Examining the Johnson Matchbox ATU

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Walt, at 90, Now 92 and licensed 78 years

Examining the Johnson Matchbox ATU

« on: May 11, 2010, 11:45:46 AM »

Examining the Johnson Matchbox ATU

The Johnson Matchbox comprises an unbalanced input section, link coupled to the inductor, thus providing an excellent conversion from the unbalanced-to-balanced balun function; two balanced L networks that perform the impedance matching function; a main tuning capacitor in parallel with the inductor, forming an LC tank circuit that is tuned to resonance at the operating frequency; a second capacitor having four sections, all on the same shaft. The two outer sections of the second capacitor are the output capacitors of the balanced L networks that provide the balanced output terminals. The two inner sections are simply two capacitors in series, connected in parallel across the output terminals, and thus are also connected in parallel with the input of the balanced feed line.

The Matchbox is an outstanding antenna tuner (ATU), and has an excellent decades-old reputation as a low-loss tuner.

However, because the designer(s) didn’t fully understand the function of the inner two sections of the four-section capacitor, they misled the users concerning the function of those two sections. The instruction manual for the Matchbox tells us that the four-section capacitor is a voltage divider, and thus performs as an impedance divider, which is why they included it in the design. Big mistake, because it in no way performs as an impedance divider. The inner two sections are unnecessary, perform no useful function, and are in fact superfluous, and can be disconnected with no harmful results.

We’ll now examine why the inner sections perform no useful function, but actually detract from the impedance-matching function. We’ll also see why they would perform a useful function if they were on a shaft separate from the two outer sections, allowing adjustment independently from the output capacitors of the L networks.

Because the two inner sections of the capacitor are connected in parallel across the input terminals of the feed line, the output capacitors of the L networks never see the true input impedance of the feed line—they see only the line-input impedance as modified by the capacitance appearing in parallel with line-input impedance. The function of this circuitry is disturbing because the modified impedance the L network capacitors see is constantly changing during the tune-up procedure, because the inner sections of the capacitor are turning simultaneously with the turning of the L network capacitors. Thus, for every change of the L network capacitors during adjustment to obtain the match, the impedance we’re matching to is also changing. It’s similar to a monkey chasing his tail. A match is ultimately achieved, but with no help from the two inner sections of the capacitor.

We’ll now explain why the two inner sections of the capacitor could assist in the matching procedure if they were adjustable independently from the outer sections, the output capacitors of the L networks. We know that occasionally the feed-line length that reaches from the antenna to the tuner presents an input impedance that is outside the range of the tuner. This situation usually leads the operator to change the length of the line so that it does present an impedance within the range of the tuner. However, there is an alternative to changing the physical length of the line to modify the input impedance—changing the electrical length of the line by adding either series or parallel inductors (or capacitors) at the line input terminals. Whether to use capacitors or inductors depends on whether the line is too short or too long. The two inner sections of the four-section capacitor are already in parallel with input of the feed line, so if these sections of the capacitor were adjustable independently of the L network capacitors, some additional impedance range of the ATU would be accomplished by adjusting the inner capacitors to change the electrical length of the line.

I hope the comments above help in understanding the function of the Johnson Matchbox ATU.

Walt, W2DU


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