This is a project I devised to build a balanced Z-match, capable of matching an unbalanced 50 ohm transmitter output into an antenna (e.g. a doublet, G5RV etc.) fed by balanced feeders (e.g. 300 ohm, 450 ohm ribbon feeder or 600 ohm open wire feeders). The ATU covers a wide range of impedance and was built in an Elecraft EC2 enclosure (the same size as an Elecraft K2). I intend to use the ATU from a portable holiday location with a 270 foot doublet near a lighthouse in Devon.
The completed ATU is shown here:
Fig. 1: Schematic of the Z-match.
Based on the classic KW EZ match design. The variable capacitors C1 (430 pF) and C2 (split stator 2 x 290 pF) were taken from an old (unused) Yaesu FC-902 ATU. The transformers T1 and T2 were wound using 16 swg tinned copper wire on PVC formers and attached to the inside and outside of 4 x 4 mm hollow square section PVC tubes using hot melt glue. The coil details are: T1 (pri) 5 turns 32 mm dia 6mm spacing, overwound with 5 turns 40 mm dia 6mm spacing. T2 (pri) 8 turns 32 mm dia 6mm spacing, overwound with 6 turns 40 mm dia 6mm spacing. The 160m loading coil was 16 turns 3mm spacing on a 36mm diameter ceramic former. The Ant.1 (10 - 30 MHz) and Ant. 2 balanced outputs were switched in my ATU using a ceramic wafer switch operated from the front panel. The shaft extension to this switch was a 6mm diameter fibreglass rod, running through the centre of T1 to the front panel. This was done to enable all the components to fit inside the EC2 enclosure with the minimum of internal wiring lengths.
The "SWR" meter is a reflected power meter in my ATU (see below).
Fig. 2: Schematic of the reflected power meter.
Based on the "Stockton" (GM4ZNX) bridge design. The transformers TA and TB were wound using 12 turns each of 22 swg enameled copper wire on FT-50 43 toroids. The coils are linked bu a single wire passing through the cores (see schematic). Although the circuit is hard to understand, it works very well indeed, and tuning is acomplished by adjusting the Tune/Loadvernier controls on C1 and C2 until the indicated reflected power (on meter M) reads close to zero. The variable resistance was in practice replaced by a fixed 20k ohm resistor, enabling the ATU to be used for 100W power levels. Vary this resistor for other powers as needed. The germanium diode was an OA91 or equivalent (OA47 etc.). An identical second diode circuit and meter can be connected symmetrically to the right hand side of the 2 x 100 ohm paralleled resistors, to indicate forward power if needed. In practice the reflected component gives all the information necessary to achieve matches of 1:1 SWR (as checked via an external commercial SWR meter).
In practice ...
The unit operates extremely well. With a 60 foot span doublet antenna at a height of 30 feet and fed with 300 ohm ribbon feeder, I can obtain matches on all bands 1.81 MHz - 29 MHz with SWR values of 1:1. The use of 6:1 vernier knobs on C1 and C2 is highly recommended as sometimes the resonances can be extremely sharp. The 60mm diameter Jackson Brothers 6:1 reduction vernier dials were obtained from http://www.mainlinegroup.co.uk/jacksonbrothers/4080.htm. I keep a record of the TX frequency and C1 and C2 vernier settings (on a scale of 0-100) as they make returning to a match rapid. The day-to-day settings are surprisingly reproducible.
Thanks to Neill G4HLX for loan of a KW EZ match and Derek M0BNZ for a meter!