"No, parallel-fed antennas do NOT suffer uptilt/downtilt as frequency is varied unless the harness was special-ordered for factory downtilt. If the antenna wasn't ordered with downtilt, all of the elements are fed in phase, and they will always be in phase regardless of frequency."
Jeff, the pattern depends on both phasing and spacing. As frequency drops, the interelement phasing, expressed in degrees, remains the same, but the spacing, expressed in degrees or wavelengths, drops. If you model a colinear array of parallel-fed dipoles in an antenna software program, and don't scale the dimensions as you scale the frequency, you'll see the main lobe shift up or down, and "butterfly" lobes appear, as you get a few per cent off-frequency. In an extreme case, a pair of vertical colinear dipoles fed in phase with half-wave spacing has the familiar big lobe toward the horizon. As frequency rises, the pattern degrades until, at a frequency of 2X, it becomes an end-fire array, with most energy directed straight up and down. This happens with no change in phasing or spacing. 73, Paul, AE4KR
