Hi Shahar,
It is my understanding that you are able to plot return loss in dB for dipole antenna. However, you are uncertain about sign of the values. The magnitude of s-parameter S11 for passive device like antenna is expected to be a negative number in dB. However, return loss |Tin| as defined below is always a positive number for passive device.
RL = −20 log |Tin|
For a matched load (|Tin| = 0) has a return loss of ∞ dB (no reflected power), while a total reflection (|Tin| = 1) has a return loss of 0 dB (all incident power is reflected). Note that return loss is a non-negative number for reflection from a passive network or device.
Example :
If |Tin| = 0.452
return loss is RL = −20 log |Tin|=−20 log (0.452) = 6.9 dB
Please, refer to the return loss documentation for more information. From above link, its seen that return loss for loopCircular antenna is positive.
