The L-A simple - A Simple but Effective Matching Network
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L1, the variable coil should be at least 25 microhenries or more, and can either be a rotary or tapped inductor. The variable capacitor should be at least 1000 pF and in case this is not large enough. More capacitance should be available. This is primarily in case you have to match a very low impedance antenna. If you never have this problem, then skip it. This coupler is primarily for wire antennas, such as short or long wires.
The tuning capacitor should have a voltage rating of at least 1,000 to 1,500 volts if you run more than 500 watts and the coil should be a high quality ceramic or plastic form with provisions for tapping. If you use switches, be sure the switches are of high quality ceramic. You also need an SWR bridge in line to determine when the VSWR is 1:1 The L network is found in several commercial tuners, and in some cases, there is a balun included to enable the tuner to match balanced lines.
Tuning consists of tuning the coil and capacitor for minimum VSWR, as well as watching a field strength meter if you have one. If you are not able to get the antenna to load up correctly, then switch the capacitor to the other side of the coil and start over. Sooner or later you will find the correct settings and the antenna will start to radiate. The field strength meter insures that you are not getting a false tuning indication caused by circulating currents in the tuner. This can happen in certain circumstances, and will cause your signal to be rather puny. A good field strength meter is a very handy device to have around the ham shack for determining when your antenna is radiating at its maximum. I have used an RF ammeter to indicate the maximum current in the antenna wire, but this is only if you run at least 25 watts or more. It is not required, but mentioned in passing only.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 16th May 2024