The Ultimate ‘L-Match’ ATU based on an original design by Frank J. Dziurda Jr, K7SFN
Over the years I’ve always kept an eye on the ham radio hardware scene, particularity tuners and amplifiers. I’ve owned an MFJ-989C 3Kw Series-Roller Inductor Tuner since the early 90’s. OK, I’m hearing some people squealing quite loudly but in all and for the outlay, the MFJ has actually done a pretty good job.
I had to basically sort out all the rubbishy assembly issues as soon as it arrived. Things like cold solder joints and MFJ’s poor quality control were expected, but after I’d corrected all of these issues – the tuner actually worked quite well.
They say 3Kw in MFJ’s manual, but it’s actually more rated at about 1500w. In reality, anything over about 600w and the inductor started to complain quite badly and spitting fire. It’s not that I’m running 1500w day in day out but I wanted a unit that would loaf along whatever was thrown at it. Apart from that, the UK license doesn’t allow anything near this, so I was determined that at some point in time that I’d make my own bullet-proof ATU that was better in everything and every area that the MFJ offered.
Looking around the marketplace, there’s not much to choose from in top-end ATU arena and most operators who want ‘Real’ quality end up building their own units. Palstar have a couple of high-end units but they are expensive [£1500+]and both are ‘T-Match’s’ which I didn’t want. I felt I could do a better job building my own.
A few boxes however do stand out. The Ten-Tec 238C for instance along with the James Millen 92220 and some of the LL Tuners by PA0LL [sk] are considered some of the best around.
Tuners come in many variants and there’s more than one design. There’s quite an array from the ‘T’ Match, the ‘Differential Tuner’, the ‘Link-Coupled Tuner’, the ‘LL Tuner’ and the ‘L-Match’ to name just a handful. Every design has it’s plus and minus points. Some are lossy like most of the commercial ones around today. Others have lower losses but also have more hardware installed so cost more.
The ‘T Match’ is widely used mainly because it’s cheap to make, will tune a 6-inch nail and has a wide tuning range. Most commercial ‘ham’ tuners today are ‘T’ Matches. They are also very lossy, especially when impedances drift a long way from resonance. The data looks good on paper, but certainly is not going to be the tuner of choice for aficionados.
Good low-loss tuners are expensive to make and require quality components so a really ‘good-one’ that will handle QRO without breaking a sweat won’t be cheap, but will be the only tuner you’ll ever need. Construct once then forget.
So, after trawling the net for what seemed a decade, I came across a design by US Ham
Frank J. Dziurda Jnr – K7SFN
Frank’s design is a nice low-loss ‘L’ tuner with super heavy-duty components and having much lower losses than the common ‘T’ match. After looking through the design, I found myself making a few amendments, mainly because of available hardware. For instance, Frank uses ‘Jennings’ relays – freely available in the USA but pretty expensive here in the UK. As alternatives, I purchased [back in 2010] 20 x Russian B2B-1B Relays [for silly money] from a ham in Friedrichshafen. These were originally manufactured for the Russian military and I still had 12 in the workshop sitting around doing nothing. Online even today, they look to still be freely available, even from Ukraine for under $30.00 a piece. Additionally, the Russian relays were tested by the US at Los Alamos many years ago and were said to handle about 8kv so about 7kw on 10m and nearer 10kw on the lower bands.
I decided to ‘loosely’ follow Frank’s design, adding my own updates and ‘design license’ along the way and in early 2023 had come up with a first revision. In January of this year, I managed to secure a very nice 10-1000pf English Electric 10kv Vacuum Capacitor from a G3 here in the UK, so that’s one expensive component sorted. I intend to add a 1:1 current Balun at the output and also include a QRO dummy load. Another option is to cannibalise a quality SWR/Power Meter and build it into the unit, but that will be for later. It may also be useful to add more than one coaxial output and put in a balanced feeder arrangement as well. As you can see – it’s a real free-for-all when it comes to what features you want to include.
In the workshop, I’ve plenty of Russian ‘Doorknob’ caps purchased from ham rallies over the years, so we’ll find some to suit – even if they are not exact values.
Next on the main list is an edge-wound variable inductor – so the hunt is on.
Do you have a 20-30uh Edge-Wound Inductor capable of at least 20 amps – if so – drop me a line.
The relays are now sourced and apart from a few ancillary components – we’ll be ready to start building at some point this summer – so that’s where I’m currently at.
Check out the initial schematic below. I’ll be updating this article during the Summer of 2023 as the build progresses – so stay tuned!
Thanks to Frank K7SFN for the additional help so far and giving me the ‘push’ to have a go.