Sec 12.1: Introduction

This chapter is directed to the person who does not have access to sophisticated test equipment and must rely on simple equipment which can be constructed from readily available parts. The material focuses on homemade test gear that can give surprisingly good results. Measurements made on this equipment checked very closely with high-precision laboratory test sets (refs 34, 35, 36). ^[1]

All of the important transformer characteristics can be measured using this basic apparatus. Parameters such as transformation ratios, high- and low-frequency performance, optimum impedance levels for the types of windings used, and the characteristic impedances of bifilar windings can be readily determined. Each parameter can be evaluated with a simple resistive bridge and a general-coverage signal source that uses junction field-effect transistors (JFETs). That elusive parameter known as efficiency can also be obtained satisfactorily by a simple comparative technique described in Sec 12.5. This is an indirect measure of the in-band loss (only 20 to 40 millidecibels). (Direct measurements can be made only with highly complex laboratory apparatus.) As an added bonus, this simple equipment can be effective for measuring the important parameters of vertical antennas, that is, the resonant frequency and the resistance at resonance.

Sec 12.2: The Wheatstone Bridge

The simple resistive bridge, known as the Wheatstone bridge, is shown in Fig 12-1A. The bridge is balanced when no voltage exists between terminals D and B. At this point of balance, no...