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The book formula is 468/freq in mhz gives the total length of the antenna. Take half of that for the length of each leg. Then some say to take off a small -% for an inverted V.
The above formula assumes you antenna is at least 1/2 wavelength above ground, which would be 40 meters which is impractical in most cases.
So for a flat top dipole you need 130 feet of wire or 65 feet for each leg.
This can drop to 62 feet for an inverted V.
Here is a website with a built in calculator function.
http://www.hamuniverse.com/dipivcal.htmlYou will need a tuner to be sure you are resonant on the freq you want to transmit on.
here is another page that will let you input the angle as well.
http://www.ccdxc.org/ant_calc.htmRemember to add some extra to terminate (wrap) your ends and allow for trimming is necessary.
Just build and try it.