KC8AON's QRP PROJECTS
COMPACT 12 thru 40 METER ENDFED HALFWAVE WIRE TUNER

A POCKET SIZED TUNER FOR ENDFED HALFWAVE WIRES


A GREAT COMPANION FOR COMPACT QRP RIGS

 
COVERS 12 THRU 40 METERS WITH HALFWAVE ENDFED WIRE
 
SMALL ENOUGH TO GO ANYWHERE
 
HAS RESISTIVE SWR BRIDGE WITH LED INDICATOR
 
IF YOU LIKE COMBINING BACKPACKING AND QRP, YOU NEED THIS LITTLE DEVICE !
 
This is a very compact halfwave endfed wire antenna tuner that works 12 thru 40 meters with a halfwave wire for the band of interest attached. It is built into one of the small plastic Radio Shack project boxes that fits in the palm of your hand. It is basically a parallel resonant circuit that transforms the hi Z of an endfed halfwave wire (around 5000 ohms) down to the 50 ohms that your transmitter likes. It consists of a transformer of 27 turns of #22 wire wound on a T50-2 core, and a 3 turn link wound over the ground end of the main windings. The 50 ohm side goes to the 3 turn link, and the hi Z antenna connects to the ungrounded end of the 27 turn winding. The variable capacitor connects across the antenna side from the ground to the antenna terminal. The variable capacitor in this tiny tuner was salvaged from an old AM broadcast band transistor radio that I picked up at a fleamarket for 50 cents ! If you see these old radios cheap at fleamarkets, grab them - the vari caps are valueable ! To use the tuner, first attach the endfed halfwave wire for the band of interest to the antenna terminal and get it up as high as possible. I like to put mine up as an inverted L if possible with as much wire as possible in the vertical plane, and the balance of the wire running horizontal. This gives both vertical and horizontal radiation that works well both in close and far out. Next, attach the transceiver to the 50 ohm input on the tuner. With the swr bridge switch in the operate position, adjust the capacitor for maximum reciever signal or noise, then put the switch in the tune position, key the transmitter and watch the LED as you fine tune the antenna to get the LED to go out. Once the LED is off, put the switch back into the operate position, and you are ready to transmit. Refer to the schematic below to help explain how to build the tuner. It is a very simple circuit to reproduce.

 
HALFWAVE WIRE LENGTHS 12 THRU 40 METERS

 
12M - 19', 15M - 22', 17M - 26', 20M - 33.5', 30M - 46', 40M - 67'

 



 


73

 

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