well....a coaxial cable workes because the outer shield is a relatively solid piece of metal, and forms a faraday cage around the internal RF. In other words, there is RF energy on the INSIDE surface of the coax shield, but after a few skin depths there is no longer any RF current flowing in the shield metal. This by the time you reach the OUTSIDE surface of the coax shield, the RF energy is way down...maybe 100 dB lower.
So now put a slit in that shield. So the RF current has to hit the slit, and go around it to continue on down the cable. When it hits the slit, SOME of the RF current can now ride up the slit edge, and end up on the shield OUTSIDE surface. Once it is on the outside surface...it radiates into the air like an antenna would.