What is antenna null?

Vanessa1402

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Hello everyone, I heard this forum has many knowledgeable users so can someone explain to me:
What is antenna null?. What are the different between antenna sidelobes and antenna null?.
In case of AESA and PESA radar, why radar doesn't listen to signal comming from null direction?. How can the radar distinguish ?.
 
No bumping. See rules.

Every antenna has a radiation pattern. They don't perfectly transmit in one desired direction. It transmits strongly in the main direction, and then less strongly in the sidelobe directions. In the areas between main lobe and side lobe and between separate sidelobes, transmission is close to zero. This is an antenna null. You fix this by scanning the antenna to place the main lobe on the target. Exactly the same applies to receiving. You rotate the antenna so the main lobe direction is pointing where the target is. You can't point the antenna at an angle where the null faces the target and expect to receive a signal.
 
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No bumping. See rules.

Every antenna has a radiation pattern. They don't perfectly transmit in one desired direction. It transmits strongly in the main direction, and then less strongly in the sidelobe directions. In the areas between main lobe and side lobe and between separate sidelobes, transmission is close to zero. This is an antenna null. You fix this by scanning the antenna to place the main lobe on the target. Exactly the same applies to receiving. You rotate the antenna so the main lobe direction is pointing where the target is. You can't point the antenna at an angle where the null faces the target and expect to receive a signal.
If the antenna is a parabolic antenna then I understand your point. There are some direction where signal can't enter the antenna. I don't understand how antenna null work on an electronic scanned array such as PESA and AESA?
Their aperture is at a fixed position and they steer their beam by wave interference. Radars have receive and transmist phase. In the transmit phase, I get how they form the null. But how the antenna null work in the receive phase of AESA and PESA?.
 
*I am not a radar expert. This is from my understanding.*

The electronically steered antenna still forms a beam which is steered and hss a main lobe, sidelobes and nulls. You could imagine it as steering a virtual antenna. Early phased arrays often had larger sidelobes than a well designed planar array which could limit maximum range or require additional sidelobe suppression techniques.

The receiver works analogously to the transmitter. A phased array receiver compensates for the time delay difference between the signals from different antennas and combines the signals coherently to enhance the reception from the desired direction(s), while rejecting emissions from other directions.

WIth a phased array receiver you can create virtual "nulls" (by signal processing) in desired directions to counter for jamming for example where your antenna just won't recieve from, making the jammer useless.

So there is no place where there is a permanent "null" in the phased array design, but at any given moment the radar is forming a beam with main lobes, side lobes and nulls just like a parabolic antenna. Unlike a parabolic antenna these nulls aren't in fixed places but vary according to how you process and combine the signals from multiple elements.
 
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*I am not a radar expert. This is from my understanding.*

The electronically steered antenna still forms a beam which is steered and hss a main lobe, sidelobes and nulls. You could imagine it as steering a virtual antenna. Early phased arrays often had larger sidelobes than a well designed planar array which could limit maximum range or require additional sidelobe suppression techniques.
I understand this

The receiver works analogously to the transmitter. A phased array receiver compensates for the time delay difference between the signals from different antennas and combines the signals coherently to enhance the reception from the desired direction(s), while rejecting emissions from other directions.

WIth a phased array receiver you can create virtual "nulls" (by signal processing) in desired directions to counter for jamming for example where your antenna just won't recieve from, making the jammer useless.
I don't understand this part. How can the electronic scanned radar know which exact direction the signal come from to reject signal from the null?.
If the radar can know which direction the signal comming from to reject then why don't all side lobes become null on PESA and AESA?.
 
So think about this -- every element in a phased array is itself a small antenna. So each element has nulls due to the geometry of the antenna and the interference patterns between that antenna element and the ones around it. As a result, phased arrays also have nulls resulting from the overlapping patterns of the many array elements.

However, phase arrays can do "virtual" null steering, changing how each element feeds into the overall array and effectively moving the combined null of the whole array.
 
However, phase arrays can do "virtual" null steering, changing how each element feeds into the overall array and effectively moving the combined null of the whole array.
Can you please explain this deeper?
What stop the array from nulling everything except the main lobe in receive phase?.
 
However, phase arrays can do "virtual" null steering, changing how each element feeds into the overall array and effectively moving the combined null of the whole array.
Can you please explain this deeper?
What stop the array from nulling everything except the main lobe in receive phase?.

Anything you do to move the nulls in phased array also affects the size/shape of the mainlobe, so it's all about tradeoffs. You can suppress sidelobes to an extent (quite a lot, with phased arrays). But you can't eliminate them entirely because that's just a how RF waves work.

Beyond that is unfortunately outside my understanding. The physics behind wavefront interference and antenna design involves very heavy math that I can't handle anymore.
 

Anything you do to move the nulls in phased array also affects the size/shape of the mainlobe, so it's all about tradeoffs. You can suppress sidelobes to an extent (quite a lot, with phased arrays). But you can't eliminate them entirely because that's just a how RF waves work.
In the active beamforming phase, I understand how moving the null affect the size and shape of main beam. But if signal in receive phase is rejected by software based on their direction then how can that affect the main beam shape?. Logically, you can remove signal from all direction except where the main lobe point at?
 
The thing is, the antenna doesn't necessarily "know" the precise direction the signal came from.
 
The thing is, the antenna doesn't necessarily "know" the precise direction the signal came from.

If you accept the outbound case, the inbound case is the same. You seem to think the receiver can just suck in all signals from all directions arriving in all antenna elements and figure out where they came from simultaneously, but a BF (beam forming) phased array radar has to do the same signal processing maths in reverse on the incoming signal to "receive" from the currently scanned direction (potentially removing mathematically other signals from non-scanned directions recieved at that time). The phased array scans a lot quicker than a physical antenna but you are still looking for a signal in a specific direction at a specific time.

In a passive DF (direction finding) setup like you find on an RWR you can use time-of-arrival processing to predict the angle from which the signal has arrived instead.

Now its possible to do both these functions with the same hardware to some extent. APG-77 has direction-finding passive modes.
 

I don't understand this part. How can the electronic scanned radar know which exact direction the signal come from to reject signal from the null?.
If the radar can know which direction the signal comming from to reject then why don't all side lobes become null on PESA and AESA?.


You do this by 2 way. First is to use external antenna. to cover the necessary angular sector and then compare the incoming reflection or signal to where the mainlobe of the antenna is actually pointing. This way one would now the originating point of the jammer and therefore adjust the antenna pattern to put some nulls.

This is example of the sidelobe/mainlobe canceller. This one is in AN/APG63V3 or APG-82. Notice the small group of 3 horns below the main array. That is the sidelobe/mainlobe canceller antenna.


EpIMxZIUcAI7LZa.jpg

Another example is in this EL/M-2052 antenna. Notice the 4 Vivaldi antenna at top of the array

ELM-2052.png

If anyone ever ask what is the differences between Sidelobe and Mainlobe canceller ? They are the same except the external antenna. Sidelobe canceller can have Low gain antenna as the sidelobe gain is weaker while the mainlobe canceller or notcher have higher gain antenna to cover for mainlobe's stronger gain.

Another way is to sacrifice or set some elements in Receive mode to "dedicate" them to sidelobe/mainlobe cancellation group. That is when one concerned about Size and weight + power requirement for the additional receiver antenna outside the main array.

This AN/APG-83 doesnt have dedicated sidelobe canceller.


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Nulls or antenna Nulls here is simply the "valley" of its radiation pattern while lobes are the "peak"

Nulls.png
Nulls and peak like that are of course generated by natural interference happening in any antenna or any object exposed to or emitting electromagnetic radiation.
 

I don't understand this part. How can the electronic scanned radar know which exact direction the signal come from to reject signal from the null?.
If the radar can know which direction the signal comming from to reject then why don't all side lobes become null on PESA and AESA?.


You do this by 2 way. First is to use external antenna. to cover the necessary angular sector and then compare the incoming reflection or signal to where the mainlobe of the antenna is actually pointing. This way one would now the originating point of the jammer and therefore adjust the antenna pattern to put some nulls.

This is example of the sidelobe/mainlobe canceller. This one is in AN/APG63V3 or APG-82. Notice the small group of 3 horns below the main array. That is the sidelobe/mainlobe canceller antenna.


View attachment 655036

Another example is in this EL/M-2052 antenna. Notice the 4 Vivaldi antenna at top of the array

View attachment 655037

If anyone ever ask what is the differences between Sidelobe and Mainlobe canceller ? They are the same except the external antenna. Sidelobe canceller can have Low gain antenna as the sidelobe gain is weaker while the mainlobe canceller or notcher have higher gain antenna to cover for mainlobe's stronger gain.

Another way is to sacrifice or set some elements in Receive mode to "dedicate" them to sidelobe/mainlobe cancellation group. That is when one concerned about Size and weight + power requirement for the additional receiver antenna outside the main array.

This AN/APG-83 doesnt have dedicated sidelobe canceller.


View attachment 655038
Correct me if I'm wrong, but according to your explaination, side lobe blanker SLB and Side lobe canceller SLC use the same way to determine if jamming signal is from the side lobes and null?


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Nulls or antenna Nulls here is simply the "valley" of its radiation pattern while lobes are the "peak"

View attachment 655035
Nulls and peak like that are of course generated by natural interference happening in any antenna or any object exposed to or emitting electromagnetic radiation.
I understand this, I also fully understand in case of average parabolic radar. I just don't understand how the "null" work in the receive phase of PESA and AESA. When they transmit, the wave interference can make the null, but how that work when they receive?. I imagine the null is fixed in receive because the T/R are fixed and they don't make any wave interference when they don't transmit.
 
Correct me if I'm wrong, but according to your explaination, side lobe blanker SLB and Side lobe canceller SLC use the same way to determine if jamming signal is from the side lobes and null?
Yes they do. Blanker and canceller are the same thing.

Tho null basically means no signal or very weak signal can be received from there. So jammer wont jam null part of the antenna.

Jammer alike will always try to insert their signal to sidelobe or mainlobe.

I understand this, I also fully understand in case of average parabolic radar. I just don't understand how the "null" work in the receive phase of PESA and AESA. When they transmit, the wave interference can make the null, but how that work when they receive?. I imagine the null is fixed in receive because the T/R are fixed and they don't make any wave interference when they don't transmit.

The antenna reciprocity theory states that radiation pattern during transmit and receive will be the same. So for conventional reflector or slotted array antenna, the location of nulls and peaks are the same.

Phased arrays however have the advantage of having phase shifters inside their antenna elements so it can dynamically change the antenna radiation pattern. E.g unweighted during transmit to maximize the power aperture product while for receive it can change into something maybe like Taylor -40dB to reduce sidelobe which in turn reduces the amount of noise jamming that can get in.

Having Sidelobe canceller or mainlobe canceller or having some antenna elements in blanking duties allows the phased array radar to dynamically put nulls in the direction of the jammer, thus it wont receive the jamming signal.
 
Yes they do. Blanker and canceller are the same thing.
Just my 2 cents
SLB and SLC aren't actually the same thing, they are a bit different

Thanks.

The thing however.. They can be combined into 1 Hardware and sharing common antenna

Combinations.png

So the differences between them may no longer be obvious. It's now pretty much like differences between FLIR vs IRST... they used to be different back then... but nowadays we have IRST with TV/FLIR channel or differences between ESM and RWR.. The RWR term used to only be used for a system which only picks the mainlobe of the threat radar and use simple Crystal or pre-amplified crystal receiver. Nowadays RWR... can have multiple types of receivers in it and also do job of ESM's.
 
Yes they do. Blanker and canceller are the same thing.
Just my 2 cents
SLB and SLC aren't actually the same thing, they are a bit different

Thanks.

The thing however.. They can be combined into 1 Hardware and sharing common antenna

View attachment 655089

So the differences between them may no longer be obvious. It's now pretty much like differences between FLIR vs IRST... they used to be different back then... but nowadays we have IRST with TV/FLIR channel or differences between ESM and RWR.. The RWR term used to only be used for a system which only picks the mainlobe of the threat radar and use simple Crystal or pre-amplified crystal receiver. Nowadays RWR... can have multiple types of receivers in it and also do job of ESM's.
Two words from me
The left diagram is just simple guard channel. It used commonly in PD (not only AESA) radars to protects against natural and artificial interference.
Idea is simple. Interferences leaks to main array by side lobes, which gains is much lower than 1 Use omnidirectional antena with gain close to 1 to compare returns between main antena and aux antena. to conclude which returns comes from side lobes. It operates on "blips" (detected returns over thresholds). It compares returns in the same places of time and Doppler frequency. Rule is simple: if amplitude of "blip" in aux antena is higier than in main it is external interference that goes through sidelobe, and should be not take in account. In other case it is useful return from sth that is in mainlobe.
 
The right approach is to make some nulls in direction of interference. The idea is just to cancel interference by subtract signal of interfence from all signals registered in the main array. To do this you need to register the interference signal, and this is done by aux array. Next you need to adjust gain of the registered signal (make amplitude lower) and shift in time to adjust the same time wave elements in both paths. This time depends on physical placement both arrays and angle of arrival of interfering signal. Gain and time (phase) is somehow adjusted in iterative manner to get minimum of interference in some approach (adaptive fiter)
 
And to do this you need main and aux antena(s).
In AESA especially in digital once, you have many many individual antennas so you can shape revive path as you want with multiple nulls and main revive beans by sophisticated signal processing.
But you need to have revcive path that produce digital signal in each array element. So far it is not feasible in fighter array X band radars. But for lower freq (like UHF E-2D) it is possible. Signal from each array is converted to digital form so, having enough processing you can do whatever you want including space time adaptive processing (optimal processing in Doppler - time "axis" and in "space" what corresponds to beam forming)
 
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Yes they do. Blanker and canceller are the same thing.
Just my 2 cents
SLB and SLC aren't actually the same thing, they are a bit different

Thanks.

The thing however.. They can be combined into 1 Hardware and sharing common antenna

View attachment 655089

So the differences between them may no longer be obvious. It's now pretty much like differences between FLIR vs IRST... they used to be different back then... but nowadays we have IRST with TV/FLIR channel or differences between ESM and RWR.. The RWR term used to only be used for a system which only picks the mainlobe of the threat radar and use simple Crystal or pre-amplified crystal receiver. Nowadays RWR... can have multiple types of receivers in it and also do job of ESM's.
From little of what I can understand according to these post above side lobes canceller are very much superior to side lobes blanker?. Why is there a need to combine the two together?
 
Is there any way for jammer to counter sidelobes blanker and sidelobes canceller?. If there isn't any does that make standoff jammer useless in most case?
 
Is there any way for jammer to counter sidelobes blanker and sidelobes canceller?. If there isn't any does that make standoff jammer useless in most case?
stand off jammer can stay in main lobe too
 
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