Scan angle of an antenna array

[generalcustard]

Hey guys, just a brief(ish) question regarding the scan angle of an antenna array ... as far as I understand it, the scan angle is the angle with respect to the axis of the array where you get maximum gain, i.e. the direction in which the main lobe of the pattern is directed. Is this correct?

Also, I realise that to prevent grating lobes appearing in the radiation pattern you should have an element spacing of <lamda/2, but if it isn't possible to keep the element spacing this low I was trying to determine what the effect would be on the scan angle. I think the following equation relates element spacing and scan angle:

element spacing < lamda/(sin(theta)+1)

, where the array is required to scan over a scan volume of plus/minus theta.

Say the element spacing is equal to 0.7*lamda, this gives a value of 25.4 degrees for theta; does this mean that the array will only be able to scan to an angle of plus or minus 25.4 degrees from it's axis and therefore not permit broadside radiation? I think my understanding of this may be a bit flawed, would anyone be able to explain this further maybe?

 

[VEBill]

Broadside radiation is ALWAYS possible by simply feeding the array elements in phase.

 

[Higgler]

when you scan that far, another beam pops up from imaginary space from the opposite direction of your scan. i.e. scan to +25 degrees and at -90 degrees, a second beam starts to be formed. So your original beam starts to drop in amplitude and it looks like a far away sidelobe starts to grow. Active VSWR suckouts start to be a serious concern too, you can get some energy trapped on the surface and burn up an array. That's why giant high power arrays have fire suppression systems.

Your max. angle scan can be increased by having elements whose individual patterns rolloff in one plane. Like a V pole wave guide element scanned in hpole plane, the pattern of the element rolls off alot in the hplane and if you overscan scan that direction, you won't lose too much gain until the individual waveguide element's pattern increases.

check out Pozars books, lots of good info there. Mostly about patch antenna arrays.

k

 

[generalcustard]

Thanks for your help guys (or girls of course!).