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LOW-POWER ANTENNA TUNER AND SWR METER


1N34A
The SWR meter is a variation of a resistive bridge. This circuit has the advantage of providing high sensitivity at low power inputs. Its disadvantage is that the maximum power that can be applied to the circuit is limited by the power dissipation of the 50-Ω bridge resistors because 75 percent of the transmit power is absorbed by these resistors under perfectly matched conditions. In this circuit, a DPDT switch adds in a 100-Ω resistor only for SWR readings, allowing the circuit to provide a reasonable SWR for your transmitter even during severe tune-up conditions. Additionally, you have minimum insertion loss in the FORWARD position because the l-kΩ bridge resistors permit you to always indicate forward relative power and calibrate the full-scale reading for SWR measurements. The antenna tuner itself is a standard T-type tuner utilizing a pair of 335-pF miniature transistor radio variable capacitors and a toroidal inductor, tapped as shown in the figure. To actually make the taps, simply wind the toroid with 40 turns of #24 wire, and then scrape the outer surface of each appropriate tum with a hobby knife to clean off the enamel insulation. Then tin each wire turn, and tack-solder tap wires to them.
               
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