How to turn on a point sensor midway through an EM FDTD Simulation

Good day!

I'm working with a simple EM FDTD simulation set up in Sim4Life where I have a plane wave source that emits a Gaussian pulse and a PEC sphere some distance away from the plane wave source but directly in front of it. Finally, I have a point sensor directly behind the plane wave source. My setup can be seen in the following image:


My goal is to capture the back-scattered far-zone e-field that results when the plane wave impinges on the PEC sphere. Ultimately I want to generate a plot of normalized amplitude vs frequency of the back scattered field. As I you can see in the following plot I have successfully done this:

Frequency Response.PNG

However, as you can see from the following graph, I not only capture the back-scattered field (from about 6-9ns) but also the transmitted pulse from the plane wave source (from about 0-4ns).

Time domain response.PNG

I don't want the transmitted pulse in my recorded data because then the amplitude vs frequency graph of the backscatter is hidden under the much larger transmitted signal. The second image is what I get but what I'm expecting is the black line in the following plot:

Sphere Magnitude vs Frequency.PNG

My question is this: is there a way to turn on the point-sensor midway through the simulation? This way I could wait until the transmitted pulse has been sent out and then turn the sensor on at about 4 ns and only record the reflected signal. Is this possible? If not is there another way to get what I need? I tried exporting the time-domain data into matlab where I chopped off the first 4 ns and then did the fourier transform on the remainder of the signal but the results I get are not what I should be getting--possibly because I'm doing the FFT wrong...

I've been stuck on this one for a while and would appreciate any help that can be provided!


Sorry but I am thinking of using a time-gate for my own experiment and wondered if you managed to get it working?