This is a discussion on New J-10 Thread III within the Air Force forums, part of the China Defense & Military category; Originally Posted by crobato What happens when cancellation results? The Law of Thermodynamics states that energy cannot be created nor ...
I am having reservation with this particular explanation. From my interpetation, your explanation would imply that there would only be one point of destructive interference: EM wave goes in an interferencing point, becomes heat, and nothing would come out to create further interference, which would violate observation.
The point is, that in this radar example where there is destructive interference, constructive interference also takes place. Energy wise, this becomes a "zero sum game". Hence no violation of conservation of energy or change in temperature.
Constructive and destructive interference happens separately. When destructive interference happens, you get a null signal. It by itself does not reinforce constructive interference elsewhere.
Lets say you get four waves. Wave 1 and 2 are constructive, 3 and 4 are destructive. 1 and 2 therefore creates Beam A.
The energy and amplitude of Beam A is equal to the energy of Wave 1 and 2, but not with 3 and 4, so that Beam A = Wave 1 + Wave 2 + Wave 3 + Wave 4. Its just Beam A = Wave 1 + Wave 2.
Interference of Waves
The principle of linear superposition - when two or more waves come together, the result is the sum of the individual waves.
The principle of linear superposition applies to any number of waves, but to simplify matters just consider what happens when two waves come together. For example, this could be sound reaching you simultaneously from two different sources, or two pulses traveling towards each other along a string. When the waves come together, what happens? The result is that the waves are superimposed: they add together, with the amplitude at any point being the addition of the amplitudes of the individual waves at that point.
Although the waves interfere with each other when they meet, they continue traveling as if they had never encountered each other. When the waves move away from the point where they came together, in other words, their form and motion is the same as it was before they came together.
Constructive interference occurs whenever waves come together so that they are in phase with each other. This means that their oscillations at a given point are in the same direction, the resulting amplitude at that point being much larger than the amplitude of an individual wave. For two waves of equal amplitude interfering constructively, the resulting amplitude is twice as large as the amplitude of an individual wave. For 100 waves of the same amplitude interfering constructively, the resulting amplitude is 100 times larger than the amplitude of an individual wave. Constructive interference, then, can produce a significant increase in amplitude.
Destructive interference occurs when waves come together in such a way that they completely cancel each other out. When two waves interfere destructively, they must have the same amplitude in opposite directions. When there are more than two waves interfering the situation is a little more complicated; the net result, though, is that they all combine in some way to produce zero amplitude. In general, whenever a number of waves come together the interference will not be completely constructive or completely destructive, but somewhere in between. It usually requires just the right conditions to get interference that is completely constructive or completely destructive.
Now to understand how a phased array works, lets say you got two waves. These waves are coming out from separate elements but they superimpose each other.
To get constructive and destructive interference in a controlled manner, you adjust the latency of each wave as they emit. Some go sooner and some go later. That is what you call phasing. There are algorithms that work with that to determine the phase value depending on the angle of the beam steer. The part where the waves become in phase forms the main beam, while the part where the waves becomes out of phase forms the null area, all in varying degrees so that you shape the lobe.
Sonar arrays as a note, also use the same phasing and beam steering principle mentioned.
Now if you understand this, you will understand a bit of ECW too.
Now once this idea sinks, you will also understand why phase arrays need more electrical energy than mechanical arrays, resulting in greater power sources. Null signal is just wasted energy. However, its partly offset with the lack of any mechanical energy involved in scanning.
Now you get better RF reception gain in theory over a mechanical antenna because you don't have a receiver plate in front of the receiver. True on AESA but not on PESA. However, temperature variances does affect the gain on the module due to electrical resistance brought upon by heat. Hot chip -> bad reception. Thus you need to keep it cool. Hot chips also lead to chips to breaking down.
Nothing beats the good old parabolic antenna though for RF gain, that's why you use them in astronomy. Its the sheer act of the antenna that is able to gather all the RF and focus them into a single point.
To be honest, I'm not completely sure what happens if two radar or EMF waves (including infrared, light, UV, gamma) cancel each other out. We need to deal with quantum mechanics here. I do know that if sound waves cancel each other out, they produce heat so I may got something mixed up.
Last edited by crobato; 03-25-2009 at 08:56 PM.
"Lets do a thermal sweep."
However, when it comes to EM waves, the cancellation does not create heat, since there is no energy transfer. Suppose you are right, if two beams of radio waves interfere with each other in vacuum, where does the heat go?
The cancellation effect of destructive interference of radio waves are only in the eye of the observer (radar receiver) at the interference point. Each individual beam does not get weaker or stronger. A good experiment is to shine two beams of light using flash lights. Have their beam paths cross each other. You won't observe a difference in the spotlight they each create on a reflective surface.
Two beams of flashlight don't make the experiment. White light consists of a myriad of spectral frequencies all mixed together to create the appearance of white. All these frequencies are being emitted in all sorts of random phases. Cancellation is impossible. For light to cancel, you would need to use lasers.
Here is an example of constructive and destructive interference on light, and its applications.
How interferometers work: a simple introduction to interferometry
"Lets do a thermal sweep."
Last edited by crobato; 03-26-2009 at 09:25 PM.
"Lets do a thermal sweep."
are you refering to radar cancellation technique?or RAM material/
I recalled a back issue of Miltech,reported that some RAM material and plasma convert RF signal into heat.
Radar cancellation technique remain highly confidential topic.during the bombing of serbia air defence,NBC news claim B-2 bomber carry new ECM technique to cancel out RF.it is possible that Israel F-16 may also use same technology to bomb syrian nuclear reactor near the turkish border.
You can also find radio wave cancellation even in microwave ovens. Its not hard to explain how. In fact I already explained how radar cancellation works with the explanations above. The trick is to get the second wave be a clone of the first wave and time it to be completely out of phase. Meaning what is the peak or the crest of the first wave is the complete opposite on the second wave, being the trough or valley. The concept with structural RAM is that the second wave is not an emission but a reflection; you want to make a second reflection out of phase with the first reflection. If its an emission the topic moves to ECW.
Many RAM coatings are ferric, which is fancy word for iron fillings. If radio waves strike a magnetic material, they are absorbed and converted to heat. That's why effective antennas have to made with an iron material. If you notice why slotted arrays have this orange color, that's the color of iron oxide, or rust.
"Lets do a thermal sweep."
Don't want to really drag on this discussion but, actually, using the words "cancellation" or "destructive" can be confusing since the energy states of the individual wave components remain the same during the interference whether it be destructive or constructive. This shouldn't be confused with the dissipation of EM wave energy as it interact with a medium, like for example the atmosphere or even a solid object.
No energy is actually lost during an EM waves' destructive interference as the same wave components would, in another instant of time, be involved in a constructive interference.
I think I have enough of this discussion even if I can't get the point across.
Last edited by Quickie; 03-27-2009 at 01:40 PM. Reason: addition
according to a chinese web site, adapting DSI inlet reduced the overall weight by 300 kg.
Although F-16 first tested flight with DSI inlet way back in 1995,is there any reason why it never adapted by USAF?