1. If the antenna is to be used for multiple bands, keep it nonresonant to
> avoid feed point impedance extremes.
>
If "feed-point impedance" means the impedance at the center of the dipole
with open-wire feedline, it doesn't matter--it'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 easily-matched impedance at the
shack (for a dipole with open-wire 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 multi-band 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 quarter-wave 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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|