L - Network Tuner
This page consist of two versions of the L - Network Tuner. The first design being that of
a higher wattage one, then as you look further down this page you will find a much
lower wattage version.
In the past all my amateur radio stations operated with a maximum power of
100 watts, This all changed this past Father's Day when I was presented
with an Ameritron AL-811H amplifier.
Since all my antenna tuners were built to handle 100 watts it was time to buy
a tuner that could handle the 800 watts or build one. The obvious choice was
to construct a new tuner.
My antenna system covers 80 through 10 meters with a SWR of 2.2 to 1 or
lower. It was plain to see a simple tuner could easily bring the SWR down to
acceptable levels. I also wanted to construct a tuner that was smaller than
most commercial made kilowatt tuners.
The L-Network tuner is a simple impedance matching network that consists
of one variable inductor and one variable capacitor. The L-Network tuner has
often been described as a one band tuner made to work with random wire
or vertical antennas. With proper components it will match a wide range of
impedance. The L- tuner can match high to low or low to high impedance
simply by reversing the coax cable connections. Since I was only concerned
with a match on the 80 and 40 meter bands, the L-tuner seemed the prefect
My tuner is built in an old Heathkit audio generator cabinet with measurements
of 13 in X 7 1/4 in X 7 1/4 in. The front panel is made from aluminum stock.
The Hammarland capacitor measures 500 pf. with both sections tied together.
Plate spacing is .100 of an inch. The coil form is 3 inches in diameter and
wound at 8 turns per inch with 12 ga. wire. Total coil is 26 turns. .
I made my coil taps at 1 turn starting at the end closest to the antenna connector.
I did this for 6 turns and then made taps every 2 turns. This was done purely on
guesswork. You could use an alligator clip and low power to find that perfect
spot for your antenna system and then make your tap with a switch. I was lucky
and only had to change one tap. My vertical antenna is easily tuned for all bands
in the 80 to 10 meter range.
The band switch is a 17 position Centralab and utilizes silver contacts. Silver braid
measuring 3/16 inch wide was used for tap switch connections. All other wiring was
done with 12 ga. bare wire.
The L-Network tuner was perfect for my situation. It handles the amplifier's 800
watts using very little inductance and capacitance. I am not sure of the tuners
limitations, but it seems to work well when SWR is within a reasonable range.
Simple in design, low on parts count and with the ease of operation makes this tuner a real winner.
A Homebrew Coax Switch
After building the L - Network tuner I decided it would be best that I had a coax switch box
that could be safely used with the 800 watt amplifier. Time to dig into my junk box once again.
This is what I came up with and if you have any questions about it's construction, drop me an
E-Mail and I will gladly provide you with more details.
73, James, KC5LDO
The Transmatch or Tuner Revisited
Transmatch is actually the proper name. It's an L version built just like the one above
on this webpage. The only difference is that this one is made for 200 watts maximum.
So if QRO is not your method of operating, then this little brother to the big one
shown above just might be the one that will meet your needs.
Here are the specs:
Chassis size: 10" X 7.5" X 3"
Variable capacitor: 500 pf. with .020 of inch plate spacing.
Coil information: 2 inch diameter, 42 turns of #16ga. silver tinned wire
tapped at 3,8,16,19,23,27,30,33,35 turns.
The finished project.
So, no matter if your pleasure is QRO or QRP, if you need a real good L Transmatch, then either one of these projects should meet your needs.
Foremost is to enjoy the building of projects and I hope we meet on the air.
73, James - KC5LDO
Articles written by James Tobola - KC5LDO
and reproduction, publication, or duplication of this article, or any part thereof,
in any manner is prohibited without the express written permission of the author.
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