Broadband Microstrip-to-Coplanar Strip Double-Y Balun

NASA’s Jet Propulsion Laboratory

A new version of the double-Y balun, transitioning from an unbalanced microstrip to a balanced coplanar strip (CPS) line, has been designed to feed a complementary spiral antenna with an input impedance of 100 Ω. Various versions of the double-Y balun have been investigated in previous literature for use with balanced mixers and pulsed antennas. Of the previous versions, the double-Y balun transitioning from a coplanar waveguide (CPW) to CPS was found to exhibit the widest bandwidth of operation while having little metal content (attractive for use in ground-penetrating radar applications). However, the double-Y balun transitioning from a CPW to CPS requires coplanar waveguide bridges at the junction; the inductive behavior of the bridges, in addition to CPW parasitic resonances, degrades the passband performance of the balun.

Figure 1. The New Double-Y Balun, shown here in top and bottom views, is designed to feed a 100-Ω complementary spiral antenna (balun is manufactured on 0.635-mm thick substrate). The panels on the right are expanded views showing greater detail.

The new double-Y balun transitions from a microstrip line with truncated ground plane to a CPS line. The balun does not employ CPW lines; hence, CPW bridges are not required at the junction. In addition, the balun does not exhibit CPW parasitic resonances, thereby improving passband performance.

Figure 2 Preliminary VSWR and Insertion Loss Data: (a) Plot of measured vs. computed VSWR is shown for the double-Y balun in Figure. 1 terminated with a 100-Ω load resistor and (b) a plot of measured vs. computed insertion loss is shown for the balun in back-to-back configuration.

Figure 1 illustrates the new version of the double-Y balun designed to feed a complementary spiral antenna. Panels on the right illustrate an expanded view of the balun junction. Preliminary voltage standing-wave ratio (VSWR) and insertion loss data are illustrated in Figure 2. Measured data were compared with numerical results computed using Momentum. It is seen that the balun exhibits a VSWR of less than 1.5 from 400 MHz to 8 GHz and a VSWR of less than 1.8 up to 13 GHz. The VSWR can be reduced further by reducing reflections from the balun junction and load resistor. Also, the balun is seen to exhibit an insertion loss of less than 1.5 dB up to 12 GHz. Further work involves characterizing the balun’s performance when feeding a complementary spiral antenna.

This work was done by Jaikrishna Venkatesan of Caltech for NASA’s Jet Propulsion Laboratory.

NPO-42763