Log Periodic Stacking Notes

Measurement notes: to ensure fair comparison, all parameters are modeled in free space and forward gain is expressed in dBi.

To establish a single antenna reference, a 13-element (14-30 MHz) antenna was modeled in free space. The forward gain is 7.15 dBi with a F/B ratio of 26.77 dB.

To establish a stacking reference, two identical 13 element (14-30 MHz) log periodics were modeled. Stacking distance is 44-feet apart. The graphic below shows that forward gain on 14 MHz has increased to 10.26 dB, for a stacking gain of 3.11 dBi. The F/B value has degraded to 12.43 dB. This F/B stack degradation is normal with stacked antennas, and often occurs regardless of the antenna type. It is possible to stack antennas without degrading the F/B ratio, but the antennas must be designed from the outset as a dedicated stacked set.  

Two different antennas were then stacked. The 15-element (10-30 MHz) antenna was placed on the top, with the 13-element (14-30 MHz) antenna spaced 44-feet below.

The forward gain and F/B values are essentially the same as stacking two identical antennas, with forward gain now projected at 10.26 dBi, and F/B at 12.78 dB. To provide an equivalent forward gain at 14-MHz would require an optimized monoband yagi of five-elements on a 48-foot boom [1].

Antenna file downloads:

Download stacked 15 and 13 element log periodics in Nittany .NWP format

Download stacked 15 and 13 element log periodics in *.NEC file format

Notes

1. R. Dean Straw, ed., The ARRL Antenna Book, 18th Ed. (Newington:ARRL, 1997), pg 11-18. Figure 16 illustrates the forward gain of this optimized yagi to be about 10.5 dBi.