There is no formula for calculating the resonant length of a half sloper.
That's because the tower and everything on the tower is the other half of
It's sorta like trying to calculate leg length of a dipole, when the other
half of the dipole is a random mess of metal of totally unknown impedance.
The tower impedance at the point where the sloper feed is connected could
raise the resonant frequency, lower it, or even make a low SWR impossible
Perhaps they intended for people to cut their antenna far too long, so they
would have some assurance as the wire was trimmed it would eventually hit
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