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Old computer's PSU gives useful parts for antennas

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ANTENTOP- 02- 2003, # 003

Old computer's PSU

Table 2 RF- autotransformer loaded to 450 Ohms

 Frequency, MHz 1.9 3.7 7.1 10.1 14.2 Input resistance, Ohms 100 65 30 25 20 VSWR relative to 50 Ohms 1:1.92 1:1.25 1:1.74 1:2 1:2.61 VSWR relative to 75 Ohms 1:1.33 1:1.16 1:2.51 1:3.01 1:3.76 Efficiency, % 67 69 60 45 12

Table 3 RF -  autotransformer loaded to 600 Ohms

 Frequency, MHz 1.9 3.7 7.1 10.1 14.2 Input resistance, Ohms 110 70 18 15 8 VSWR   relative to 50 Ohms 1:2.11 1:1.34 1:2.9 1:3.48 1:6.53 VSWR   relative to 75 Ohms 1:1.46 1:1.08 1:4.18 1:5.01 1:9.4 Efficiency, % 61 87 65 52 12

Using data from Tab. 1- 3, I made diagrams for three transformer's loads- 300, 450 and 600 Ohms. The diagrams show:

-                  An input resistance vs. frequency (Fig. 2),

-                  A VSWR relative to 50 Ohms vs. frequency (Fig. 3),

-                  A VSWR relative to 75 Ohms vs. frequency (Fig. 4),

-                  The efficiency vs. frequency (Fig. 5).

As it was mentioned above, the efficiency is the most important parameter of any RF transformer. The RF autotransformer has a high efficiency (87 percent) only on a range of 160 meters while this one is loaded to

600 Ohms. The efficiency falls sharply on other amateur HF ranges.

As for a range of 40 meters the RF autotransformer has the efficiency of 65-35 percent (it depends on the transformer’s load). As for a range of 30 meters the RF autotransformer has the efficiency of 52 and 45 percent while this one is accordingly loaded to 600 and 450 Ohms. The efficiency is more in comparison with RF- transformers shown in Fig. 3 and 7, Part- I. Alas, while the autotransformer has a high efficiency, it has a low input resistance and accordingly, a high VSWR! Therefore it is impossible to use a 50 or 75-Ohm coaxial cable together with the autotransformer.

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