Active Return Loss

The performance of an individual antenna element can change drastically if it is placed into an array. Since an antenna radiates into freespace, it's impedance is dependent on its surroundings. The radiated power, separation and orientation of surrounding elements all contribute to this.

Quite often, the Antenna Array design engineer will optimize these parameters to improve the return loss when compared to a single element. As a result, antenna arrays can operate over a broader frequency range then single antennas.

Definitions:

Simply stated, Return loss is the ratio of power received at a port relative to the power delivered from that port.

Mutual coupling is when RF power from one element leaks into another element. In the simple case of two elements radiating power with equal amplitude and phase, this can be easily measured with a 2-port Network Analyzer. Below, the mutual coupling between elements 1 and 2 (M12) is measured as S12 on a Network Analyzer.

Passive Return Loss is the return loss seen at an antenna element when all surrounding elements are terminated into RF loads. In the above example, the Passive Return Loss at antenna #1 (S11,P) is measured as S11 on the Network Analyzer only when Antenna #2 is not radiating (terminated into RF load).

• Network Analyzer port #2 should be disconnected for this measurement.

Active Return Loss (or Active impedance) is the return loss seen at an antenna element when surrounding elements are also radiating power.

• It will change depending on the magnitude and phase radiated from each element.
• (Magnitude and phase are often controlled to steer the direction of radiation of the entire array.)

In the diagram above, lets assume that both elements are identical and radiating the same power. We can measure the Active Return Loss at Antenna #1 (S11,A) by simply measuring the S11 on the Network Analyzer. This result should be different from a Passive Return Loss measurement since BOTH elements are now radiating; the mutual coupling between the two elements will change the power delivered back into antenna #1. In fact,

S11,A = S11,P + M12.    (components in Real, Imaginary form. NOT dB!)

So, now what if we have more than two elements in the array?

The Active Return Loss at Antenna #1 is the sum of its passive return loss, plus the mutual coupling between Antenna #1 and all other elements in the array:

S11,A = S11,P + M12 + M13 + ... M1N ,

where N is the number of elements in the array, and the other elements are terminated into RF loads during the M1N measurements.

A note on Measurement setup: 

• This test should take place in an anechoic chamber to reduce background noise and adversely affect the results.

• Ensure all other elements are terminated when measuring Passive Return Loss and Mutual Coupling.
• For arrays with many elements, measuring the mutual coupling between every element can be burdensome.
• Furthermore, elements far from the element-of-interest will contribute negligible power to the Active Return Loss.
• It is advisable to only include M's that contribute non-negligibly to the Active Return Loss.
• 10 dB is typically a good cut-off point, but it depends on the Passive Return Loss.

Finally, Active Return Loss will change with scan angle of the array. That is- it will change for varying magnitude and phase delivered by the elements. This can be simulated using our homogeneously-radiated measurements as a baseline. I will write another post on this in the future. Or you can contact me if you are dying to know how.