CHAPTER TWENTY-SEVEN

The Lecher Wire

Up to this point, we haven't devoted an entire chapter to any particular device. It is much preferable to present principles instead, which allows one to explore the practical applications at one's leisure (we would hope).

However one radio device is so simple, informative, and profound in its native state, that we feel it deserves a special place in the Opus of Amateur Radio Knowledge and Lore.

This device is called the Lecher Wire, or Lecher Line. It was the creation of a rather obscure German electrical engineer by the name of Ernst Lecher. We know a lot more about his invention than the man himself.

There are a few things we can guess about the man, however, and one of these is that, in all likelihood, poor Ernst is spinning in his grave.

The reason for this is because, if you do an Internet search on Lecher Wire, you will be inundated with all kinds of bogus, tree-hugging, posy-sniffing, tin-foil-hat-wearing, Mother-Earth-suckling-hippie buffalo snot about the Lecher Wire being some sort of New Age dousing rod.

Let us set the record straight, once and for all. Nothing in Mr. Lecher's writings even remotely suggests such an application of this fine, highly-scientific, respectable instrument. The fact that a bunch of loony-bin losers have latched onto this device and commandeered it for their own miserable existence in no way detracts from the instrument's true brilliance and simplicity.

The Lecher Wire in its basic form is nothing more than two parallel wires. Its purpose is to demonstrate electromagnetic wavelength. It does so in the simplest means imaginable, and yet, if properly interpreted, can eliminate almost all misunderstandings about transmission lines, and a great number of those associated with antennas.

For a Lecher wire to be effective, it needs to be at least a half wavelength long at the frequency of interest, though it's best if it's several wavelengths. Generally, a practical instrument consists of a six to eight foot plank with a small riser at each end, and couple of 8 or 10 gauge copper wires stretched between the risers, separated by an inch or two. You feed one end of the parallel lines with some R.F. from an oscillator or small transmitter. Most classroom setups use a frequency in the 450 Mhz range. A voltage detector, usually a germanium diode and a microammeter is slid along the line, and the voltage is noted. (The microammeter, being many times higher in resistance than anything we're measuring, is effectively a voltage detector, though if you're really persnickety, it's still a current detector). If you place the detector at the far end of the lines, you will have maximum voltage. As you slide the detector toward the generator, the detector will decrease until, at some point, it will read nothing. If you continue to slide the detector, the voltage will rise again, and then drop to another zero-voltage point. The difference between these two zero points is precisely half a wavelength. As mentioned above, if your Lecher Wire is more than a few wavelengths, you can find several more nulls in your detector voltage.

What happens if you put a short circuit at the end of the Lecher Line? Well, obviously you will have a voltage null at that end. As you slide the detector toward the generator, you will see a voltage maximum at one quarter wavelength from the end, and another null one-half wavelength from the end.

What we have demonstrated is standing waves, arguably the most unnecessarily mystified concept in all of “hamdom.”

Now, we have demonstrated what happens on a transmission line with two extremes: an open end, and a shorted end. Both of these conditions give us an INFINITE standing wave ratio, that is, at some point along the line the voltage goes to ZERO.

Now, let's put a resistor at the far end of the Lecher Wire of some value other than 0 ohms, and run the experiment again. Unless we really got lucky and grabbed just the right value of resistance, we will find that our R.F. detector changes value as we move it along the line, but we will find that the value never drops to zero. The voltage minimas will still repeat themselves at precisely half wavelength intervals, but they will be shifted relative to our open-ended, and short-ended situation. As we slide the detector along, the ratio between the maximum voltage and our minimum voltage is the standing wave ratio, VSWR.

Now, there's one factor we've not touched on yet, and that is the characteristic impedance of the Lecher Line. Let's say in the last experiment we grabbed a 1000 ohm resistor. Let's say that the minimum voltage we found along the line was 1/3 of the voltage maximum we found along the line. We know we have a standing wave ratio of 3:1. Our transmission line characteristic impedance is either 3000 ohms or 333 ohms. (A little experience tells us that the characteristic impedance is more like 333 ohms, with typical bench type Lecher Lines. If we guessed wrong, we will find out soon enough). Based on this guess, let's rummage through our parts drawer and find a 330 ohm resistor. (It's not too likely you're going to find a 333 ohm resistor). Let's replace our 1000 ohm resistor with our 330 ohm resistor. Now we run the experiment again. We find, much to our astonishment, that the detected R.F. voltage is the SAME EVERYWHERE! We have a condition known as a flat line, one with a VSWR of 1:1. And we now know that the characteristic impedance of the Lecher Line is 330 ohms. (Plus or minus a few). If we had guessed wrong, and put a 3000 ohm resistor on the end of our Lecher Line, we would have measured a VSWR of 9:1.

Now, obviously, you can't determine wavelength on a perfectly flat Lecher Line. This is one condition where a perfect 1:1 VSWR match is not a good thing. As long as the VSWR is something other than 1:1, you can find the wavelength by looking for nulls. But the lower the VSWR is, the shallower the nulls are, and therefore harder to find.

Now, my favorite Lecher Wire is one I built using a couple of half-inch copper pipes. Instead of having the ends of the lines fixed, I cut a couple of long horizontal slots in my end risers. I can slide the lines in so they can almost touch each other, or move them apart nearly a foot.

The characteristic impedance of a Lecher Line, (or any parallel transmission line) is lower when the lines are fat and close together, and high when they're skinny and far apart. My high-falutin' Lecher Wire has an adjustable characteristic impedance of between about an ohm and about 6000 ohms. I can also taper the impedance, by scooting one end close together and the other end far apart. It's an amazingly versatile tool for showing off all kinds of R.F. principles. It's such a cheap and dirty project that every ham should have one. It really takes the mystery and baloney out of transmission line theory.

Now, in addition to the Lecher Wire, we have a close companion, the Smith Chart. I hear groans of agony from even seasoned engineers when I invoke the name of Smith, and it's all so unnecessary. The Smith chart is only as complicated as you make it. Lots of Smith Charts have “bells and whistles” you'll probably never use, but the fundamental principles of the Smith Chart are not beyond the grasp of any interested ham. In fact, if you have a Lecher Wire handy, the Smith chart quickly becomes quite intuitive. If you work with transmission lines at all, you'll wonder how you ever lived without it.

The fact of the matter is, if you did the above experiments, you have already seen the bulk of what the Smith chart can show you. However, in addition, the Smith chart can predict what the transmission line will do with various complex loads, that is, loads that have both resistance and capacitance or inductance at their far ends. (You can also use the Lecher Wire and R.F. probe to analyze reactive loads, but it's a bit trickier).

We will not take valuable space here to give a tutorial on the Smith chart, as the ARRL Handbook, the ARRL Antenna book, and numerous Internet sites do a marvelous job of this. It's well worth the time to beat this wonderful chart into submission, so that it becomes a tool, not a terror.

If you are a ham for any length of time, you will sooner or later hear all kinds of unmitigated balderdash, alternatively known as “B.S.” when it comes to transmission lines. With your Lecher Wire and your Smith Chart, you can shut the mouths of the idiots forever, and astonish them in the process. Try it; you'll like it!

The Opus of Amateur Radio Knowledge and Lore 3