Clarks Mountain is just south of Seaside Oregon. The trail is about 1.8 miles of one-way hiking starting at a clearly marked trailhead on the very south end of town on Sunset Blvd. There's a large parking area that may fill up on sunny summer weekends. 

The rainforest is very close at the summit and a long wire-type antenna may be troublesome to set up compared to a vertical (my EFHW snagged and broke there). You should wear things you don't mind getting muddy - think rubber boots more than hiking boots. And a GPS is pretty much necessary to tell where the summit is - it is not at all defined by the terrain. Both Google Earth and Google Maps show the trail.