resonance vs resonator in antenna

Question

Resonance in electronic means: reactance of zero. (maybe wrongly/correctly someone's calling the minimum VSWR, the resonance of antenna! and this cause confusion)

I've heard in Antenna Design for Mobile Devices book: "If your antenna does not have good resonator then matching circuit will not really help". Or look at this question on stackexchange-ham : "What is the best resonator in Yagi antennas?". I think they must say "radiator" instead of resonator! isn't that?

Because the resonance is not important in hypothetical radiator if we have no limitation in value of feeding/source impedance. For example: Radiator con have any reactive/resistive impedance, who care about it, if our source have impedance conjugate to radiator impedance, since there is not reflection.

But maybe the resistive impedance of radiator is physical characteristic of good radiator, but why? (and this is philosophy of calling the radiator, resonator!). Any idea about it?

Or maybe this (resonators are equal to good radiator) is conflicting concept, because if we use conjugate of pure reactive radiator, all of energy will transfer and we then have no reflection, then all energy have to radiate, isn't that? (If this is really conflicting then the quote from "Antenna Design for Mobile Devices" from Zhijun Zhang must be wrong!)

But if we deny the equality, more confusion will occur, since in this manner everything can radiate in our band of interest efficiently, (through matched source/line) and there is no lambda/2 limitation.

I need idea to stop being confused!

Question in glance: 1.Is there a different between definition of resonance in electronic and antenna? 2.Is the resonance is the characteristic of good antenna? why? 3.If yes, then what will happen to the signal, that transferred to antenna completely, through conjugate matching? Since it have to radiate but you told it is not a good radiator, what's different. 4.Is radiating efficiently mean having no reflection from antenna?

Answer 1

The word Resonator is used to mean the loading coil in a short vertical whip antenna, that makes it resonate at the frequency of interest.

The Hustler brand, for example, calls its coils resonators.

I don't know or any other meaning of a resonator in antennas. I've never heard of it in a yagi or other "full size" antenna. I would use *resonant circuit" for anything made of discrete components. There is a Dielectric Resonator Oscillator in a satellite TV LNB, and it has a characteristic resonant circuit response.

Now for efficiency and matching of antennas, what matters is the amount of current you can drive into the radiation resistance of the antenna. All the capacitive or inductive impedance just has to be cancelled out by a conjugate matching network, otherwise it reduces the current flow.

The matching components will have some loss, so if they can be kept as small as possible the antenna will be more efficient. Often, making the antenna the right size and shape will result in a radiation resistance closer to the driving impedance, making the matching circuit's job easier.

Finally, if you try to match a highly reactive antenna with another large reactance, the bandwidth of the antenna will be small. Again, an antenna with a smaller reactance to start with, will generally have a wider bandwidth when matched.

Answer 2

A "resonator" may be used in a "resonant" circuit. A resonator is a circuit; resonance is a condition that exists at the natural frequency of the resonator.

In order to resonate, a resonator, whether it contains "lumped" circuit elements (e.g, resistors, capacitors, inductors, etc.) or distributed circuit elements (e.g., transmission lines, antennas, etc.) must comprise positive and negative reactances which cancel at the resonant frequency.

For example, a simple series resonant circuit comprises a capacitor (negative reactance) and an inductor (positive reactance) which resonates where the values of the capacitive and inductive reactances are equal and opposite, so they cancel to zero. At the resonant frequency, such a perfect circuit passes the input signal without attenuation.

Conversely, a simple parallel resonant circuit comprises a capacitor (positive susceptance) and an inductor (negative susceptance) which resonates where the values of the capacitive and inductive susceptances are equal and opposite, so they cancel to zero, equivalent to infinite reactance. At the resonant frequency, such a perfect circuit completely blocks the input signal.

Some hams refer to the frequency at which an antenna system presents the lowest SWR to a transmitter as the antenna's resonant frequency. This is a colloquial generalization of the term. As indicated in the question, a "resonator" added to a non-resonant antenna will change the impedance measured at the driving point, but it can also change the current distribution on the radiator and the current induced in nearby objects (like ground, radials, vehicle body, etc.), which can effect efficiency and other parameters. Whether it produces "resonance" in the purest sense of the word is up to the discretion of the designer.

Answer 3

The real definition of a resonator is to store energy in each cycle of excitation. It is common to misuse that term to mean zero reactance, but that is the beginning of the confusion. Once you distinguish resonance from the surface impedance phenomenon, the confusion will go away.

Most antennas, like dipoles, monopoles, and full wavelength loops, are not resonators. Most antennas are standing wave devices. Therefore, the feedpoint impedance may exhibit a "resonance-like" appearance without storing energy.

LC-tuned circuits are like a pendulum; they store energy in two forms, and the two forms of energy cycle at each cycle of excitation at the resonance frequency. At steady-state of excitation at resonance, resonators do not withdraw or dissipate any significant amount of energy. However, antennas do not store energy. Antennas have to withdraw energy from the feedline at each excitation cycle and radiate it.

There are proper resonant antennas, like tuned loop antennas (so-called "magnetic loops" and loopsticks). They are good antennas for what they are, and the resonator inside the antenna is an integral part of their operating performance, but that discussion does not apply to dipoles, monopoles, larger loops, etc.

So, resonance is not a characteristic of good antennas. And that is true even if you adopt that "resonant" means zero-reactance. For example, 3/8 wavelengths vertical monopole is an antenna that has a messy complex impedance at the feedpoint. However, those antennas work very well as long as the feedpoint impedance is matched.

A good antenna must radiate efficiently, and to the direction that matters for the desired communication (very low takeoff angles for HF high bands and above, could be high takeoff angles for HF low bands if NVIS is desired). In other words, radiation efficiency and the radiation pattern are the key factors. "Resonance" by any definition is a factor in those.