f = 18.0 MHz 13.06 dBi Azimuth Gain @ 23° Elevation,Why? A solitary dipole has its lowest take-off angle when situated lambda/2 above ground. the advantage of this position is that the radiation pattern consists of only two lobes. As height-above-ground increases, a third lobe begins to develop, aimed straight up. Higher yet, and this third lobe will split flatten out, then split into two (for a total of four). At infinite hight, the lobes of a dipole will be many.
Beam antennas likewise exhibit this same property, but to a significantly lesser degree. And equally likewise, a beam antenna can enjoy increased gain via ground reflection. Said benefit is strongest (same as with a dipole) when situated lambda/2 above ground. A quandry, then, for wide-band antennas such as log-periodics, since lambda/2 varies throught their range. Tilting the boom will solve this dillema, but comes at a cost in gain (since the antenna is pointed ground-ward).
Plots below quantify the trade-off of so doing for an LPA whose boom measurs 16m. For a longer boom, loss-of-gain would be less, owing to a shallower tilt. With a shorter boom, the opposite would result.
Effect Shown: Splitting and narrowing of main lobe into two as frequency is increaed.
f = 18.0 MHz 13.06 dBi Azimuth Gain @ 23° Elevation,
f = 19.0 MHz 13.06 dBi Azimuth Gain @ 22° Elevation,
f = 20.0 MHz 12.9 dBi Azimuth Gain @ 21° Elevation,
f = 21.0 MHz 13.19 dBi Azimuth Gain @ 20° Elevation,
f = 22.0 MHz 13.45 dBi Azimuth Gain @ 19° Elevation,
f = 23.0 MHz 13.49 dBi Azimuth Gain @ 19° Elevation,
f = 24.0 MHz 13.28 dBi Azimuth Gain @ 18° Elevation,
f = 25.0 MHz 13.41 dBi Azimuth Gain @ 17° Elevation,
f = 26.0 MHz 13.66 dBi Azimuth Gain @ 17° Elevation,
f = 27.0 MHz 13.84 dBi Azimuth Gain @ 16° Elevation,
f = 28.0 MHz 13.72 dBi Azimuth Gain @ 16° Elevation,
f = 29.0 MHz 13.41 dBi Azimuth Gain @ 15° Elevation,
f = 30.0 MHz 13.34 dBi Azimuth Gain @ 15° Elevation,
Effect Shown: Maintenance of single-lobe pattern accross whole frequency range.
f = 18.0 MHz 11.5 dBi Azimuth Gain @ 27° Elevation,
f = 19.0 MHz 11.37 dBi Azimuth Gain @ 27° Elevation,
f = 20.0 MHz 11.12 dBi Azimuth Gain @ 27° Elevation,
f = 21.0 MHz 11.31 dBi Azimuth Gain @ 26° Elevation,
f = 22.0 MHz 11.41 dBi Azimuth Gain @ 26° Elevation,
f = 23.0 MHz 11.33 dBi Azimuth Gain @ 27° Elevation,
f = 24.0 MHz 11.11 dBi Azimuth Gain @ 27° Elevation,
f = 25.0 MHz 11.26 dBi Azimuth Gain @ 26° Elevation,
f = 26.0 MHz 11.41 dBi Azimuth Gain @ 26° Elevation,
f = 27.0 MHz 11.45 dBi Azimuth Gain @ 26° Elevation,
f = 28.0 MHz 11.33 dBi Azimuth Gain @ 26° Elevation,
f = 29.0 MHz 11.14 dBi Azimuth Gain @ 27° Elevation,
f = 30.0 MHz 11.15 dBi Azimuth Gain @ 27° Elevation,