Why when I stand before a radio broadcast signal is blocked?

i hope all radio broadcast antennas tend to be omnidirectional.
it also applies to receiver antennas.
Then why does, when i stand in a particular direction before the radio, the signal is completely blocked and it makes a hiss noise.
I feared my radio as unidirectional. or any other reason.

thanks

I assume that this is FM radio ie VHF 88 to 108MHz.
You are a refelector of the radio waves and if you stand in the right place you will refelct a signal onto the receiver antenna that is of the same level with a 180 degree phase shift. This reflected signal cancells out the signal you are trying to hear.
This is the way directional antennas work.
YOU are part of the antenna whe you are near you radio.