The trace form is a perfect pairing in the sense of harmonic analysis, i.e. it identifies $\Phi_n$ with its portyaginPontryagin dual, see Weil's "Basic Number Theory" or Tate's thesis. Therefore, your statement follows from the Pontryagin duality, for example Proposition 3.6.1 in the book "Principles of Harmonic Analysis" by Deitmar/Echterhoff.