ANALOG ADVENTURES: Bridging the Gap

Technology Special:

analog adventures

BY ERIC P. NICHOLS, * KL7AJ

Ihave always been a fanatic about old / vintage / ancient / prehistoric electrical instruments. After our recent QSY (the physical type, not the frequency type), I “inherited” yet another huge collection of semi-vintage Hewlett-Packard test devices. It will be one of my dark winter projects to restore some of these gems to their former functionality, if not their former glory. I have some friends who are FAR more skilled at cosmetic restoration of boat anchors and such than I will ever be.

One of my recent non-HP hamfest acquisitions is this nice Heathkit RLC bridge (PhotoA), not necessarily a precision instrument, but very useful, nonetheless. I can never resist adopting anything Heathkit.

Now a large percentage of electrical and electronic instruments incorporate some sort of bridge circuit. The classic Wheatstone bridge has countless varieties and variations, but they all perform the same basic function: Comparing the voltages between two different points in a circuit. The RLC bridge in Photo A is no exception in this regard. In my June 2022 Analog Adventure column, “Using Vitamin K, ” I presented a simple bridge circuit as well as a challenge to solve a bridge that wasn’t really a bridge. Here is a nice note (in part) I got from Rick Peterson, WA6NUT:

...You mentioned thatyou’d like to see some solutions to the problem (finding the voltage across R3 in Figure 2). So here’s mine. Back in the day, it seemed like I could alwaysanalyzeacircuitusingThevenin equivalents. So that’s how I analyzed thebridgecircuitinFigure 2. R1andR5 become a 750-ohm resistor from a 9volt source, and R2 and R4 become a 1.333K-ohm resistor from an 8-volt source. Bothresistorsfeedthe5K-ohm resistor. With0.1411765mAofcurrent flow, thevoltageacrossR3iseasilycalculated to be 0.70588 volts. I’vebeenretiredover25years, soIwas surprised that I could still remember how to do Thevenin equivalents!

* 138 Shenandoah Drive Fairbanks, AK 99712 email: <KL7AJ@cq-amateur-radio.com> Photo A. One of Eric’s most recent additions is this Heathkit RLC bridge, which he picked up at hamfest. (Photo by KL7AJ)

Figure 1. A simple circuit to measure radiation resistance. (Image by KL7AJ)

Bridging the Gap

Thanks to Rick for the nice comments, and he indeed came up with the right answer.

Of course, as often happens, reader comments suggest ideas for subsequent Analog Adventures, and this is certainly no exception; we will need to explore Thevenin (and Norton) Equivalents in a near-future piece. This is far too rich a topic to cover in the remaining space this month.

Now if you haven’t worked with a bridge before, one obvious question is, “why would I use a bridge when a simple ohmmeter will do the job?”

This is best answered by giving a common example of when a simple ohmmeter will not do the job: When we need to measure the value of a resistor that is not a resistor. We’re talking about that mysterious entity known as radia-

tion resistance. You just can’t stick an ohmmeter across the feedpoint of an antenna and measure radiation resistance. Nor can you measure inductance or capacitance with such a simple device, but for now we’ll stick with radiation resistance because it’s so interesting.

Figure 1 shows about the simplest method of measuring radiation resistance. (Note: this won’t tell you anything about reactance or resonance; for that we need a more elaborate bridge). V1 is an RF generator, operating at the frequency of interest. R4 is our unknown radiation resistance. R3 is a potentiometer that covers (hopefully) the range of the anticipated radiation resistance. And finally, we need an RF voltage detector between Y1 and Y2. This can be a simple RF voltmeter, or, more elaborately, a receiver with an S-meter.

To use this bridge, connect the antenna to the R4 position, a generator as shown at the frequency of resonance of the antenna, and a detector between Y1 and Y2. Adjust R3 to get a null. When a null is achieved, R3 is the same as the radiation resistance. Of course, it’s convenient if R3 has an actual calibrated dial, but if not, the value of R3 can be measured after the fact with an ohmmeter.

Pretty slick, eh? Well, I think so, but then again, I’m easily impressed.

Now, here’s a challenging homework problem. If the antenna is NOT precisely at resonance, will this bridge still tell you the correct radiation resistance? Why or why not? Like many such problems, the answer can be deceptively elusive. Don’t answer hastily. P.S: It’s not cheating to actually build the circuit and see.

If you go back through ancient electrical engineering literature, you will find virtual libraries of complex bridge equations; this was one of the most advanced topics in the fledgling field of radio technology. And unfortunately, a lot of this ancient lore has been lost. Of course, with modern devices like really cheap and plentiful VNAs you don’t have to know all the math, but you should be aware that even the most advanced such instruments are bridges at their core.

Revisiting our beloved Heathkit RLC bridge ... there were a number of similar devices available; one of the more precise (and pricey) instruments of the ilk was made by Leader. These RLC bridges are not actual RF bridges; they have internal oscillators operating at a couple of kilohertz. They are fine for determining component values at moderate frequencies, but if you work with mainly radio frequencies, you will want something like a noise bridge, at the very least, and sometimes a little something more.

Which brings us to another interesting topic. How do you accurately measure extremely low resistances, say, on the order of 0.001 ohms? You certainly can’t do this with your average digital multimeter (DMM), or even an above average DMM. The answer, not too surprisingly, is another bridge, in this case a Kelvin double bridge. We’ll go into the Kelvin double bridge in some detail in a future article.

Just Compensation

Aside from the bridge’s ability to measure quantities that just can’t be measured any other way, the bridge can make better measurements of quantities that can be measured by simpler methods. One of the inherent properties of a bridge is that it can cancel out certain errors. For instance, we know that many circuits can be temperature sensitive, which is not generally a good thing (unless we’re specifically attempting to measure temperature). However, a bridge circuit can be used to perform temperaturecompensationin a number of interesting circuits, which we will explore in detail before too long. Notice that those digital folks don’t think about these things very much.

Neutrality

Most of us hams of a certain vintage have had to deal with neutralizing RF amplifiers. Neutralization is generally lessof an issue in solid-state amplifiers, but is not entirely non-existent. The neutralizing circuit of a vacuum tube RF amplifier is actually a bridge. It may take a bit of redrawing the schematic to recognize it as such, but it is one, nonetheless.

And More

We should close by mentioning just a few more applications of bridges in passing. One of my favorite bridges is the noise cancelling antenna box, which has become increasingly popular in recent years. Fortunately, I live in a very quiet environment, but I have built a few of these in the past, particularly for nulling out certain AM broadcast stations. Another common bridge is the Wien bridge, used in just about every true sine-wave oscillator. HewlettPackard’s very first instrument was actually a Wien bridge, using a small incandescent lamp as a controlled current source. Needless to say, the rest is history; this was the genesis of Silicon Valley ... or rather, Vacuum Tube Valley, at the time.

One of the most interesting ... and ancient ... bridges is the Varley bridge, used by phone companies to determine the distance to a telephone line fault with a surprising level of accuracy. On a telephone line a couple of hundred miles long, the Varley bridge could narrow down a short to within three or four standard spaced telephone poles. Nowadays, time-domain-reflectometry, TDM, is used to perform such tasks.

Coming up next ... I will show you some of my restoration projects in progress ... so many boat anchors ... so little time!

Until then, keep those soldering irons hot! – 73, Eric

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