> In my example, I mean something like an industrial lead

Industrial leads are non-signalled, regardless of whether the main track is CTC, ABS, or interlocking.

> or a non CTC siding.

This would be a siding which is not within CTC limits.

>My understanding of PTC (which may be wrong) is that trains can only pass where there are wayside antennas to talk to the PTC system, IE a CTC controlled siding.

Meets can be made only at designated sidings PTC or not. A industry job getting in the clear for a main line train isn't a "meet". If the PTC system fails there are provisions for falling back to "plain old" CTC.

> In CTC territory, there are uncontrolled sidings

There are no "uncontrolled sidings" in CTC limits.

> and industrial tracks which a local,etc could clear the main for opposing movements.

For a locomotive, with or without cars, to clear in a non-controlled (industrial) track within CTC limits requires either electric switch locks on all switches connecting the track to the main line or non-controlled absolute signals at these locations, positioned so they govern movements from these track(s) to the main track. Without provisions like these, when a locomotive is allowed to clear at one of these tracks, max authorized speed by the location on the main line is restricted speed between the two control points on either side of the location of the industrial track.

> Can that still happen with PTC, or does a meet have to occur at a PTC equipped siding?

PTC is an overlay to whatever signal system is in place, although usually this is some form of CTC.