The only way I know is to build a gap filler that goes between the bottom of the window and the bottom of the window opening. You can make it out of metal, laminated wood, phenolic, or fiberglass. It needs to have properly align and machined surfaces so it fits snugly against the existing surfaces. Usually it needs weatherproof seals attached. It has to have the correct connectors installed, so that the coax from the antenna can be detached from the gap filler separately from the coax that goes to the modem, set top box, or television. You will also need to raise up or lower the window latching device on the other part of the window with a shim block and longer bolts. If you are renting and leave the place, you disconnect the coax cables, remove the gap filler, take out the shim block, screw the latching device back in, clean up the area, and no harm done to the property. If you are really careful to match the materials and coatings, a relatively thin gap filler will blend in with the rest of the window. From a short distance it will look like you have the window raised slightly. You do not want to raise it too much, as from a long distance it will look like an open window and will invite thieves to attack it as a possible entrance point that they can get into without breaking the glass.