[TowerTalk] lp V stepIR

[jimlux]

Jim Brown wrote:
> On Sun, 24 Jan 2010 21:16:53 -0800, Mike wrote:
> 
>> I agree, I am learning something here.
> 
> At the risk of being my usual PITA self, there are other good ways 
> to learn things, like buying a good book and studying it. The ARRL 
> Antenna Book and the new 2010 ARRL Handbook are really quite good. 
> TT members Jim Lux (W6RMK), Steve Morris (K7LXC), and I contributed 
> to the Handbook. 
> 
>
Yeah, but I don't think the handbook covers the esoteric details of 
phased, nonparallel gain antennas, especially over reflective ground. 
Even leaving aside the issues of interaction between antennas, when the 
boresights aren't parallel, simple pattern multiplication doesn't work.

Consider this:
if the two antennas are both horizontal, then straight array pattern 
multiplication works nicely, and you get the combination of the grating 
lobes from the separation and the antenna pattern.
However, if they're skew (the usual scheme with dual LPDAs), then not 
only do you have an interesting combination of amplitudes (e.g. when 
you're at, say, 20 degrees up, the field is the combination of more from 
the bottom antenna (because you're closer to boresight) than the upper 
(where you're more off boresight).  And that's without worrying about 
something like the relative phases, although one can probably assume 
that the phase center moves consistently on both antennas.

I can say from gut feel that a 90 degree included angle is too much 
(unless you have very long booms), because, for example, at the horizon, 
you'll be down a significant amount on BOTH antennas (about 3 dB for the 
usual 90 degree HPBW), so instead of "sharpening" up the vertical 
pattern, it actually makes it broader.

  It's quite an intriguing problem, and I should have some plots done 
tomorrow.