Rudy Severns wrote:
> Pattern distortion is one consideration Rick, but if you're talking about an
> array things get a lot worse.
>
Now you're in my favorite area..
> In an array, besides the obvious distortion possible from adding another
> element (a parasitic element) you will also alter the self impedances of the
> elements. Each one probably a little differently. The result is that the
> feed network will not see the loads its design was predicated on and as you
> try to switch directions the array will no longer be symmetrical. In
> addition to the extra unintended element you will have incorrect element
> excitation in the driven array. That will really give you pattern
> distortion.
Lots of modeling, lots of measuring, and still you won't be able to a
priori predict what's going to happen unless you're set up for a sort of
idealized situation (e.g. RK's nice flat ag field).
If you're in a "cluttered" environment (like, say, a residential lot),
I've come to the conclusion that you can't adequately predict what's
going to happen, either by impedance measurement on the installed
antennas, or by modeling. You have to have something that can
essentially adapt to what the situation actually is as it changes (yep,
turn on those sprinklers, and everything changes).
And that needs some way to estimate the far field pattern.
>
> My thought was that you would want to measure your tower(s) right up front
> before you even made a final decision on a new antenna much less built
> anything. That way you know what to expect and that might change you
> decisions on location, etc. A present people tend to put up the new antenna
> and if they notice a problem (they may not even if it's there!) go to the
> nearest tower and put some kind of skirt on it and tune around until you
> reduce the effect. That can work but I would prefer something a bit better.
>
> As I mentioned earlier, I have a method to make those measurements but it's
> not practical for most hams. That's why I've been asking if anyone else has
> a less complex solution to this kind of measurement.
I think if you're serious about phased arrays, then the few hundred
bucks for the VNA probably is the least of your problems. And, in any
case, the VNA costs about the same as the building permit for the base
for the tower on which one would put a Yagi (at least here in So Cal).
I think there's sort of two classes of phased arrays for amateurs.. the
first is the 4-square or other simple schemes, where you get moderately
improved performance, with an implementation that is relatively tolerant
of component variations and actual feedpoint Z or mutual coupling.
The other class is a serious array, where someone has measured element
currents, probably built custom phasing networks, and for which knowing
the element coupling and Z is important. Rick N6RK, Greg W8WWV, Victor
K1LT, etc are in this class. But for this class, you've already bought
into a fairly complex project, and whether you fork out the $500 for a
VNA widget is just another expense among many.
That said, for the last few years I've been working on algorithms to
(automatically) calibrate a field deployed array, but it's a tough
problem, and, unfortunately, not one where there's a heck of a lot of
literature to look to. It's not a "commercially valuable" application,
either. I did get funding for an R&D project to look at self
calibration of an L-band array, which has some applicability, but in the
HF world, most users of arrays have large budgets (broadcast, HF OTH
radar, etc.) AND fixed installations, so they can spend the time and
money on careful measurements of whatever is needed, including running
far field patterns using slick stuff like RELEDOP.
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