J-10 Thread III (Closed to posting)

[crobato]

Its possible the KJ-2000 is an AESA, but I like to be convinced with some reasonable evidence that it is. The KJ-200 and the Type 346 radar has reasonable evidence to be so. For instance the Type 346 radar, when it was on the 891, was surrounded by all sorts of refrigeration pipes. Only an AESA produces that much heat to warrant such an arrangement. On the KJ-200, if you notice the balance beam, it has a large air intake in the upper front, and an opening to let the air out in the lower rear. Its a pattern clearly intended for air circulation. This suggests a radar that is temperature sensitive and hot, and for me, reasonable basis to be an AESA. As for the SLC-2, it said so in the NRIET web page long time ago, calling it active phase array, about the time where I saved that picture of the oval ESA.

 

[Blitzo]

Oh yep yep I see what you mean.

 

[challenge]

PESA like any conventional radar such as simple reflector antenna,slotted planar array needed wave guide, a large transmitter and complex receiver.
RF wave has to travel thru wave guide and finally antenna,(Both transmit and receive)signal lost a total 15- DB ,this translated into 70% reduction in range.
for AESA it is complete opposite,there's no RF lost due to the fact there's no wave guide,no large transmitter,result smaller antenna,longer range.

 

[crobato]

Actually RF lost is the highest for PESA and AESA. It has nothing to do with wave guides. It has to do with interference and heat.

In order to bend the beam, you need to create a constructive interference on one side, destructive interference on the other. This interference is created by the interaction of the waves themselves by slightly altering the timing of the emission. This changes the 'phase'. A constructive interference happens when two waves act to reinforce each other, when they're on phase, and a destructive interference happens when the waves cross each other, when they are out of phase. This is cancellation.

What happens when cancellation results? The Law of Thermodynamics states that energy cannot be created nor destroyed. The RF energy of canceled waves becomes heat. That's why things get hot.

It gets hotter when you understand the concept of _semiconductors_. Unlike pure conductors, electricity passing through a semiconductor which makes up these elements, produces heat due to the electrical resistance of the material. So you have two heat sources here.

RF gain and minimal loss is the highest when the beam is on center. But the further to the side the beam scans, the more interference is needed, more heat is created, more RF loss happens. That's the fundamental weakness of ESA designs. The best way to approach this is to put the ESA with minimal mechanical assist, in order to reduce the need for scanning wide.

Now as the radar gets hot, heat increases electrical resistance. The result is signal loss. That's why you need to keep the array at a certain temperature.

 

[challenge]

actually,what I know about PESA and AESA is based on past issue of Military technology magazine.

 

[Engineer]

What happens when cancellation results? The Law of Thermodynamics states that energy cannot be created nor destroyed. The RF energy of canceled waves becomes heat. That's why things get hot.

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.

 

[Quickie]

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.

Agree. Only in the infra-red region (and thereabout) does EM wave generate the kind of heat that we normally deal with. And, to make things clear, that only happens when the waves interact with another medium other than themselves. The waves' construction or destruction pattern does not by itself generate heat.

 

[Scratch]

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.

 

[crobato]

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.

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Linear superposition

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

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

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.


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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.

 

[oringo]

Constructive and destructive interference happens separately. When destructive interference happens, you get a null signal. It by itself does not reinforce constructive interference elsewhere.
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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.

You are right about constructive and destructive interference. That rule works for waves like sound waves and mechanical vibrations.

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.