Explaining Half-Wavelength Resonance

[Martian2]

This is my first attempt at explaining half-wavelength resonance to detect a stealth fighter.

An aircraft is comprised of many pieces of metal with different lengths. The fuselage is the longest metallic piece. The fuselage is basically an antenna. The other pieces of metal inside the aircraft are also antennas.

Now, all solid objects have a sonic resonance frequency. Similarly, all metallic objects have an electromagnetic resonance frequency.

The fuselage has an electromagnetic resonant frequency. The ailerons, elevators, vertical stabilizers (probably metallic ribs), metallic wing ribs, metallic landing gear, etc. have their own resonant frequency.

With modern amplification and digital processing software (to exclude background noise), the detector is able to isolate the signal return from the aircraft due to half-wave and quarter-wave resonance of those metallic pieces.

There is no known way to stop a resonant frequency. It is a physical property. When the electromagnetic wave hits the piece of metal, it creates a standing wave. The electrons wiggle at the resonant frequency. This turns into a transmitter like an antenna. This is exactly how an antenna works. You pump a signal into an antenna. In the past, you couldn't distinguish the signal coming off resonance because it was near the noise level. Now, you can.

The only method is to stop the electromagnetic wave from reaching the piece of metal. The stealth coating is designed to interfere with the electromagnetic wave from reaching the piece of metal. However, the stealth coating only works at one (or maybe two) frequency band.

 

[gambit]

There is no known way to stop a resonant frequency.

Not true.

There is ALREADY a built-in mechanism that will reduce the resonance behavior itself: Damping.

Damping is cycle-to-cycle loss.

Damping - Wikipedia, the free encyclopedia

The damping of a system can be described as being one of the following:

• Overdamped: The system returns (exponentially decays) to equilibrium without oscillating.
• Critically damped: The system returns to equilibrium as quickly as possible without oscillating.
• Critically overdamped: The system returns to equilibrium instantaneously.
• Underdamped: The system oscillates (at reduced frequency compared to the undamped case) with the amplitude gradually decreasing to zero.
• Undamped: The system oscillates at its natural resonant frequency (ωo).

For example, consider a door that uses a spring to close the door once open. This can lead to any of the above types of damping depending on the strength of the damping. If the door is undamped it will swing back and forth forever at a particular resonant frequency. If it is underdamped it will swing back and forth with decreasing size of the swing until it comes to a stop. If it is critically damped then it will return to closed as quickly as possible without oscillating. Finally, if it is overdamped it will return to closed without oscillating but more slowly depending on how overdamped it is. Different levels of damping are desired for different types of systems.

Is it technically possible to overdamp as complex a body as an aircraft ? Absolutely. Technical feasibility is a different beast. Electrical damping circuits are common in electronics engineering, so as far as 'stealth' goes, active cancellation at the material level will accomplish most, if not all, effects of electrical damping of the oscillation of the body.

Another item that was (conveniently) omitted is that under radar bombardment, resonance level depends on physical aspects to the seeking radar, meaning a rod will resonate less from the tip perspective than from the length perspective.

IET Digital Library: On the application of pattern recognition to identification of simple targets based on resonance and polarization diversity

In this paper, target radar features based on extracting the polarization matrix of each of the radar target's complex natural resonances (CNRs) in the time domain is investigated. These resonance frequencies and their complex residues (amplitudes) in a co- and cross-polar configuration form a polarization matrix decomposition of the target in late time and are also related to the principal dimensions of the target and their relative physical orientation. We have developed and investigated new radar target features, whereby, for incident circular polarization we generate horizontal and vertical complex residue patterns for each known target at the first few dominant resonance frequencies over a number of aspect angles.

The B-2's avionics can calculate the best possible flight profile to present the least radar reflectivity, which includes the possibility of resonance, to any seeking radar.

 

[Shotgunner51]

This is my first attempt at explaining half-wavelength resonance to detect a stealth fighter.

An aircraft is comprised of many pieces of metal with different lengths. The fuselage is the longest metallic piece. The fuselage is basically an antenna. The other pieces of metal inside the aircraft are also antennas.

Now, all solid objects have a sonic resonance frequency. Similarly, all metallic objects have an electromagnetic resonance frequency.

The fuselage has an electromagnetic resonant frequency. The ailerons, elevators, vertical stabilizers (probably metallic ribs), metallic wing ribs, metallic landing gear, etc. have their own resonant frequency.

With modern amplification and digital processing software (to exclude background noise), the detector is able to isolate the signal return from the aircraft due to half-wave and quarter-wave resonance of those metallic pieces.

There is no known way to stop a resonant frequency. It is a physical property. When the electromagnetic wave hits the piece of metal, it creates a standing wave. The electrons wiggle at the resonant frequency. This turns into a transmitter like an antenna. This is exactly how an antenna works. You pump a signal into an antenna. In the past, you couldn't distinguish the signal coming off resonance because it was near the noise level. Now, you can.

The only method is to stop the electromagnetic wave from reaching the piece of metal. The stealth coating is designed to interfere with the electromagnetic wave from reaching the piece of metal. However, the stealth coating only works at one (or maybe two) frequency band.

 

[Martian2]

Not true.

There is ALREADY a built-in mechanism that will reduce the resonance behavior itself: Damping.

Damping is cycle-to-cycle loss.

Damping - Wikipedia, the free encyclopedia

Is it technically possible to overdamp as complex a body as an aircraft ? Absolutely. Technical feasibility is a different beast. Electrical damping circuits are common in electronics engineering, so as far as 'stealth' goes, active cancellation at the material level will accomplish most, if not all, effects of electrical damping of the oscillation of the body.

Another item that was (conveniently) omitted is that under radar bombardment, resonance level depends on physical aspects to the seeking radar, meaning a rod will resonate less from the tip perspective than from the length perspective.

IET Digital Library: On the application of pattern recognition to identification of simple targets based on resonance and polarization diversity

The B-2's avionics can calculate the best possible flight profile to present the least radar reflectivity, which includes the possibility of resonance, to any seeking radar.

I was going to cover broadband stealth and the differences between the F-22 and B-2. I will discuss damping in that post.

My original post had the F-22 in mind. It is the primary threat. There are only 20 subsonic B-2s and they have their own problems in detection.

Anyway, my thread topics are covered in multiple posts. I start with the general rule and slowly work my way through the exceptions.

To give a preview, the thousands of F-35s that are expected to be built are in the same situation as the F-22. Damping isn't really an option due to the weight and lack of space requirements.

I will write five more posts to cover:

1. Three more things about resonance
2. Other methods of detecting stealth fighters: Quantum Well Imaging Photodetectors (QWIP) and Schilieren Optics
3. Explaining broadband stealth and the differences between supersonic F-22 and subsonic B-2 flying-wing. Damping isn't worth the trouble.
4. Detecting B-2 with new drone technology and using satellites to search the fly-path. China's six multi-layered defense against stealth aircraft.
5. The latest in what China and the US have to say about stealth

Since this is a hobby, I will write these posts over time. If I try to write them all at the same time, it feels like work.