A brown beetle with long antennae is likely a longhorn beetle (family Cerambycidae). Adult longhorn beetles typically have elongated, cylindrical to flattened bodies and long, distinctive antennae that are often longer than their body length.
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Prioninae of the World I.
Cerambycidae of the Western Paleartic I.
Their coloration is often brown, reddish-brown, or black, sometimes mottled or banded with other colors, but many species are plain brown. Sizes range from a few millimeters to 1-2 inches long. Some species have small spines on the thorax, and a few mimic bees, wasps, or ants.
Brown beetle with long antennae
For example, the Brown Fir Longhorned Beetle is chestnut brown, about 6-12 mm long, with long antennae that are longer than the body in males and shorter in females. It has a prothorax wider than long, rounded sides without spikes, and is covered with fine grayish hairs. This species is native to China and has been found in the U.S., posing a threat to trees like sequoia and redwood.
In general, if you see a brown beetle with very long antennae, it is very likely a species of longhorn beetle, which are wood-boring insects often associated with trees and logs.
