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The effect of a short discontinuity in transmission line characteristic impedance

At the junction in connection of two physically dissimilar transmission lines, the fields are distorted by the geometry, and a small mismatch occurs, even for lines of like Zo. Terman describes this and suggests that it can be represented by an equivalent shunt capacitance, the Discontinuity Capacitance, and that it might be of the order of pF. The effect at HF is usually insignificant, but at 1GHz, 1pF of shunt capacitance in an otherwise matched system causes VSWR of 1.37.

Another discontinuity is where connectors might not have a ‘through’ characteristic impedance the same as the transmission line, and not only may there be sharp geometric discontinuities, but the length of the different Zo section may create significant reflection. Exploring the latter effect, a section of mismatched line in an otherwise matched system changes the impedance looking into the mismatched section. The change for very short mismatches can be thought of as shunting the nominal Zo with:

Implicit in this is that a short section of high Zo line can largely compensate a short section of low Zo line in an otherwise matched system, and vice versa.

For example, if a connector section of length 20mm has a ‘through Zo’ of around 100Ω was inserted in a matched 50 ohm system at 144MHz, the 0.01λ of 100Ω line would approximately shunt the line with about -j0.002S susceptance causing a VSWR of about 1.1. The effect of the same discontinuity on an already mismatched system may be much less obvious. For example, if the load VSWR in the previous case was 1.5, then the worst case degradation by the discontinuity is to VSWR=1.65, but for most load impedances, the effect will much less, even an improvement.

The obsession by some with the discontinuity caused by connectors such as ‘UHF series’ at VHF and below is not rational, though those connectors are poor for other good reasons.


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