Wednesday, 22 June 2011

Switched inductor ATU

This is a work in progress.

This is my attempt at documenting the build of a small low power LC-Tuner.

The concept is nothing new, but I made a few changes to suit my way of thinking. The inductance is provided by bunch of inductors (toroids) connected in series. Switches are placed over each inductor that allows the inductance to effectively be shorted out or to remain in circuit in series with the other selected inductors.

The clever bit is the choice of inductances used and arranging the switches in such a way that by using Hex code one can quickly determine which switches to open or close as required to increase or decrease inductance. Each inductance is double the inductance of the previous one. For example the inductors can be / 16 / 8 / 4 / 2 / 1 / 0.5 / micro Henries respectively. This will effectively give 0.5 to a total of 31.5 micro Henries in 64 steps:  ‘00’ through ‘3F’

The capacitor will still be a normal air variable unit.

It might be a little time consuming to determine the correct settings for each band initially, but once the best match is determined and noted down, it will be very easy to change from one band to the next.

I did some number crunching and the results can be viewed at the link below:

From this data it is clear that if the ATU will be used for the 80 metre band and higher the range of inductance required is between zero  and  23 micro Henries and the range of capacitance is between zero and 1000 pF. The values of both L and C for 40 metre and above tuner will be half of this.

This presents a new obstacle in the design. This means additional high voltage padding capacitors are needed to switched in to extend the range of the normal 1-140 pF air variable capacitors. Note from the table provided this high value of capacitance is required when the impedance is around 100 Ohms. This situation won’t occur if the wire is cut to a length anywhere near a multiple of a half wave length long.

With all the extra components I am starting to think it might be better to limit the matching range and stick to the high impedance EFHWA antenna.

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