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Q&As Say the Most Basic How-to-Dos Remain Elusive, Part 1

The more things change, the more they stay the same… forever ? A dozen years ago, an E-T-A Circuit Breakers survey found that designers who specify circuit breakers didn't understand the basics of circuit protection. That survey ruffled some feathers. Reader response to my post back then on EE Times (a sister site) expressed doubt and perhaps a little disdain mixed in with some hidden self-embarrassment. But Planet Analog's recent live chats on such subjects as circuit protection and resistors tell me more of this brand of education is still needed. So let's deliver it.

Ever-expanding engineering curricula have future engineers racing through all the technologies in the classroom but may not leave them much time for learning how to apply them at the ground level. Some letters and emails over the past months and years from both beginning and experienced engineers (and as emphasized in a tech note last year) tell me we can never learn too much about making well-reasoned practical judgments.

In this series, we will address four recurring questions that resulted from an April tech note. The questions were asked by some technicians and engineers who I believe readily know the answers — only they haven't thought about them in simple terms for a very long time. You may not be too familiar with a given subject area, but it's not so much about the subject as it is approaching the problem from scratch. So try these — you'll get the idea.

The first two questions appear below.

Can I use coax traps in your five-band antenna?

Perhaps, but I won't recommend them in this no-frills design. Ask this: What's the nature of this component? The coaxial trap, perhaps first described by Robert H. Johns in 1981, is a combination inductor and capacitor made from several turns of coaxial cable wound on a plastic coil form. In practice, the user connects the coax shield at one end to the center conductor at the other (Figure 1), and there are several ways to wire the trap to the outside world (antenna's flattop).

Figure 1

The coaxial trap configuration.(Image: John DeGood)

The coaxial trap configuration.
(Image: John DeGood)

But it's coax, so you have to wonder if the trap is going to perform like one with fixed-value inductors and capacitors. These days, the coax trap is increasingly recognized as an element with transmission-line characteristics. And so one thought that comes to mind is that the coax trap can't provide a fixed value of, say, capacitance over a wide band of frequencies.

That implies the impedance of the coax trap at frequencies sufficiently removed from its resonant design frequency might be far different than for the traditional trap that uses “pure” L and C elements. And our chosen design method relies on simple elements such as fixed-value L and C components.

Some five-band antenna designs using coax traps report success with the addition of compensating sections of wire (roughly analogous to capacitance hats). But in the last reckoning, the coax trap doesn't meet our original design objective for a no-frills approach intended to eliminate extra elements and/or a tedious cut-and-try procedure.

I don't have the space for a straight single-wire antenna. Can I run the antenna in a Z-shaped (or whatever) configuration?

You can try, but don't expect it to work well. If one side of the dipole “looks” at the other to any extent, the corresponding fields from each section will interact electromagnetically. This coupling will likely disrupt the relatively simple impedance profile we've modeled for a straight long-wire “transmission-line” dipole. Beyond that, one might conclude that solving five nonlinear equations in five unknowns is enough of a challenge for the $12 piece of software we're using. Adding a sixth “coupling” variable into the equation set would not be easy to model and could well be beyond the capabilities of the software.

We'll address the other questions in (Q&As Say the Most Basic How-to-Dos Remain Elusive, Part 2).

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