The activation zone for Cooper Mountain is a (very) short hike above the dog park at Winkelman Park, 10139 SW 175th Ave, Beaverton, Oregon. There is no AZ access from Cooper Mountain Nature Park to the west and the summit proper is posted "No Trespassing."
For "sidewalk" activations with a HT, you might use the high point across the street from the water provider property (parking is possible down SW 182nd Avenue). Also, this earlier trip report describes another public space in a greenway that is also in the activation zone. However, Winkelman Park with public access and off-street parking is recommended.
In the image below the recommended activation location is circled at the upper left (near 45.4475° -122.86°) with the parking in the center. There are some small trees for antenna support.
