As far as I am aware, there is no "cone". On my aircraft (and many others) there is a transmitter and a seperate reciever for each RA system(cpt and fo). The transmitter sends a beam of radio activity towards the earth, but only the ground directly below bounces the radio wave back to the reciever unit. This diagram below should help.Think of it like shinning a torch onto a mirror and back into your eyes.
https://www.google.co.uk/imgres?imgu...9WAZsQ9QEIIzAA
Over uneven ground the radio alitmeter will be all over the place, it reacts very fast. On approach into my base we often get the "1000ft" radio-callout 3 times. I hope that helps, (I could be a different system in a helicopter though to be fair...!)