How can I explain the reason for the asymmetric radiation pattern (vivaldi)?

imtiaz369

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I've attached a photo of two models. Model 1 does not have a slot, whereas model 2 does. If you look at both of these models' radiation patterns, after adding a slot, the radiation pattern in the xz plane or phi=90 appears asymmetric! There is no form of asymmetric behavior without a slot! Also, I've attached a surface current distribution; I understand this is due to the slot, but how can I explain it properly?


 

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For antenna 1, the open end of the excitation line is above solid ground, so currents in the bottom metal are mostly symmetric.

If we look at the composite picture with both layers of antenna 2, we can see that one bottom cutout is near the end of the excitation line, so that currents are distorted there. This is much less symmetric than antenna 1.


 

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Slots in Vivaldi antenna are used to improve gain, VSWR, and efficiency over frequency range.
I think the placement of the bottom slot (model 2) is wrong. Is to close to the excitation line.
Also, if you check for similar antenna designs, you will see that the slots (which are more than two in most designs) don't have equal dimensions (most designs), but progressive dimensions. This will be important to improve antenna performance vs frequency.
 

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A few more question:

1. I'm trying to see the effects of these slots near the feed line. So, how do you define the current distortion there? Is it due to a different direction?

Or, in theory, why is this happening if there is an open spot near the excitation line?



2. For a more accurate understanding, I have simulated two other models in the attachment.


--- Updated ---

Can I define asymmetry behavior using the concept you mentioned?
 

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imtiaz369 said:
I'm trying to see the effects of these slots near the feed line. So, how do you define the current distortion there? Is it due to a different direction?
Click to expand...
If you look at return currents in the original model, you will see where the ground plane current flows away from the open end. In the slotted version, it can't take that path because of the slots, and interferes with the current flowing on the slot boundaries. I think this is where it gets rather asymmetric.

 

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