REbuild the Z-match ATU

A couple of years ago I built this Z-match ATU mainly just to test and see how the Z-match works. It was done in a hurry so there were a few thing I don't like in the build. All the components were pretty much just slapped together to get it working. The plan came from several different sources. If you are not familiar with Z-match ATU or you are planning to build one that handles 100W with reasonable impedance please read the following articles, it sure helps a lot!

• AD5X Z-match
• WB3GCK Z-match tuner
• G3VGR Z-match tuner
• Also the article by VK5BR
• Stockton type SWR bridge by KC8AON 

Z-Match ATU built in 2014

Even though it has an SWR bridge built in showing the reflected and forwarded power but the LED indicating light just won't give me an accurate reading on exactly how much power I am seeing. So it's time for giving the ATU a facelift!

Stockton Type SWR bridge

Planning
I began with sourcing a large enough enclosure to house a analog meter as well as all the other components. And I can across this enclosure on ebay. As well as this 100 uA meter. The new enclosure has more height than the old one too so the rotor plates of the 2 gang capacitor will stay away from top cover unlike the old one. The rest of the arrangement of the ATU circuit is almost the same except the Lo/Hi impedance output switch.
For this build I chose to move the switch to the front panel so I can avoid reaching back to the ATU every time I want to switch the output coil. By achieving so I used a 2P2T rotatory switch with ceramic insulator.
Other than that this built will include a 100W/10W switch for the power meter. I simply just using a DPDT on-off toggle switch to shunt two 50K ohm resistors(one for refelected one for forwarded) to the meter to be able to get into 100W range.


Building

First step is to take everything apart which took me about an hour or so to finish. Lots of de-soldering and unscrewing. The new build began as soon as the new enclosure came in the mail. It took about 7 days to get here from HongKong. First step is to arrange the inductor and the variable capacitors. once these components find it's place I then started putting the meter in place. Here's a picture of the final arrangement.

Final arrangement of all the components

I highly recommend this enclosure, both front and back panel as well as top and bottom cover are removable. It just makes things like measuring and drilling much much easier. The only struggle in this build was to cut open the front panel so the meter can be mounted. But in the end it was done ok.
One last improvement I made to the ATU circuit was to add a fixed value (430pf 500V) mica silver capacitor to the input capacitor. And made it switchable by using a DPDT On-Off-On toggle switch. (see fig1.)

Picture from WB3GCK's website  (fig. 1)

This build took me about 20 hours from start to finish, which includes taking the old unit apart then calibrating the power meter and found out the shunt resistor value etc. But it was well worth the time. The ATU handles 100W and matches the Loaded OCFD Antenna on all bands from 6-80m. A big swinging needle meter is much easier to read.

Big Swinging needle!

Meter showing roughly 3W of forwarded power on 80m.

Instead of just being a test build this ATU has now permanently sitting on top of my Yaesu FT-847. And the old T-match MFJ-941e has to find a new home.

Here's the closeup details of what's inside the enclosure. Top left: Inductor, Top right: Rotary Switch for Hi/Lo impedance, Bottom left: SWR Bridge, Bottom right: 6:1 reduction vernier drive.

TNX ES 73 DE KD2AVU