What is the function of the long stub?

[tony_lth]

Hi, Gurus,
Attached picture is the 77G mmwave radar antenna.
For the red marked, there is very long stubs for one of the TX antenna.
I wonder what is the function of the very long stub? And what rule is for the so long stub in the antenna design?
Sincerely,
Tony Liu

 

[vfone]

Amplitude tapering in binomial array is used to get a low Side Lobe Level (SLL).
This is done by varying the effective width of the radiating antenna elements.
The long stub that you mentioned, actually is the last antenna element, which have a very narrow width.

 

[volker@muehlhaus]

I think he refers to the two long lines that turn away 90 degree, marked with the red symbols

 

[tony_lth]

yes, volker is right.
Why need so long (marked red) stub? That is about 2 or 3 lamda length.

 

[vfone]

I missed them. Those lines are even more than 3 lamda, which doesn't make any sense.
The two series elements rows where they are connected looks that are dummy elements, so the entire structure could be the same thing.

 

[tony_lth]

That is a BOSH mmwave radar, which has 3TX and 4 RX channels.
The right most 10 stubs antenna is the far field TX anetnna.
Then the antenna which has very long stubs should be the near field TX antenna.
The 3rd TX is the ref back to RX.
The left is the 4 RX antennas.
I noticed the height of aforesaid antenna, which has very long stubs, is different from other antennas.
Do the very long stubs have the function of elevation capability, i.e. 4D radar?
The very long stubs are very near to the ground GND, so it should be a small caps to adjust the electrical length?
Best,

 

[vfone]

The long stubs could be used for low range, wide angle, radar detection.