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Ham radio hobbyists, as the former term indicate, have been all over the time great technicians, but with exceptions, never professionals on electrical or electronics works and related fields.

There are aspects of the radio where the hobbyists rub the professional turf. One of this aspects is antenna design. Until present, the biggest difference between a professional and a hobbyist was the amount of technology available to them, especially measurement equipment. General knowledge was already at hand, as books and encyclopaedias were available to both.

More in depth on antenna design, both professionals and hobbyist manage similar concepts, like SWR (stationary wave relationship), capacitance or inductance of circuits.

Nowadays, it’s easy to see a professional using expensive equipment, mostly software based, performing complex calculations, but, there was a point in time where those calculations were done manually using simple tools like a Smith Chart.

A bit of history

Smith charts were originally developed around 1940 by Phillip Smith as a useful tool for making the equations involved in transmission lines easier to manipulate. With modern computers and the use of software, the Smith chart is no longer used for the calculation of transmission line equations, however, their value in visualising the impedance of an antenna, or a transmission line, has not decreased.

The Smith chart is a graphical calculator, or nomogram, designed for electrical and electronics engineers specialising in radio frequency (RF) engineering to assist in solving problems with transmission lines and matching circuits. The use of the Smith chart, and the interpretation of the results obtained using it, requires a good understanding of AC circuit theory and transmission-line theory, both of which are prerequisites for RF engineers and, why not, for general radio hobbyists.

The Smith chart is a fantastic tool for visualising the impedance of a transmission line and antenna system as a function of frequency. Smith charts can be used to increase understanding of transmission lines and how they behave from an impedance viewpoint. Smith charts are also extremely helpful for impedance matching. The Smith chart is used to display an actual (physical) antenna's impedance when measured on a Vector Network Analyser (VNA).

Network Analysers

In general, a network analyser is an instrument that measures the parameters of electrical networks. Analysers could be of two main types, Scalar Network Analyser (SNA), that measures amplitude properties only, and Vector Network Analyser (VNA), that measures both amplitude and phase properties.

A Vector Network Analyser (VNA) is a form of RF network analyser widely used for RF design applications. A VNA may also be called a gain–phase meter or an automatic network analyser. An SNA is functionally identical to a spectrum analyser in combination with a tracking generator.

Nowadays, VNAs are the most common type of network analysers, and so references to an unqualified "network analyser" most often mean a VNA.

With the advent of software and the cost reduction on materials crafted on China, entry-level devices and do-it-yourself projects have also been available, some for less than $100, mainly from the amateur radio sector. Although these have significantly reduced features compared to professional devices and offer only a limited range of functions, they are often sufficient for private users, especially during studies and for hobby applications up to the single-digit GHz range.

S-Parameters (scattering parameters)

Scattering parameters, or S-parameters, are popular in RF/Microwave circuit design and testing. Following are scattering parameters used for measuring/characterising an RF device.

From the Figure 1, beside, following two equations are derived:

b1 = S11 x a1 + S12 x a2 b2 = S21 x a1 + S22 x a2

S11 → Reflection coefficient at Port1 S22 → Reflection coefficient at Port2 S12 → Isolation (Reverse) S21 → Insertion loss (passive device case)

Figure 1: S-Parameters

Figure 1: S-Parameters