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YAGICAD question
Am trying to design a 3 element tape measure antenna for 2 meters
with a 10x10 mm aluminium boom. After calculating the dimensions with Yagicad 6.1 the tapes get cut and mounted. http://img837.imageshack.us/img837/8209/snc14096a.jpg (Before cable and hairpin match) The driven element sits on a slider which allows to adjust for best F/B ratio over the working frequency span of the antenna. Normally the F/B degrades quickly when tuning off center. http://img90.imageshack.us/img90/7112/fbratio.jpg With the driven element on a slider you can move the peak to the desired frequency, within a certain range. About 10 MHz span is possible with such a 2 meter antenna Measuring with a scalar analyzer and tests with actual radio signals reveal that YAGICAD does calculate too short elements. Everything is a few MHz too high in frequency, the gain maximum, the F/B optimum, the resonant frequency, on the antenna, when built with the YAGICAD dimensions. The only parameter I can vary in YAGICAD is the element diameter. The tape measure is iron, 19 mm. Iron can be selected in YAGICAD, but that only affects little. I optimize the design for best F/B ratio, because that is what you want for a direction finding antenna. I have to add 4 cm to the passive elements and 8 cm to the driven dipole to get a working antenna, having selected 0,02 m element diameter in the YAGICAD program. Entering less diameter worsens the situation. The boom adds only 1 or 2 mm to the elements, but the final dimension results are several centimeters off! The SWR is excellent with the hairpin calculated by YAGICAD. At least something that works. Luckily I have a good test equipment and see live on the screen what the antenna does when I change elements. But that YAGICAD is a mysterium to me. After so many revisions I would expect some more realistic results. w. |
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