1. If the antenna is to be used for multiple bands, keep it nonresonant to
> avoid feed point impedance extremes.
>
If "feedpoint impedance" means the impedance at the center of the dipole
with openwire feedline, it doesn't matterit's only part of the equation.
What does matter is the impedance at the shack end of the feedline. The
impedance at shack is a function of dipole length, feedline length, and
feedline characteristics.
To avoid impedance extremes and to get easilymatched impedance at the
shack (for a dipole with openwire feedline) make the length of one leg of
the dipole plus the length of the feedline an odd multiple of a quarter
wavelength at the frequency (frequencies) of interest. Not always easy for
a multiband antenna. Life and ham radio require compromises.
For instance, on forty meters, an eighty meter dipole has an impedance
extreme at the center feedpoint. A quarterwave length (at 7 MHz) of
feedline will transform that extreme impedance to a reasonable value at the
shack.
Ski Exuberantly,
Hank, W6SX
Mammoth Lakes, California
Elevation 8083 feet in John Muir's Range of Light
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