# Phased Antenna Array Directivity

Preface:

I have done multiple searches trying to determine the directivity of a phased array antenna. I do realize that a single number for directivity is not applicable to all phased arrays since you can have different scan angles and use an arbitrary amount of elements in an array. In light of this, I won't ask for an empirical, single, numerical answer, but rather I'd like to determine a general intuition for phased array antenna directivity .

Question:

Assume one has chosen correct phasing weights for each element in a phased antenna array to achieve maximum directivity at some angular coordinate $$(\theta,\phi)$$ Also, the total radiated power of each element is given by $$P/n$$ where

P: Sum of power radiated by all elements and

n: number of elements

Is said array's directivity better or worse, at that same angular coordinate, than using a single element with $$P$$ as the total radiated power? $$\$$

NOTE(1): If it is simpler, assume each element is a perfect isotropic radiator.

A different question that I could ask which would also satisfy my post is the following:

Do phased array antennas offer the advantage of creating a very selective, narrow, main lob that can be steered alone or do they also allow for a greater directivity compared to an isotropic radiator with equal total radiated power?

## 1 Answer

Generally, the directivity of an antenna or array does not depend on $P$ or $P/n$.

The main contributor to directivity is $L$, the diameter of the array. The advantage of a phased array is that the array diameter can often be much larger than the diameter of any individual element. For example, one element can be in Hawaii and the other one in the Virgin Islands, giving an array diameter of 8000 km for the Very Long Baseline Array, with angular resolution around 10 micro-arcseconds.

• That's angular resolution, not directivity. I wonder how he's defining directivity? – Neil_UK Oct 7 '16 at 16:06
• @Neil_UK, according to Wikipedia, directivity "measures the power density the antenna radiates in the direction of its strongest emission, versus the power density radiated by an ideal isotropic radiator (which emits uniformly in all directions) radiating the same total power." As such, it's intimately related to the angular resolution of an antenna used as a receiver. – The Photon Oct 7 '16 at 16:07
• Oh, OK, I'd been calling that the antenna gain for the duration of my engineering life. – Neil_UK Oct 7 '16 at 16:09
• @Neil_UK, yeah, they're closely related. The next sentence in the wiki article is, "An antenna's directivity is a component of its gain; the other component is its (electrical) efficiency." – The Photon Oct 7 '16 at 16:11
• I included information about the total radiated power of each element in both the array and single-element case to compare two antennas with equal "total radiated power" i.e. (P/n)*n = P and P*1 = P. – ElecEng2016 Oct 7 '16 at 16:30