by CHfour:
"The QDM/QDR is known from the VOR receiver info so it's not difficult to calculate the relative brg if the hdg is known."
I don't see what VOR has with magnetic bearing (qdm/qdr)? VOR just receives two signals - one electronicaly directed to cover 360 radials and other none directed - VOR receiver just calculate the time diference betwen two signals - this gives you the info on whic radial you are - then a VOR gauge shows the deviation from the preselected radial. mining that if your VOR receiver calculates that you are on 90° radial and you have selected 100° radial, than you are 10° from the radial, and by DME you can determine right-angled lenght to the selected radial. This is how I understand the VOR.
I thought your question was about the mechanics of the VOR needle on the RMI. If your aircraft was on (say) the 270 radial from the selected VOR then the tail of the associated needle would be placed over the 270 mark on the compass card. No extra info or antenna is reqired as the radial (QDR) is known and the heading so the two are combined for display on the RMI.