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Analog Angle Article

Simple example shows benefits of coaxial shielding

I recently connected a DTV converter box to my 10-year old, 2-inch Radio Shack pocket TV (discussed here), to provide a small TV monitor or TV “radio” (depending on how you think about it). The connection between the two units was a 2-foot (60 cm) piece of coax cable that I had handy; one end has a Type-F connector and the other now has a 1/8-inch regular audio plug, to fit the TV's “external antenna” input.

The piece of coax I used was only shielded for half its length. For reasons I don't remember, I had previously used it for some other project and stripped away a lot of the shield, although the center-conductor spacing insulator was still there, protecting the core wire. I soldered a piece of wire to the remaining shield, then spiral-wrapped that wire loosely along the center conductor's insulation, and connected it to the ground of the audio connector. Thus, what I had was good electrical continuity for the shield, but poor physical-shield coverage for half the cable's length.

As I used the little TV, it was pretty clear that there was noise and distortion on the screen. Was this due to some sort of signal mismatch or discontinuity through the cable and connectors, and resultant impedance bumps? Was it a weak signal? Or perhaps it was just too much noise pickup, degrading the SNR?

I couldn't really do much about measuring or improving the impedance situation, nor could I easily (and cheaply) boost the signal, which would be contrary to having a low-cost upgrade to DTV reception. But I could do something about the shielding. I bought a 3-foot (1 m) RG59/U coaxial cable with Type F connectors, replaced one end's connector with the necessary audio plug, and gave it a try. The reception was now excellent, steady, sharp, and consistent.

Admittedly, this is not leading-edge engineering R&D, nor have I discovered something that we all don't already know. But it is a clear, causal demonstration of how even a modest amount of shielding can affect analog link performance. Sometimes, you just have to relearn old lessons by going through them again.♦

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