The rule of thumb to find a quarter wavelength monopole antenna is using 234/frequency (in MHz). For 1/2 use 468/f and for 5/8 use 585/f. So a 1/4λ antenna on 2m is 234/145 = 1.61 feet and a 1/2λ is 468/145 = 3.22 feet. If you are making a dipole, then each half would be 1.61 feet and the overall length would be 3.22 feet, if you are making a 1/2λ vertical then the whip would be 3.22 feet. Keep in mind that only the 1/4λ monopole and 1/2λ dipole have the correct intrinsic feedpoint impedance to match a coax, the 1/2- and 5/8-wavelength antennas have much higher feedpoint impedances and will need to have a matching network to work. So if the antenna in your hand does not have a matching network, auto-transformer or something to match the coax impedance then the antenna can only work as a 1/4λ whip.
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