Posted by
Ron D'Eau Claire-2 on
May 14, 2006; 11:44pm
URL: http://elecraft.85.s1.nabble.com/Antenna-Feedline-Question-tp389612p389615.html
Ken K3VV wrote;
...one of the best pieces of advice was Don's suggestion to make the antenna
a bit longer than the ~66 ft required for a resonant half-wave dipole on 40
meters. As Don mentioned making the antenna longer will make it easier to
tune on 20 and 10
meters where a 66-ft antenna has an extremely high feedpoint impedance.
----------------------------------------
Avoiding a close encounter with 1/2 wavelength is important, Ken, but
cutting the antenna to a length other than 66 feet won't necessarily help
and might make it worse. That's because the feed line acts like an impedance
transformer. The impedance at the feedpoint at the antenna will appear at
the terminals of the feed line *only* if the feed line happens to be exactly
1/2 wavelength long, electrically.
It can be rather difficult to predict the actual feedpoint impedance without
some careful measurements and calculations. Usually it's easier to see if
the ATU doesn't have enough range, then adjust either the feed line length
or the antenna length a bit until it works on all the bands.
Ron AC7AC
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