At 04:41 PM 7/29/2006, Howard Klein wrote:
>Jim wrote:
>*The challenge will be in getting practical experience. Modeling it is
>*simple enough, but with those "extra knobs to twist" it's not clear yet
>*what will prove to be the most useful types of pattern.
>
>Jim, W6RMK
>
>That is exactly the problem for an average ham station with average ham
>instruments.
I think that in the long run, just as with Yagi design, a few hams will do
lots and lots of experiments and try lots of things, and those will filter
out to the ham community at large.
The most challenging part (of an already big challenge) is figuring out how
to manage the effect of the local earth properties within, say, a dozen or
so tower heights. That is, assuming Ham W6XYZ figures out how to make
their totally adjustable stack play at his QTH, how do you transfer that to
Ham W1ABC, with totally different topography, a different tower height, and
different earth properties. The problem is sort of like effectively using
HFTA, but an order of magnitude more complex (since HFTA assumes constant
ground properties, if nothing else).
> In tweaking a four 4 element SteppIR or even more complicated a stack of
> four elements, how can you determine where you are and which
> dimensions will give a desired effect? I've modeled a stack of 2 four
> element SteppIR's best I could and then entered those dimensions into YO7
> Optimizer to optimize, for example, gain. The results for me don't
> necessarily follow my empirical results. It is difficult to know if I
> have improved anything or just screwed it up. No big news but I have
> definitely proven to myself that lowest SWR does not mean best gain. I
> have found that on 20 meters both my four at 45 feet and my four at 85
> feet do better than both. Seems like a phasing problem but on the other
> bands results are as expected.
The trade between SWR and gain isn't unusual. In the general "passively
excited" phased array thing, for a three element SteppIR you've only got 3
degrees of freedom, to try and set 4 independent variables (relative phase
and magnitude of the front and back element current). Usually you wind up
with the choice of getting the element currents in the right relation
(which determines gain) OR the feedpoint impedance nice (SWR), but not
necessarily both. The SteppIR is better than a fixed element antenna, since
you can tolerate a very narrow band design, as long as that design can be
"moved".
But as for testing.. I think that is the real chore. Perhaps effective use
of things like the NCDXF beacons and some automated measurements (think
fancy BeaconSee) is the most practical in the short run. The other
interesting alternative is something like RELEDOP (google for it), which
lets you do a far field pattern of an HF antenna. I can envision doing
something like hanging an instrument package on a big RC plane and flying
it around the antenna under test. This is hardly the casual ham thing, but
hey, there ARE hams out there with time and money to do stuff like
this. Just like hams who have access to antenna ranges to develop antennas
or access to network analyzers for microwave development.
Jim, W6RMK
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