1/radius cubed, others as 1/radius squared. These can't carry energy away because they contain less energy as the radius increases Only the component that decreases as 1/radius represents power radiated, and this is much smaller than the other components close to the antenna.
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