If I remember correctly from Upmans antenna tests the stock Uniden 6 inch antenna works equal or better than most 3:rd party antennas in the 155-160MHz range. Of course if you get a full length tuned antenna to a single frequency like 160MHz, something like 18 inch long, it will be much better and will also function well for the airband. The 15 inch one that Whiskey suggests are probably also fine to receive the whole hi-vhf band.
Usually the more gain an antenna has the more crucial it is to hold it vertical and a less gain antenna are more forgiving at what angle it is in.
/Ubbe