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.
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|