LPDA Bow-tie hybrid with square boom
Image of the LPDA Bow-tie hybrid with square boom.

The LPDA-bow-tie hybrid is - as the name suggests - an antenna combining an LPDA and a bow-tie. This antenna is widely used in EMC (electromagnetic compatibility) testing and is commonly known in the EMC industry as a BiLog, BiConiLog or bicon/log hybrid. The primary reason for combining a bow-tie with an LPDA is to eliminate band breaks in frequency sweeps during EMC testing, thus reducing test time and effort.

At the lower end of the operating band the performance is that of an electrically small bow-tie (the S11 will be very high and the radiation pattern will be omnidirectional). At higher frequencies, the antenna will operate more like an LPDA (a higher stable impedance that is easier to match, with an end-fire radiation pattern).

The LPDA bow-tie hybrid antenna with typical total gain patterns at fmin, fmin_lpda, 2.5fmin_lpda and fmax for an optimum performance.

The S11 at the low frequency end can be improved through various means, such as resistive loading. When resistive loading is used, less elements are needed for the LPDA design. This approach, however, is not feasible for EMC testing as it introduces noise to the measurement setup. For this reason Magus allows two design options: ‘Optimum arrangement’ which uses resistive loading and fewer LPDA elements and ‘Optimum performance’ which has more LPDA elements, but no resistive loading. These two options are illustrated in the figures below, where the impact of the resistive loading can clearly be seen in the gain and reflection coefficient at the lowest frequencies.

Typical on-axis gain versus frequency.
Typical reflection coefficient versus frequency in a 50 Ohm system.