Z-match Tuner

[Rosy Demorest]

Forthis project, I decided to try my hand at building a Z-Match tuner from scratch. This type of tuner has been around for a while. While the Z-match can take on several variations, what distinguishes it from other circuits is that it is a resonant circuit that uses a fixed inductor.

Z-Match tuners became very popular within the QRP community years back, thanks primarily to articles in QRP journals by Charlie Lofgren W6JJZ and the emergence of Z-Match tuners in kit form. Emtech sold its wildly popular ZM-2 kit commercially and the NorCal QRP Club began selling their BLT tuner kit (a W6JJZ design) like hotcakes.

I make no claims of originality for anything in my version of the Z-match. I based it on a classic design which was first appeared in SPRAT #84 (see the G3YCC web site for a schematic of the original design). This design, by the way, is similar to the one used in the Emtech ZM-2.

The coil was wound using some #22 solid hookup wire (from Radio Shack) which I had laying around. The secondary winding is wound between the turns of the primary to ensure tight coupling. I added a toggle switch to ground one side of the secondary winding to accommodate single-ended loads (like a random wire). A piece of styrofoam was glued to the bottom of the enclosure to provide some support for the toroid and to keep it away from metal surfaces.

Another W6JJZ modification I used was the inclusion of a DPDT (center off) toggle switch to provide some flexibility with the input capacitor. Using this switching arrangement, I can select between one section of the capacitor, both sections in parallel, or both sections in parallel with a fixed 470pF mica capacitor. The extra input capacitance can sometimes be helpful on the lower frequencies.

The SWR bridge I used is a Dan Tayloe LED SWR indicator from a kit that was offered years ago by the Arizona scQRPions. It uses a resistive bridge circuit with a single LED to indicate a null when the bridge is balanced. For the 50-ohm resistors in the bridge, I substituted 2 100-ohm, 1-watt resistors. The bridge will handle a typical 5-watt QRP rig without flinching and could probably handle a bit more than that.

To use the Z-Match, adjust the capacitors for a null in the background noise in your transceiver. That will get you close to a match. Then, switch in the SWR bridge, apply some RF, and tweak the capacitors for minimum brightness on the LED. There may be some interaction between the two capacitors, so you might have to go back and forth between them a time or two.

Looks like L1 is a transformer. I see 12t and 17t for a total of 29t yet I see some additional turns at the bottom that go to ground. Would this be 8t?

I see on the other side, it shows 8t so this is why I assumed 8t. One side is primary and the other is secondary.

Hello,

Sorry for being ignorance. I want to build this. I have the parts. And I already drilled the aluminum case I got from Jameco. It is bigger than I need, but it gives me room. And parts like variable capacitors, switches, connectors, and LED are in the case now. Now, it is a matter for building the toroids and soldering the components point-to-point.

Sorry for being difficult. I have not done a lot of homebrewing. But I need a z-match for my QRP radios I built and are building. But I want to make sure I fully understand before I proceed. The rest should be easy.

Years ago I built the AM-2 Z-Match antenna tuner and found that it could match pretty much any old bit of wire thrown outside. I melted the polyvaricons by running WSPR one day but they were easily replaced.

The metal case is great but it would be wonderful if the front and back panels were pre-drilled. My metal work skills are not great and I found the front and back plates to be quite hard aluminium to drill safely.

Some of the wiring, in particular the links from the panel switches down to the board is pretty tedious. I guess the solution would be a PCB behind the panel to handle that wiring but extra cost would be added by this.

Only after wiring up front and back panels did I realise that you need to slide the board in to the bottom of the case before connecting the panel wiring. I should have tried fitting the case before this but also the instructions should have mentioned it.

I unscrewed the BNC connectors and luckily had enough length of wire to the switches to be able to lift the panel out of the way and slide the board in to the lower part of the case which I had already lined with gaffer tape to try to minimise likelihood of shorts to the case as warned in the manual.

The Z-match is an amazingly versatile tuner.

The T match will also match just about anything if it uses a roller inductor. The tapped coil versions are a compromise but do pretty well. Changing the tap point would probably give a good match in your case.

35 ft is close to a quarter wave on 40 m and a half wave for 20 m. It would not be my first choice of length for an 80 wire antenna. However sometimes we must use a wire that is long enough to fit between two supports and it will present a challenge to resonate.

As indicated my preference would be to use some tuned feeder, offset from the end, to change the reactance values and try to match that. Running twin feed line all the way to the ATU is a simple option but you need to use the Z-match or a 4:1 current balun with the T-match. It is often inconvenient to bring twin feeder inside so putting the balun outside (in a rain proof box) and using coax to the ATU is often done.

What do you need for that?

2 small variable capacitors from old transistor radios with rotary knobs

1 Amidon toroidal core T2-100 or slightly smaller or larger

2 BNC sockets

enamelled copper wire 0.2-0.5 mm diameter and isolated wire

The attitude is sensitive and you need a little patience.

First put C1 in the middle position.

Then find the minimum SWR value with C2. There can be two minima.

Then change C1 and C2 down to the smallest SWR

Add a small switch on the earth side of the secondary coil and a pair of banana plug sockets, one each side of the secondary, and you have a matcher suitable for both balanced and unbalanced antennas.

Switches give you options to match a variety of impedances, across many frequencies. If you understand your tuner, and build it yourself, the advantages of switches are obvious. Certainly too many switches can be confusing, and they may lead to bad matches, without knowledge of their function. If you have too many variables in a system, there may be unexpected consequences.

Unwanted resonances usually cause loss and lead to incorrect settings, with little power going to the antenna. You can often find them using an analyzer connected to the input of your tuner, using very low power. Connect no load to the tuner, or a short circuit, and you can still find a match or matches at incorrect settings. These are caused by unwanted coupling, inevitable series resonances, and other parasitic behavior among the components. They often occur when trying to match a high-Z load near, or above, the upper frequency limit of a tuner.

Significant tuner loss is often caused by using a design that is not appropriate for the impedances involved. Often this is because the taps on the the main inductor are not quite right, or because the secondary circuit has too few or too many turns for an optimum match. This is almost inevitable when using a single antenna or wire on many bands. Switches or manual taps may be needed!

In my experience, the Z-match and BLT-type tuners are very useful and wonderful, but they may not be ideal for feeding high-Z end-fed antennas, like the 20M wires we use for SOTA on 40-20-15M and other bands. The simple tuners I use and have described are easier to tune with fewer issues, and probably have less loss when feeding impedances above 1000-2000 ohms.

Thanks for this Chris interesting method will work through it and try myself! My previous efforts have been based on using transformers to provide the higher impedance load which proved to be very problematic and produced inconsistent results.

But since I learned that my new QDX Digital Transceiver from QRP-Labs doesn't like transmitting in high SWR, I wanted to have something like a ZM-2 or ZM-4, to be able to use the QDX on a piece of random wire, when a perfectly tuned antenna is not available. Probably also useful for my 3 QCX transceivers.

First I mounted all the switches, connectors and the two tuning capacitors (polyvaricons).The BNC connectors were from old Ethernet cards, and were fixed with some drops of superglue to both pieces of the chassis, after which I soldered the PCB's toghether to form an L-shaped chassis.

Then I added the SWR bridge, and finished with the rather complex coil with different taps and extra windings, and connected the tuning capacitors. I basically followed the circuit diagram of the ZM-4 , except that I did not include the 160m band.

And best of all .. I tested the tuner on my "random wire endfed" (9.15m wire with 4m counterpoise + 9;1 UNUN), and I was able to tune it on all bands 80m - 20m. Higher bands were not tested because of lack of time, but probably will work as well.

For portable operations, I was looking for a multiband compact Tuner able to tune both symetrical and coaxial feeders and that would be easy to build. After many searchs on the Web, I finally found few Web pages about the Z-match design (see my "Links" page for some usefull sites and for the schematics).

Additionally, I have incorporated a SWR bridge. This very simple circuit is lighting a LED when the bridge is not in equilibrium. In one leg of the bridge is the load of the antenna (through the tuner) and in another leg, there is a 50 ohms load. When the tuner is well tuned versus a given antenna, the load reaches 50 ohms, the bridge is at the equilibrium and the LED turns off. Very simple system but it can only work for very low power, otherwise the ferrite core, the diode and the load may vanish in great smoke !

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