> 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.
This is true. However, the antenna was originally designed for no downtilt,
and all of the elements are in phase. Typical peak gain on the horizon
usually ends up occuring somewhere around 0.9 wavelength spacing (varies
depending a lot of other things, but consider this a nominal value), with
little change in peak gain when varied up or down a tenth of a wavelength or
so. Minor variations between inter-bay spacings will also not shift the
major lobe off the horizon, but it will affect the elevation angles of the
minor lobes (think "null fill").
> 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.
At some point you've gone so far away from the design frequency that what
had been the "major lobe" no longer is the "major lobe". Until you get to
that point, the major lobe will remain on the horizon as long as the
elements are fed in phase. It won't shift up or down. At least it never
has in any array I've ever modeled for two-way, broadcast, or otherwise.
> 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.
A pair of vertical half-wave dipoles spaced a half-wave would be touching
each other. That puts the feedpoints 180 degrees apart. Or am I not
understanding what you're saying...?
> 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.
Yes, but the point is that what had started out as the "major lobe" was on
the horizon, but you've gone so far away from the design frequency to the
point where what started out as the major lobe no longer is the major lobe.
You're talking about a 2X change in frequency - the antenna is no longer the
antenna it was designed to be. We're not trying to use a 220 dipole array
on 440 here, we're talking about relatively minor excursions from the design
frequency when using "commercial" antennas out of band by a few percent or
so.
--- Jeff WN3A
