Friday, April 2, 2010

Antenna Tuner Matching & Feedline Lengths

This is a recurring popular topic in our morning QSOs on 75 m.  Multiband operation of a single antenna system results in a wide range of impedance at the transmitter end of the feedline. The Hamuniverse web site provides this article with a discussion of some common scenarios that can result in a very high impedance being presented to an antenna tuner. A few good thumb rules are provided along with a table of feedline lengths to avoid and lengths that are acceptable.

The table focuses on the 160, 80, and 40 m bands. It is worth noting that the listed lengths to avoid are computed for one frequency on each band. For example, the 130 ft and 260 ft lengths to avoid on the 160 m band relate to 1.8 MHz (assuming a feedline velocity factor of 0.95). At 2.0 MHz you may want to avoid feedline lengths of 117 ft and 234 ft. Thus, considering the entire 160 m band you may want to avoid feedline length ranges of 117 - 130 ft and 234 - 260 ft. Similarly, on the 80 m band you may want to avoid length ranges of 58 - 67 ft, 117 - 135 ft, and 175 - 200 ft. On the 40 m band you may want to avoid 32 - 33.5 ft, 64 - 67 ft, 96 - 100 ft.

A table footnote says that "some trimming or adding of line may be necessary to accommodate higher bands". If one desires to use the antenna on ALL bands through 10 m, the set of feedline lengths needed to satisfy the guidelines on all bands may become more restrictive. Careful testing on all desired frequency neighborhoods should reveal any problem areas for matching. Then, hopefully, minor adjustments of the feedline length can accommodate the range of the antenna tuner.

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