Feeder Matching by Means of Tapped Lines

Author: Edmund A. Laport

Figure 4.36 shows schematically a simple method of matching a feeder of characteristic impedance Z₀ by tapping it onto another mismatched feeder of the same characteristic impedance some distance above a short-circuiting bar which is located at a maximum current point in the vertical feeder. The distance above the short circuit to obtain a match depends upon the standing-wave ratio in the secondary feeder. For a very high Q the tapping point will be quite near to the short circuit, and as the Q is smaller and smaller, the tapping point increases in distance.

When the secondary feeder has no standing waves, the main feeder is placed one-quarter wavelength above the short circuit, so that the impedance of the lower quarter-wave section is infinite. This is shown in part B of this figure. This quarter-wave section acts as a filter for even harmonics of the working frequency by placing effectively a short circuit across the main feeder at the tapping point for the even harmonics where this section has an equivalent length of an integral number of half wavelengths for the even harmonics. In Fig. 4.36B there is shown an additional quarter-wavelength section below the short circuit. This is shown to bring out the fact that the one-half wavelength below the tapping point forms a short circuit to ground for any parallel currents that may be flowing in the two wires against ground, while the upper quarter-wavelength section presents a virtually infinite impedance to the desired balanced currents in the feeders.

FIG. 4.37. Location and length of impedance-matching stub line when the feeder and stub have the same characteristic impedance.

In tapping into lines in this manner, care must be exercised to avoid mutual couplings between lines. This is best accomplished by bringing the lines together normal to each other.