2SA13. Final geometrical dimensions in free space

(May 02 2006)

The original design of 2SA13 has 10/8 mm aluminium tubes for the feed element. It has turned out to be more practical to use 6 mm rods and therefore the length of the feed element has to be increased to compensate. As a second order effect the coupling to surrounding elements is changed and therefore a rerun of the optimisation is motivated although the improvements by allowing the other elements to readjust is extremely small.

Reoptimisation for 6 mm feed element.

The original design assumed the conductivity of the aluminium elements to be 27*106. The alloys used for production will be 6082-T6 for the 6 mm rods and 6063-T6 for the 10/8 mm tubes. The conductivities are 26*106 and 30*106 respectively.

Since the changes are very small with respect to the original optimisation, no control parameters were changed, OMNEC was just allowed to find a new optimum with the same control parameters. The element positions were rounded to one mm and after that the element lengths were allowed to readjust, a procedure that does not change the antenna properties at all in the NEC2 simulation. Table 1 shows the final dimensions of 2SA13 for OMNEC with 19 segments per element. The NEC2 input file of the four-stack used for optimisation can be downloaded here: INPUT.NEC

Position Length Diameter (mm) (mm) (mm) 0 (0) 1027.636(1.46) 6 230(-10) 996.972(12.37) 6(-4) 330(-10) 953.380(-2.82) 10 753 (-8) 933.950(0.46) 10 1455(-10) 904.232(0.19) 10 2254 (-8) 886.384(-0.05) 10 3097 (-6) 896.084(-0.03) 6 3964 (-6) 887.688(0.08) 6 4853 (-5) 877.558(0.06) 6 5797 (-4) 870.630(-0.18) 6 6726 (-4) 873.296(-0.43) 6 7560 (-3) 884.586(-0.38) 6 8350 (0) 888.484((0.16) 6
Table 1Dimensions of the 2SA13 with 6 mm diameter for the radiator. The change with respect to the original design is given within parenthesis. This table shows the dimensions to use with the modified NEC2 model of OMNEC

The 2SA13 physical dimensions in free space

From studies of the impedance vs frequency for an antenna with accurately known physical dimensions we know that 6 mm rods should be made about 2.03 mm shorter than the NEC2(mod) lengths while 10/8 mm tubes should be made about 0.08 mm longer. Table 2 shows the real world free space element lengths for the 2SA13.

Position Length Diameter (mm) (mm) (mm) 0 1025.6 6 230 994.9 6 330 953.5 10 753 934.0 10 1455 904.3 10 2254 886.4 10 3097 894.1 6 3964 885.7 6 4853 875.5 6 5797 868.6 6 6726 871.3 6 7560 882.6 6 8350 886.5 6
Table 1Dimensions of the 2SA13 in free space.

VSWR, pattern and gain in free space

Figure 1 shows VSWR, G and G/T for a four-stack of the 2 SA13. These plots are generated from the data in result_eplane.nec and result_hplane.nec

Figure 1 Gain, VSWR and G/T for a four-stack of 2SA13.

G/T differs between vertical and horisontal polarisation by about 0.1 dB for all frequencies. The performance data on 144.1 MHz for a four-stack of the 2SA13 at a stacking distance of 4.2 meters is like this (according to NEC2):

Gain 20.40 dBd
Half Power Beam Width 13o (E and H planes)
F/B 21.4 dB
G/T -1.44 dB horizontal, -1.34 dB vertical.

Figure 2 shows the H pattern and figure 3 shows the E pattern of a four-stack of the 2SA13. These figures are generated from the data in patt_eplane.nec and patt_hplane.nec

Fig. 2. The H-pattern of a four-stack of 2SA13 at 144.1 MHz.

Fig. 3. The E-pattern of a four-stack of 2SA13 at 144.1 MHz.

This file contains the full pattern in azimuth and elevation: pattern_3d.nec

This file shows the amount of power that is radiated within an angle w from the forward direction: powinteg.nec The data shows that 69% of the power is radiated within 10o from the forward direction while 23% of the power is radiated outside 14o, the angle of the dip between the main lobe and the first sidelobe that originates in the stacking at 4.2 meters.