Hybrid biconical/log-periodic EMC antenna

Known by names such as Bicon, Biconicalog, Biconilog, and Bilogical, the hybrid biconical/log-periodic antenna cuts EMC emissions and immunity testing time compared to using separate antennas—with a hybrid, engineers don't have to stop a sweep to change antennas. The hybrid antenna operates at frequencies from approximately 25 MHz to at least 2 GHz; the biconical portion covers frequencies from roughly 25 MHz to between 200 MHz and 300 MHz, while the log-periodic portion covers frequencies from around 200 MHz to 2 GHz.

Figure 1

Typically, biconical EMI antennas use element angles at roughly 30° to 35°. Military EMC engineers originally defined that angle, and manufacturers stayed with it until recently. Now, manufacturers use other angles. The EMC industry has settled on 19-in. elements, which limits the low frequency response of the antenna to about 25 MHz. Longer elements will reduce the low-frequency end, but shielded-room sizes place practical limits on antenna size.

The log-periodic antenna consists of a series of dipoles of increasing length. The ratio of length between one dipole and any adjacent dipole is the same. Because of those lengths, only one or two of the dipoles respond to any transmitted or received frequency. Shorter dipoles—those for use at frequencies higher than the active elements—direct the signal to the active elements, while elements longer than the active ones reflect the signals back to the active elements.

Because the biconical and log-periodic antennas operate at different frequencies for the most part, they don't interfere with each other. But some overlap frequencies excite both antennas. Antenna manufacturers calibrate each antenna and give you a plot showing the antennas' frequency response.