Hi Sheena,
I understand that you want to plot the radiation pattern for a half wave dipole antenna working at 144 Mhz but not able to do the same.
According to the definiton length of this dipole antenna is equal to a half-wavelength at the frequency of operation. With that we do our required calculations to find the length of the dipole antenna working at 144 Hz using 
where c is the speed of light.
where c is the speed of light.Doing the calculations we get the Length of dipole as 1.04m.
Just make a dipole in matlab at the required frequency and change it's length to the above mentioned.
Hope it helps!
