Wires & Pliers

Ye olde L-network

Kent Bailey, WA4DQU

Over a doorway in my shack is an L-network mounted on a board. I have used the L- network tuner for many years. It's similar to the one in the photograph.

The one in the photograph is the one I use for my outdoor work. I'm cleaning it up for another year of outdoor operating. You can hook a one-half wave length of wire to the top button, lay the wire out on some bushes, (provided the wire is insulated), and work a lot of Hams. In fact, I have worked several hundred miles with a one-half wave insulated wire just laying on the grass. It will work well with other antennas as long as you follow the rules and keep the output impedance higher than the input. You can use open-ended transmission lined to end-feed antennas for example. Just connect the wire from the antenna to the top button and the free wire to the bottom button. Boy, how simple can home-brew be?

You start with a short length of 50-ohm coax from your rig to the coil. I use about 14 feet of coax so I can tie the board to a tree. The inner wire of the coax goes to the beginning of the coil, and the braid goes to the rotor of the capacitor. (The rotor and the frame of the capacitor are already connected together). The knob is insulated from the capacitor so that you don't get burned while adjusting the capacitor (see the dowel between the capacitor and knob). The capacitor is about 180pF with a 1/8" spacing between plates. I don't need that much spacing for low power, but it's lots easier to clean the bugs out on campouts. A broadcast capacitor will work up to 100 Watts or so. If it arcs over, just get one with larger spacing.

The coil is 3" PVC pipe cut 6" long. 3/8 inch holes are drilled a 1/4" from each end. The holes are connected by lines and a slot is cut out. I used a saber saw blade bound to a stick to saw the slot out. Wind 21 turns of #12 house wire on the coil form in about 5 1/2 inches. Space the wires equally. I use 1/8" self-tapping screws at each end to hold the wire. Also leave a little wire sticking up to solder to. Then coat everything but the slot and ends with clear fingernail polish to hold the wire in place.

Mount the coil a little above the board with two screws with PVC stand-offs. Solder the inner wire of the coax, the end wire of the coil and a jumper wire with an alligator clip together at the beginning of the coil. A wire is soldered to the other end of the coil and goes to the stator connection of the capacitor and from the stator connection on the capacitor to the top output button.

The rotor-frame connection of the capacitor goes to the bottom button. That's all there is to it!

The top button could be red for single wires. The bottom button could be black for add-ons and transmission lines.

There's a multitude of things you can learn from this tuner. I use a relative field-strength meter for tune-up. Just tune for maximum output and back off just a hair.

Now the question is, why such a large tuner? It's true that my rig is one-third the size of this tuner and it seems like overkill. It just works better for me. I don't have the equipment to prove it, but I believe I get better output with the large coil. It's a blast to find out what this L-network will radiate.

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