How to model a repeating sinc pulse in LTSPICE?
Just similar to what I wrote in this answer you can do almost anything repetitive by using the time variable and a modulus. So let us set this up step by step. First just a sinc function for convenience:

.function sinc(x) { sin(x)/x }


Using this together with the time variable and a little scaling we can get this b source:

V=sinc(time*100)


to produce this here:



This isn't repetetive so let us create a modulo function and use that to bring back the parameter into range:

.function mod(x,y) { (x/y)-int(x/y) }
V=sinc(mod(time,1)*100)




Better, but not quite nice as the "pulse" starts at 0 of the sinc again. It would be better if it started a bit lower, so lets shift and align it a little bit nicer:

V=sinc(mod(time,pi/3)*100-pi)




You should now be able to take this as a starting point to calculate a waveform that matches your needs.