Hi Ron,
So (you probably know this) if you want to lower the resonant freq of your antenna you need to make it longer.
There may be an inch or so of adjustment if you loosen the set screw at the base of the antenna.
Although I might add that you have swr of 1.5 at 155 which is still quite good. And if you are rx only you likely would not notice any gain or improvement at all.
73
dwight
