John E Davis (jedsoft.org)  has several antennas based on gray-hoverman.  It appears that the frequency can be adjusted by altering all dimensions by the ratio of new/old frequency.

Look at [Link: jedsoft.org] and search for the heading "A Narrow-Band Hoverman"

I have had a number of requests for this antenna for use at other frequencies. If you want to adapt this design for use at a frequency other than 575 MHz, then simply scale the geometric measurements in the tables below by 575 MHz divided by the desired frequency. For example, to use the antenna at 525 MHz, multiply all the measurements by 575/525 = 1.095. In this case, a value such as 300mm in the table would become 329mm, or 12 and 15/16 inches. This scaling applies to all the numbers.