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238b [2018/10/30 01:59] n5na |
238b [2018/10/30 02:03] (current) n5na [Description] |
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| The first, for impedances greater than 50 ohms, requires the capacitor to be across the antenna. The second requires the capacitor to be placed across the transmitter when the antenna impedance is less than 50 ohms. In the 238B tuner, this function is performed by the 11 position switch in place between the roller inductor and variable capacitor on the front panel of the unit. At HF, as the load impedance approaches 50 ohms, (i.e. the antenna has a fairly low SWR already), the values of L and C in the network required for a perfect match become very small; smaller than the stray or minimum values of the components used. To circumvent this problem, a small fixed compensating capacitor or inductor is placed into the circuit depending on whether the network is configured for high or low impedance respectively (HI or LO positions on the 11 position switch). At low frequencies, the value of the network capacity needed to match some loads is quite large, requiring a large and expensive capacitor. To provide for this, fixed capacitors are placed in parallel with the variable capacitor to obtain the value needed. Further rotation of the main switch from the center position increases the value of capacitance in the circuit. There are five possible tuner configurations for L and C that are switched in and out by the main switch. | The first, for impedances greater than 50 ohms, requires the capacitor to be across the antenna. The second requires the capacitor to be placed across the transmitter when the antenna impedance is less than 50 ohms. In the 238B tuner, this function is performed by the 11 position switch in place between the roller inductor and variable capacitor on the front panel of the unit. At HF, as the load impedance approaches 50 ohms, (i.e. the antenna has a fairly low SWR already), the values of L and C in the network required for a perfect match become very small; smaller than the stray or minimum values of the components used. To circumvent this problem, a small fixed compensating capacitor or inductor is placed into the circuit depending on whether the network is configured for high or low impedance respectively (HI or LO positions on the 11 position switch). At low frequencies, the value of the network capacity needed to match some loads is quite large, requiring a large and expensive capacitor. To provide for this, fixed capacitors are placed in parallel with the variable capacitor to obtain the value needed. Further rotation of the main switch from the center position increases the value of capacitance in the circuit. There are five possible tuner configurations for L and C that are switched in and out by the main switch. | ||
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| + | Here are the tuner configurations possible: | ||
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| + | {{238_tunercfg1.gif|}} | ||
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| ===== Specifications ===== | ===== Specifications ===== | ||
