Why does a T shaped copper pipe act as antenna on only 2 of the three sides?

I have held a battery operated AM-FM radio close to a non-grounded water pipe. I get a stronger signal near the left (26‘)and bottom (24‘) legs of the T but no signal near the right side(4.5‘). All sides of the T are running in horizontal plane.

Cool, I had no idea it did that.

The 26' and 24' legs are closer to the wavelength that you are bringing in and thus work as an antenna.