At 08:28 AM 3/22/2005, Bill Coleman wrote:
>On Mar 17, 2005, at 7:24 PM, Brian Lambert, N1IK wrote:
>
> > The only thing that would be better at my QTH would be a STACK OF
> > STEPPIRS.
>
>Now there's an interesting idea -- have the lower antennas mounted on
>something like a hazer, so they can be raised or lowered as you change
>bands to always have the optimal stack spacing for a given band.....
>
>Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
>Quote: "Not within a thousand years will man ever fly!"
> -- Wilbur Wright, 1901
Or, use 3 stacked antennas (the tradeoff is between the cost of a hazer
(and its mechanical complexity) and another antenna.
Bear in mind also that what counts (to a first order) is the total number
of elements involved. A stack of 3 2-element antennas might do as well or
better as a stack of 2 3-3-element antennas. Look at what the EME arraying
folks are doing or, for that matter, arrays of helices (16 turns on one
helix is about the same as 4 turns on each of 4 helices)
From a directivity standpoint, you get maximum directivity from an end
fire array, but that's a theoretical maximum and implies a lot of
superdirectivity and potentially high losses. If you don't do much
superdirectivity (i.e. the gain is equal to the number of elements), then
some combination of broadside and endfire arrays could have real potential.
Ultimately, with a SteppIR type element (or, for that matter, a fixed
element with a variable tuning network in the middle), you do have a lot of
flexibility, even with fixed spacing. You can change the relative phases
of the top and bottom antennas over the entire 360 degrees, without having
to switch feedlines, etc. The basic approach is like that used in
turnstiles and quad helix antennas. Connect the two together, then tune
one a bit high and the other a bit low (so that the reactance at the
feedpoint is equal and opposite). This is like putting an inductor or
capacitor in series. Yes, there is some reactive current that will
circulate through the feedline, but presumably the feedline is fairly
short, so the loss will be low. You'll also need to fool a bit with the
power dividing problem (hook a pair of 50 ohm antennas in parallel and you
need to convert 25 ohms back to 50 somehow).
Numerical example:
put 50j in series with one antenna (which is 50 ohms) so the reactance is
50+50j.
Put -50j in series with the other: 50 -50j.
The current in one antenna will be 45 degrees lagging, in the other it will
be 45 degrees leading, so they'll be 90 degrees apart.
If you put 100Watts into the system, 50 Watts will be fed to each antenna,
and there will also be 50 watts of reactive power circulating in the two
feedlines. If the feedline has, say, 0.2 dB of loss, in the perfectly
matched case, you'd lose only 4 watts total. with the circulating current,
you'll have 6 Watts total loss (0.3 dB instead of 0.2 dB)
---
Interestingly, 180 degrees is actually the hardest phase shift to get (if
the antennas are purely resistive loads) because you can't do it with a
single C and a single L.
In real life, though, there will be mutual coupling between the antennas,
so you can probably get any relative phase you want.
If you wanted to do the "top only" or "bottom only" thing, you can do that
by setting the desired antenna to a real good match, and massively detuning
the other one (which essentially puts a huge reactance in series, so the
current is small)
And, of course, you could do it as an all parasitic array. Say you have a
stack of 3 antennas, each with 3 elements. You could drive only the middle
element of the middle antenna, and just adjust all the other elements to
generate the right phases to form the beam you want. You'd probably want
to stack fairly closely to make this work (1/4 to 1/2 wavelength), and
there are probably some tricky aspects to getting the phases right, a
parasitic broadside array might have some problems getting the right coupling.
Or, if you wanted to do away with your rotator, set up a pair of antennas
at right angles, and drive them with appropriate amplitude signals to point
the beam in between (you don't want too much directivity for this
approach). Sure, rotators are cheap, compared to antennas, but when you
start talking stacks of many yagis, the rotators start to get pretty expensive.
---
There are lots of fascinating possibilities with continously adjustable
elements in stacks.
Jim, W6RMK
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