There isn't a difference in construction. The difference is that Sepharadim (traditionally) do not accept communal 'Eruvin in large cities/neighborhoods, due to the position of the Mehaber (Shulhan 'Aruch O"H 345:7) that defines* a public domain by size, not by population density.
Some Many Sepharadim have accepted the Ashkenazi leniency in following the population density, but it's possible that either your rabbi does not follow that leniency or that your community does not fit within the acceptable norms for that leniency.
More details about the community in question would help our community to form a better answer.
*Please note that there are several criteria, but I'm focusing on the main sticking point.