Michael Keane K1MK wrote:
> At 10:31 AM 3/28/2008, Jim Lux wrote:
>> Nathaniel Lee wrote:
>> > This is probably cheaper than mechanically rotating a rhombic!
>> >
>>
>> Wasn't it Jansky who had the rotating array on railroad tracks?
>
> Yes, it was Karl Jansky (discover of radio noise from the cosmos),
> although Jansky's array rotated on tires from a Ford Model-T in a wooden
> track. It was comparatively lightweight consisting mostly of a wood
> frame that was only about 20 feet tall (although 100 feet across)
>
> The antenna itself was an 8-element Bruce array plus reflector for 20.5
> MHz. At the time (1930), it was the largest steerable antenna in the world.
>
> Janksy's colleague at Bell Labs, Edmond Bruce, not only invented the
> array that is named after him but the rhombic as well.
>
> 73,
> Mike K1MK
>
> Michael Keane K1MK
> k1mk@alum.mit.edu
>
So the Jansky array isn't going to hack it for top band. Google also
turned up some SW broadcast curtain arrays with towers on train tracks,
which are probably a bit larger (39 and 41m bands maybe).
Let's see.. you don't want the track to have too small a radius because
it's hard to bend the rails. Let's say, a couple hundred meters in
diameter. That would give you room for 8 bays across for 40m, and 2
bays for top band. Vertically, you'd need 150 ft or so for 40m, but
that would be doable. (Since there are road-able construction cranes
that go that high). A few engineering challenges, but, overall, not out
of the question.
Mind you, me being a phased array kind of guy (move photons not metal),
I'd go for 3-4 towers with the dipoles strung in two axes, and then
electrically rotate it, but I realize that doesn't have the sort of gut
level satisfaction of just picking the whole giant antenna up and moving it.
Which brings up an interesting idea. What if you mounted two SteppIRs
at 90 degrees to each other, in a fixed orientation. Could you
electrically steer the beam in any direction, *with the same
performance* you could get from a rotator? A big burly rotator,
bearings, masts, etc, could cost as much or more as the second antenna.
And, can you do the steering with ONLY the element lengths, hooking the
two feedpoints in parallel. I think so, but I'm not sure.
By the way, the original post (and the subject) is world's biggest Yagi,
and while this might be the biggest HF yagi array on one tower, I'll bet
it's not the biggest yagi. There was a monster 20m Yagi that someone
built about 20-30 years ago, and, of course, when you go to arrays, you
look at things like W5UN's EME array.
And then, there are a variety of non-rotatable 75/80 and 40m Yagis made
with wires suspended from cables between towers, as well as all manner
of broadcast SW log periodics (granted, not a yagi) that are
substantially larger than this beast.
Jim, W6RMK
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