Here is a handy little QRP ( up to 15 watts ) atu designed specifically for 20m, although it works fine up to 10m. This came about as I wanted something small and light to match a end fed half wave 20m antenna. Incorporated in the atu is simple VSWR bridge to allow easy set up of the atu and antenna. The atu is capable of matching any impedance in the range 100 Ohms to 4K Ohms, so it is ideal for a half wave or multiples of. As can be seen from the circuit diagram the unit uses a simple RF auto transformer made from two stacked FT50-43 torroid cores. A multipole switch selects one of the various taps on the transformer for the antenna feed. A short counter poise has on occasion been beneficial. In use, simply select a tap that gives the best VSWR indicated by the meter.In use on 20m, the transformer is very efficient, no core heating is detectable and over ninety percent of the power is transferred to the load.
The atu could be used on the lower bands, however a higher permeability core would have to be used and more turns on the transformer core would also be necessary>
Barry Zarucki M0DGQHOME