Anti-resonance!! is it ever useful?! when?!!

Hello!
I would be grateful if you could tell me what is really anti-resonance what happens or what causes this phenomena ? when it is useful and when it is harmful to the system's performance?!
Thank you !
Kind regards!

There are times when resonance is not wanted. Example, a switched-coil converter often puts an inductor near a capacitor. Values need to be chosen so the LC resonant frequency does not interfere with proper operation. I'm not sure if this is the same as anti-resonance as addressed in your question.

Audio feedback is a nuisance. (Resonant frequency between mic and speaker-box). We try all sorts of things to stop it. I once ruined a pair of speakers unintentionally by plugging in patch cables improperly at the monitor in/out jacks. It created a feedback loop, at a very high frequency. I heard a brief piercing screech, then quiet. The voice coils were burned. I'm sure they could have been saved by some kind of anti-resonance in the loop.

I would view anti-resonance as the opposite of resonance. In resonance, your energy builds up, at a single frequency, bigger and bigger.

Anti resonance would to me be something that sucks in energy at that single frequency, dampening it. Such as in a stop-band filter. Or a mechanical vibration isolation mount.

In a multi-band antenna you often put in a 'trap' (anti-resonance components) to effectively terminate the wire at the correct point for the higher frequency, yet let the lower frequency signal see the full length of the antenna.
Susan

Thank you very much for your replies,
one more question: how can we call an anti-resonance as a constructive / or deconstructive for the antenna radiation ?

Not sure I understand your question.
Any circuit (electrical, mechanical, antenna etc.) has a natural frequency at which is "vibrates" - think of a bell; when you strike it, it resonates at a given frequency. If you stimulate a circuit at it s resonant frequency, then the "vibrations" will be reinforced. If you stimulate it at a non-resonant frequency, then you will interfere with the natural resonance and the "vibrations" will be dampened, and possibly stopped all together.
Susan

I assume you're talking about this type of anti-resonance, which arises from coupled resonators forming two new peaks with an "anti-resonant" notch.

To be frank, I don't like the term "anti-resonant." You can take a resonant LC bandpass filter, then flip a couple of its components and get a bandstop filter, but you haven't created some new distinct phenomenon, it's still just resonance. Similarly, by measuring voltages and currents at different points in any system, you can observe peaks changing into notches all over the place, despite not actually changing the system at all.

I doubt that "anti-resonance" can be considered a commonly understood technical term in RF engineering. It has a specific meaning in antenna theory where it designates the maxima of real antenna impedance which are typically not utilized in practical operation because they are difficult to match, see e.g. Ballanis Antenna Theory. As discussed in this thread, I see the term also more unspecifically used to describe band stop filters and devices with similar function. But I agree with mtwieg that it doesn't actually explain much.

If you are missing clear terms to grasp a problem, a word doesn't help.