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According to a few descriptions of antenna fundamentals i've read recently including that in Wikipedia and the ARRL handbook, when a half wave dipole is resonant, the inductive and capacitive reactances cancel out, and that's why there is no reactance in the impedance.

My understanding is that the amount of reactance seen at the feed point of a center fed half wave dipole is determined by the phase relationship in time between the applied AC electric potential at the feed point and the resultant AC current which arrives at the feed point after being reflected back from the ends of the antenna.

So keeping in mind that a half wave dipole appears to be a series RLC circuit, what are the values of reactance that cancel out at resonance ?

And why is an antenna resonant, because the voltage and current are in phase or because the reactances cancel out, or both ?

See this question Exactly what is the cause for the zero reactance seen in the impedance at the center feed point of a resonant half wave dipole? which i have also posted which asks about the same subject.

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    $\begingroup$ It's important to note that a "disturbance" (wave) travels along the antenna, not charges (electrons). Just like an ideal wave in water, where individual water molecules are only displaced vertically, though the wave travels horizontally. The drift velocity of an electron in copper is orders of magnitude slower than the near-light-speed at which a wave (disturbance) travels along an antenna conductor. $\endgroup$ – Brian K1LI Jun 10 '20 at 10:40
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    $\begingroup$ Seems like what you're asking boils down to whether an antenna resonates in the same sense that an RLC circuit does — is that a correct understanding of your question? $\endgroup$ – natevw - AF7TB Jul 3 '20 at 17:44
  • $\begingroup$ @natevw-AF7TB Hi Nate yes that's correct. $\endgroup$ – Andrew Jul 7 '20 at 0:32
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If a half wave dipole is resonant, there is no reactance to cancel out. The definition of resonance is when there is no reactance, because there is no phase between voltage and current.

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    $\begingroup$ I think the question is not "what reactance is there to cancel out", but rather "when the reactances do cancel out because the antenna is resonant, what is the value of the negative reactance and the positive reactance?" $\endgroup$ – Phil Frost - W8II Dec 4 '20 at 14:19
  • $\begingroup$ @PhilFrost-W8II Phil That's right and the correct answer is that there are no positive or negative reactances to cancel out at resonance because resonance is almost completely determined only by the element lengths and consequent relationship between voltage and current. I'm surprised that this very valid and simple question has attracted down votes. $\endgroup$ – Andrew Dec 4 '20 at 22:53
  • $\begingroup$ @Andrew Resonance requires an oscillation between two forms of energy storage. In a swing, that's kinetic energy and gravitational potential, and in a resonant antenna it's electric and magnetic potential. If there were no reactive components anywhere, you couldn't have resonance: Q would be zero, and bandwidth infinite, like a dummy load. I think your problem is you want to think of "the antenna" as an atomic unit, but to see the reactance you have to consider the antenna as the sum of infinitesimal units, each contributing some reactance but summing to zero at the feedpoint. $\endgroup$ – Phil Frost - W8II Dec 5 '20 at 2:16
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An antenna is not a series RLC circuit, but it can be modeled with lumped circuit elements over a certain frequency range. See, for example, Tang et al, "Equivalent Circuit of a Dipole Antenna Using Frequency-Independent Lumped Elements."

A thought experiment shows why: we observe that the feedpoint impedance of a half-wave dipole exhibits periodic behavior at harmonics of its design frequency, but a simple lumped circuit does not. For example, using the formulas of Tang et al, we can model the "input impedance" of the equivalent circuit of a 20-m half-wave dipole:

enter image description here

It "resonates" at about 14.3-MHz. The same antenna modeled by NEC-2 shows the very different behavior to which we are accustomed:

enter image description here

The input impedance behavior of the half-wave dipole is more reminiscent of a transmission line, which can also be modeled with lumped elements. But, since the object of the lumped-element model is computational simplicity, the number of elements required to produce an accurate transmission line model would probably defeat the purpose.

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And why is an antenna resonant, because the voltage and current are in phase or because the reactances cancel out, or both ?

Reactance ($X$), resistance ($R$), and the phase difference between voltage and current ($\theta$) are related by:

$$ \tan \theta = { X \over R } $$

So if reactance is zero then $X/R$ is zero, so $\theta$ must be zero, meaning voltage and current are in phase.

In other words, "zero reactance" means "voltage and current are in phase". One does not cause the other: they are two ways of saying the same thing.

Likewise, resonance is defined as zero reactance. It's not "caused" by zero reactance or voltage and current being in phase: it simply is, by definition.

In other words, if you design an antenna with any one of these three objectives:

  • current and voltage are in phase at the feedpoint,
  • feedpoint reactance is zero, or
  • antenna is resonant

you will find the other two are satisfied. There is no ordering or causality between any of them: they are each mathematically equivalent.

If you are concerned only about the feedpoint, asking what the inductive and capacitive reactances are which sum to zero reactance is like solving $x-x=0$ for $x$. It's not possible, and also it doesn't matter.

Besides, a dipole isn't an RLC circuit, so there is no inductance in henrys or capacitance in farads that can be found, although as Brian K1LI explains it is possible to approximate a dipole as an RLC circuit over a limited frequency range.

There is perhaps a slightly different question you could ask which does have an answer. A dipole, like any resonant system, has a Q factor. We know that a dipole must store some energy in its electromagnetic field, and some of this energy is lost to radiation and possibly other losses, like resistive losses in the conductor and ground. The Q factor relates to the ratio of these quantities.

One way to think of Q is as the ratio of reactive power to real power. The reactive power has current and voltage 90 degrees out of phase. The energy associated with this power doesn't go anywhere and doesn't do any real work: it just oscillates between the electric and magnetic fields, forever. The real power has voltage and current in phase, and does real work: mostly (ideally, entirely) radiation.

A good half-wave dipole will have a Q of about 10. Which means if the antenna is radiating 100 watts, there is about 1000 watts of reactive power. You could measure this with electric and magnetic field probes in the near field of the antenna. With the electromagnetic fields known you can calculate the Poynting vector. In the near field of the antenna you should expect to find the imaginary part of the Poynting vector is about 10 times that of the real part, meaning the electric and magnetic fields are almost but not quite 90 degrees out of phase. Of course the precise values will depend on just where you place the probes, and the construction and environment of the particular antenna.

While this is not exactly what you asked, it does seem closer to the understanding you're seeking. Unfortunately it does require thinking beyond just what's happening at the feedpoint, and instead considering what's happening in the electromagnetic fields in space around the antenna, and this requires some more complicated mathematics.

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  • $\begingroup$ Hi Phil thanks for taking the time to provide that answer. Your answer makes complete sense. However there is one remaining mystery, when you say the voltage and current at the feed point are in phase, which voltage and which current ? The half wave dipole is series fed which means the same current that flows through the source flows into the antenna via the feed point. The value of this current must be the voltage supplied by the source at the feed point divided by the antenna radiation resistance + losses. $\endgroup$ – Andrew Nov 28 '20 at 0:44
  • $\begingroup$ The source current isn't the current of the standing wave which only exists on the antenna. The voltage and the current of the standing wave are in quadrature. At resonance the voltage of the standing wave on the antenna is much higher than the applied voltage in direct relation to the Q of the antenna. $\endgroup$ – Andrew Nov 28 '20 at 0:45
  • $\begingroup$ My understanding is the voltage of the standing wave which is 90 deg out of phase with the applied current is always zero in the middle at resonance, as you make the antenna longer or shorter the zero crossing point no longer occurs at the feed point and now the feed point has a non-zero voltage which is out of phase with the feed point current. $\endgroup$ – Andrew Nov 28 '20 at 0:45
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    $\begingroup$ @Andrew There is only one voltage and current at the feedpoint, and that's the electric potential difference between the two feedpoint terminals, or the current through either one them (which will always be equal under normal feed conditions). $\endgroup$ – Phil Frost - W8II Nov 28 '20 at 14:35
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    $\begingroup$ @Andrew Voltage and current (wherever you want to measure it) can't be exactly 90 degrees out of phase, otherwise the Poynting vector would be purely imaginary and there'd be no real power, and thus no radiation. If Q is 10, then the phase relationship is more like $\tan^{-1}(10) = 84.3^\circ$. Again, depends on just where in the field you measure it. $\endgroup$ – Phil Frost - W8II Nov 28 '20 at 14:43
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In my mind the difference is that:

  • An RLC circuit "resonates" by bouncing energy back and forth within itself. And at a particular frequency this stored energy is retained for a particularly long time. (The Q of such a circuit represents the tradeoff between ringing for an extra long time at only one particular frequency, versus the energy bouncing around with still somewhat acceptable efficiency across a wider range of frequencies.)
  • The "resonance" of an antenna usually has more to do with its radiation resistance at a particular frequency. Here's where I don't want to steer you wrong since maybe this is more conceptually related than I imagine, but the resistive energy loss in an RLC circuit is something of an inevitable "defect" whereas it's the entire point in a good antenna. You just want the energy to be lost as EMF radiation rather than thermally.

That said, a small loop antenna is decidedly both: it's an RLC circuit designed so that ideally all the R would be due to radiation rather than loss. This is again related to Q — and with resonance being at an exact "point" frequency means that any modulation sidebands need to be taken up by a lossy resistance somewhere but in practice with most antennas, and even many small loop antennas, the Q curve tends to be on the scale of e.g. the entire 80m "band" rather than anywhere near a concern for the bandwidth of a single CW/SSB transmission.

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The value of reactance varies both in magnitude and sign as functions of the operating frequency, the radiating length of the antenna, and the physical location of the feedpoint terminals along that length.

At resonance, jX = 0.

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