[werk_ag]

Hi Allan

A "monopole" antenna (e.g. a 16 cm piece of wire in the case of 433 MHz) transmits/receives most power at right angles to its length (and theoretically nothing in a direction along its length) and thus has a maximum "gain" of +3dBi (relative to that theoretical isotropic antenna).

I think this applies to a helicoidal antenna too, and it can explain why, when the receiver is placed right below the transmitter, which happens with almost all installations where the transmitter is on the roof, placing the transmitter antenna in horizontal position seems to give better results.
Now imagine the z axis as horizontal.

Example of the radiation pattern for a quarter-wavelength monopole antenna (oriented in the +z-direction)

If I'm wrong, feel free to correct me.