At 07:27 AM 3/19/02 -0500, N2EA wrote:
>
>Thanks to all who provided the YCCC link for spitfire
>array info. Turns out to be mis-identified by me.
>
>What I'm contemplating is the K8UR Sloping Dipoles Square Array,
>as described by ON4UN, in Low Band DXing,(1999), page 11-77 et. seq.
>
>In this case, full sized dipoles are sloped down from the tower,
>to the feedpoint/midpoint, and then fold back toward the tower base.
>They reach the tower a couple of meters above ground.
>
>I was initially contemplating this for 40, in lieu of a rotary.
>90 degree half power beamwidth, 4.3dBd gain @ 15 degrees elevation,
>in the 80m model provided in the book. Compares favorably with
>2 el yagi numbers, for the 75', low-load installation. The
>tower is mil-surplus, permitting install-deinstall in less than a day.
>Consequently, can be easily moved to another site.
What sort of tower is this? The only mil surplus tower I know of that can
be moved so readily is the AB-577 (I think that there's also a taller big
brother).
Something I should have mentioned that may not jump to mind, concerning
this type of array. The best way to implement it is probably as a part of
the guying assembly, but for 80 meters that requires a tower at least 100
feet tall. If you can't get the height, then you have to pull the elements
out further horizontally. There, the tradeoff is between the size of the
circle required for the tiedowns and the height of the attachment point on
the tower. Mine is attached at about the 85-foot lervel, and the required
circle is at least 200 feet in diameter. This is not a restricted-space
system.
>
>Upon reflection, I now realize that it may be possible to implement
>this on 75/80, with what I'm planning. Structural issues will gate
>the project, for the next few weeks, so I'm going to shelve the 40m/80m
>solutions decision for a bit.
>
>Meantime, I'm off to model stacked LPDA's vs. hamband dedicated yagis,
>while I work on the tower rotation challenge.
You will discover that modeling stacked LPDAs is fundamentally parallel to
modeling stacked tribanders -- the trick is to come up with a good
compromise spacing, based on the state of the sunspot cycle and the
characteristics of the stack. Given where you are on the project, I
suggest that your best compromise would shade toward the lower half of the
frequency range. My tribander stack uses 28-foot spacing, which is a
compromise between 20 and 15, shading toward 15, with short-boom yagis.
73, Pete N4ZR
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