Transformer_Match_ATU

Work on this idea was stimulated by this antenna tuner constructed by Hans G0UPL:

<G0UPL QRP ATU>

A bit of on-line research turned up some more designs that are similar:

<G4RGN "zz" ATU>

Related Information:

<Fabricating Antenna Transformers>

While these sites show the schematic and pictures of physical construction, there is only limited explanation about how this design actually works. Since I have always believed that if one fully understands the technology in use, then one can sometimes make it work better, either through best-practice operating procedures or by upgrading the circuit design to something better. Wanting to have a better understanding of how this particular ATU works is the subject of this web page.

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The original G0UPL web site ATU:

http://www.hanssummers.com/images/stories/atu

[ Click to Enlarge Image ]

The left side of this design is a relatively simple impedance bridge that reads out in watts once the impedance is accurately set to 50 ohms. This impedance bridge is interesting in it's own right because part of the metering circuit stays in-circuit to measure feedline volts even after the bridge circuit is switched off and the antenna directly connected bypassing the bridge. In most resistive bridge circuits the metering part becomes inactive when the bridge itself is switched out of circuit.

This design is also interesting because it provides a reference 50 ohm dummy load for calibrating the bridge circuit before adjusting the antenna to 50 ohms. This dummy load can be internal to the ATU for low power, or could be external for a higher power transmitter.

Now lets concentrate on the ATU itself. This is obviously a transformer with multiple taps on the secondary, but there is also a resonating function involving the transformer secondary, the variable capacitor, and the antenna itself.

First part to evaluate is the transformer function. Hans used 16 turns on the primary side and 48 turns on the secondary. The secondary is tapped at every 4 turns, giving him 12 taps connected to a 12-position switch.

Transformer impedance transformation is given as ( Np / Ns ) = Square Root of ( Zp / Zs )

where:

Np = primary turns count

Ns = secondary turns count

Zp = primary impedance

Zs = secondary impedance

This lets us make a formula for the scaling factor of primary to secondary impedance:

Zs = (Zp / (( Np / Ns ) * ( Np / Ns )))

Or, Secondary impedance is equal to the primary impedance times the square of the turns ratio. For example, if our primary winding were 16 turns (like the G0UPL example) and we select the 8-turn secondary tap, the ratio of Np / Ns would be 16 / 8 = 2. 2 squared would be 4. With a 50 ohm input on the primary the secondary impedance would then be 50 / 4 = 12.5 ohms. Going the other direction with 16 turns on the primary and 32 turns on the secondary we would have Zs = ( 50 / (( 16 / 32 ) * ( 16 / 32 )) = ( 50 / ( 0.5 * 0.5)) = 50 / 0.225 = 222.2 ohms.

Okay, now that we know how to calculate the impedances we can do a further evaluation of the tapped transformer ATU design to see what impedance matching options are available. Assuming that there are 12 switch-selected taps of 4 turns each, and the minimum tap would be at 4 turns and the maximum tap would be at 48 turns. After a little spreadsheet work we come up this table:

[ Click to Enlarge Image ]