Construction Help - Introduction

CONSTRUCTION HELP

Introduction

Transmission line transformers can be said to be difficult to construct because of the sensitivity of their high-frequency responses to the characteristic impedances of the windings (and hence spacing of the windings) and to the work-hardening of the copper wires upon winding them about a core.

At low characteristic impedance levels, a difference of only 6 mils in spacing can change the characteristic impedance from 45 ohms to 55 ohms. This could lower the high-frequency response from 30MHz to 20MHz. This would not necessarily increase the loss in the transformer since the currents are still of a transmission-line-type and hence flux canceling. If the characteristic impedance is to low, the impedance transformation can become complex (have an imaginary part at the high-frequency end) and become larger in magnitude. If it is too high, the ratio can become smaller. The primary function of the insulation on the wires is to obtain the proper spacing in order to achieve the optimum characteristic impedance of these winding (and hence maximum high-frequency response). The secondary function is for voltage-breakdown improvement.

In the winding process, copper wire (which is normally soft) can become very stiff and difficult to manage. In many cases, the windings have to be rolled-back a quarter or half turn in order to reposition them or to wind them closer to the core. A pair of pliers and a strong thumb are indispensable tools.

This write-up attempts to help the builder to construct transformers that meet the objectives they are capable of. It included the following a) wire preparation, b) winding rod and toroidal transformers, and c) connecting and tapping wires.

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