2SA13 Sensitivity to dimensional changes

---

(May 02 2006) There are three kinds of tolerances to worry about in the production of a high Q antenna such as the 2SA13. 1. Element diameters. 2. Element lengths. 3. Element positions Nominal dimensions are at 20oC. The thermal expansion coefficient of aluminium is 23*10-6 so at +40oC the geometrical dimensions have changed by a factor of 1.0005 which is a scaling effect that is equivalent to lowering the frequency by 70 kHz. Typical tolerances of aluminium tubes and rods are +/- 0.3 mm. NEC simulations show that increasing all element diameters by 0.3 mm will lower the frequency by about 150 kHz. Elements can easily be produced with a tolerance on the length of 0.5 mm. NEC simulations show that increasing all element lengths by 0.5 mm corresponds to a frequency shift of about 70 kHz. Elements are easily positioned within 1 mm on the boom tube and errors in the element positions are insignificant. Worst case performance degradation with the above tolerances is shown in the figure below where the original 2SA13 design is compared to worst case tolerance antennas.

The black curve is the nominal design. The blue curve is at +40oC with all element lengths increased by 0.5 mm and with all element diameters increased by 0.3 mm. The red curve is at -20oC with all element lengths decreased by 0.5 mm and with all element diameters decreased by 0.3 mm.

In production we will use aluminium rods and tubes with a tolerance of +/- 0.2 mm and the tolerances on the lengths will most probably be better than 0.5 mm. The blue and red curves really represent extremes, typical antennas should be affected by temperature only since both element diameters and element lengths will be above and below the nominal value in the same antenna and the influence of dimensional errors will cancel to a large extent. The numerical data for the figure can be downloaded here: 2sa13ref.res 2sa13low.res 2sa13hi.res The corresponding NEC input files are here: 2sa13ref.nec 2sa13low.nec 2sa13hi.nec