In Australia TV signal in certain locations can vary from Vertical polarisation & other areas horizontally. This help to stop signal interference, increasing the number of stations that become available to make better TV coverage. Both UHF & VHF signal can transmit in both Vertical and horizontal. So it goes without saying that it is crucial that you choose an antenna that can be set in the respective locations.
Further, the latest wave of caravan antenna now mounts on caravan roofs that are raiseable up, down and rotate from inside the caravan itself. However, these antennas not perfect as they do not take into account Australian broadcast settings. As an example, when you move from city to city or even town to town the HORIZONTAL to VERTICAL polarisation issue changes, however, the older Caravan antenna versions cannot be set in vertical mode. It is best that you check with your stockist for the latest model. This becomes apparent when they compare with a fellow caravan who is set vertically with an antenna that has a simple swing head device attached to change polarisation. Since early 2012 there is available a retrofit kit to fix this problem, and these kits are available from any good stockist.
We repeat advice from above. When ordering your new van or buying an antenna, it is critical that you choose an antenna that can be set in a vertical mode in vertical signal locations and horizontal mode in horizontal locations.
It does not make a lot of sense to having one arm of antenna vertical & the other horizontal as the signal received on the arm set can be severely affected, plus it cannot be set one up & one down and this was a deliberate design consideration. So be sure take this advice when buying a new caravan and antenna.