You can add a sector and radio to a site by invoking the "Add Sector and Radio" action. To do this, you will first need to select the site you wish to change. This will cause the site's properties to be displayed in the Properties Window:

Site Properties

The "Add Sector and Radio" action can be invoked by clicking on the Add Sector and Radio Button button. This will add a Sector, a Radio and a Connection to the current site.

Sector and Radio entities have similar actions. If you want to add a radio to a sector, select the sector and then invoke the "Add Radio" action by clicking the Add Radio Button button. Similarly, if you want to add a sector to a radio, select the radio and then invoke the "Add Sector" action by clicking the Add Sector Button button.

The number of sectors and radios attached to new sites is controlled by the Site Template that is used to create them.