Newer automatic wire antenna tuners, such as the Icom AH-4 and the Alinco
EDX-2, caution to avoid antenna lengths of even half wave multiples of the planned
operating frequency. The documentation with these tuners even contain the
formula to calculate the antenna lengths to avoid at any particular frequency
(300/MHz/2 x 1, 2, 3, etc.).
This formula works if one plans on operating on only one particular frequency.
However, the beauty of these tuners is the ability to tune a whole range of
frequencies with one antenna. From a practical standpoint, if one were planning on
operating on only one frequency, a dipole or vertical antenna cut to that frequency
could be erected and the antenna tuner dispensed with.
Emmitt thought that a chart, rather than a formula, might be more useful; antenna
length on one axis, frequency on the other and lines indicating the half wave
multiples, in feet, at any frequency. The results are shown to the left. (click on the
chart for a full size printable copy).
To use the chart:
The red lines indicate an integer half wave multiple, in feet, at the indicated
frequency in MHz. For the convenience of US amateurs, blue vertical lines indicate
their HF band frequency limits.
The dotted line is an example of an antenna of 55 ft in length. Reading across to the
intersection with each red line, you can see that even half wave multiples occur at
8.95 MHz, 17.90 MHz and 26.84 MHz. All of these frequencies are outside of the US
amateur band assignments, so a 55 ft long antenna should work with the typical
long wire antenna tuner (though 17 meter band operation at 18 MHz might be
problematic if the antenna is electrically slightly shorter than 55 ft).