Use of balun chokes in small-antenna radiation measurements

Measurements of the free-space radiation characteristics of small antennas, such as in mobile communications handsets, typically suffer from the influence of the attached radio frequency (RF) feed. A sleeve-like balun choke placed on the feed cable close to the antenna under test (AUT) prevents surface currents of the AUT from propagating onto the outer shield of the RF feed cable. Therefore, measurement results with the balun correspond well to those of an isolated AUT, much better than without any measures against cable effects. The balun can typically be used at a 10% relative bandwidth. A dual-frequency balun minimizes the cable-related effects at two separate frequency bands. This is useful in the measurements of dual-frequency antennas. The results of both simulations and measurements for a dual-band balun are presented in this paper, proving the usefulness of the proposed balun and its advantages over alternative means to decrease the cable-related effects.