The VERT1 vertical that comes with EZNEC is 1/4WL on 40m.
It will be 5/8WL on 17m. (7.2 * 2.5 = 18) EZNEC sez the
feedpoint impedance on 17m will be about 100-j470 ohms.

Hi Cecil, He was asking about 24MHZ which is 12 meters. EZNEC sez a 5/8 on
24MHZ is 63-j206, more or less. As others have pointed out a lot of factors
will enter into the final installed impedance.
Another matching method, for 63-j206, is feed the antenna with a 1.2 meter
length of 300 ohm twinlead- 1:1 balun- 50 ohm coax. The twinlead transformer
gives 43-j0. Don't tell Reg, but I did that on the Smith Chart in 48.32
seconds:-).
73 Gary N4AST