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Medium wave signals use ground propagation only during the day. The D region will absorb any signals it meets. At night the D region becomes invisible and the waves pass though to the E region. The E region reflects the signal back to the ground. Higher frequencies are not reflected by the E region. As frequency goes up, reflection goes down. E region reflection tends to make for shorter hops, meaning shorter distance DX.When frequency is high enough it penetrates the E region it then may be reflected back by the F1 region, the first part of the F region of the ionosphere.