How to Use a Semi-Automatic Antenna Tuner

There are manual antenna tuners and there are automatic antenna tuners – but what is a semi-automatic antenna tuner? I’ll explain in a moment, but first a bit of background.

For years I have relied on resonant antennas for field portable radio operations. Why carry any kind of tuner around if you don’t have to? Tuners (which should more properly be called “antenna matching units”) add weight that has to be backpacked into a site. Some also consume precious battery power. But if the antenna is resonant – let’s say it presents an SWR of 2:1 or less – we can leave the tuner at home.

How important is low SWR?

The first law of thermodynamics can be condensed into a simple statement: energy can neither be created nor destroyed, only converted from one form to another. In ham radio terms that means radio frequency energy from the transmitter that is not radiated will be converted into heat energy. How much precious RF is lost to heat?

At 2:1 SWR we lose about 11% of our RF. For a QRP transmitter (5 watts) the loss is a little more than half a watt. At 3:1 SWR the loss is 25% or about one and a quarter watts. When propagation conditions are good that shouldn’t make a whole lot of difference. But try to convince me of that when I am struggling to hear and be heard during deep QSB (signal fading) or high QRN (static) caused by the arrival of a Coronal Mass Ejection from an angry Sun!

So Why in the Name of all the Ancient Norse Gods Did I Decide to Use Non-Resonant Antennas?

Seems counter-intuitive doesn’t it? Why condemn myself to carrying extra bulk and extra weight around on portable operations? A lot of hams operate from inside the comfort of a vehicle, or on a nearby picnic table or park bench. I also do that from time to time, but I prefer to hike down a trail and find a secluded spot in the backcountry with just the odd curious black bear for company. Ok, I could live without the bears but they are a fact of life in most of North America so I am constantly on alert in case I come across one on the trails.

The fact is, the new solar cycle 25 has opened up the higher bands and made band-hopping a desirable option. So, as I have written in recent posts, random wire antennas have earned a place in my antenna bag.

Blowing the Dust Off an Old LDG Z-11

For quite a few years I have had a vintage LDG Z-11 tuner sitting on a shelf in my shack. I once actually owned two of them but parted with one in a trade with a fellow ham. I dusted it off and added it to my “QROp” field radio – a Yaesu FT-891. The coined term “QROp” refers to the power leveraging available with a field portable 100 watt radio. If conditions are good I’ll start at 5 watts, but with extra horsepower available if needed.

That extra horsepower had to be called upon during a recent POTA activation. I was doing fine business filling my log – including a European DX station – when along came a G1 class storm from an arriving Coronal Mass Ejection, pushing the planetary K-index up to 4. QSB and QRN made further contacts very difficult. I pushed the power up to 20 watts, then 35 watts and finally up into the nosebleed zone of 50 watts. I completed the activation but it was a very difficult one.

My trusty old LDG Z-11 tuner is a QRP unit but can be pushed into handling as much as 60 watts for CW operations. Beyond that is the smoke zone! The Z-11 has been superseded by the Z-11 Pro (I own one of those too) and the Z-11 Pro II. The Z-11 has some quirks. In fully automatic mode it has a very relaxed attitude towards SWR that I am not entirely comfortable with. According to the manual the Z-11 will only wake up and go to work if the detected SWR exceeds 3:1.

NB: Later models in the LDG Z-11 family permit user selection of the threshold SWR to trigger automatic tuning. For my field operations radio I work with what I have!

Fortunately the Z-11 also has a manual mode. To operate this automatic tuner in manual mode involves applying a small amount of RF and then briefly pressing the “tune” button. This action tips the internal processor out of bed and off to work. Unlike me, when I first get out of bed in the morning, the Z-11 hits the road at a fast sprint and tries multiple combinations of inductance and capacitance until it finds the best match – and it doesn’t settle for 3:1!

“There are 10 kinds of people – those who understand binary notation and those who do not.”

The Z-11 is an L-match. It can be switched between L-C and C-L modes for high or low impedance antennas. Hi-Z/Lo-Z switching can be selected manually from the front panel or left to the internal processor in automatic mode. There are 8 inductors and 8 capacitors that are tried in binary fashion. That means some combination of one or more inductors/capacitors provides 2^8, which is 256 possible values of each. Somehow LDG claims a possibility of over 30,000 combinations. I can’t follow their math on that so I’ll take it on faith. Selecting a match takes up to 3 seconds – that’s 10,000 possible L-C values per second! I also have a home-made L-match manual tuner that can do the same job – but not at that speed!

So a “semi-automatic” tuner is an automatic tuner where the operator takes control and decides that 3:1 just ain’t good enough. The slick little processor inside the innocuous black box then does the heavy lifting. It works like gangbusters. The Z-11 can tune all the bands from 20m on up when hooked up to my Rybakov or any random wire antenna. That gives me a lot of flexibility to switch bands quickly during a field operating session.

The Yaesu FT-891 is a great little radio but it doesn’t have an internal tuner. It was designed and built for portable or mobile operations so the form factor is compact. A compact form factor doesn’t leave room inside the case for extra features like batteries or an automatic tuner. Is that a drawback? In one sense it is, but internal antenna tuners often have a limited tuning range of around 3:1. It is seldom a good idea to use an antenna that is far from resonance but it is useful to be able to tune over a wider range. That is where a device like the LDG Z-11 shines.