Double-Tuned Impedance Matching

In the 1940s, Harold Wheeler developed the principles of double-tuned impedance matching in a simple form. These relationships are not well known. They are very useful for the antenna designer in providing a structured approach to obtain near-maximum bandwidth for narrowband and moderate-band antennas with a specified VSWR limit, using a practical arrangement of tuning elements. Wheeler first developed relationships for single-tuned impedance matching, which relate the fractional bandwidth to the antenna´s Q and the maximum-permissible reflection-coefficient magnitude (or VSWR). He used the results of single-tuned impedance matching to derive the relationship for double-tuned impedance matching. Wheeler´s double-tuned impedance-matching relationship is the best overall measure of the achievable fractional bandwidth for an antenna. Double tuning is practical, and provides more than double the fractional bandwidth of single-tuned impedance matching. One example of double-tuned impedance matching is presented. A comparison to recently published results for single-tuned impedance matching verifies that double tuning more than doubles the fractional bandwidth of single-tuned impedance matching.