A portable test chamber with an open top may serve as a field testing apparatus for wireless testing of electronic devices. A wireless device under test may be mounted within a cavity in the test chamber. The cavity may be surrounded by a dielectric lining of anechoic material. A layer of electromagnetic shielding such as metal foil may cover the outer surfaces of the dielectric lining. The chamber may have a box shape with a rectangular opening at its top. Satellite navigation system signals or other wireless signals may be received through the opening at the top of the test chamber during testing. The electromagnetic shielding may reduce the effects of multipath interference during field tests.