Vytenis, LY5T, wrote:
>>> We at club station going to build 80m full size vertical. We have assembled
>>> it on the ground and it is 22m (72,2ft) length. ... If I find antenna
>>> resonates lower than I need, lets say 3440 is there a practice or formula
>>> to calculate how long tip should I cut off?
I'm late to this party but thought I'd throw my hat into the ring of responses.
I happen to have a model of the DXE VA1 80m full size vertical. It has a
total of 16 taper sections ranging from 2.125 to 0.375 inches in diameter. I
"variable-ized" the model, making the base section length adjustable, the tip
section length adjustable, and *all* sections adjustable by a common factor.
Here's an overview of the wires:
https://s26.postimg.org/lsj23rakp/LY5_T-1.png
Total length as shown is 876 inches or 73 feet.
Then I made use of the AutoEZ "Resonate" button, first to find the resonant
frequency with the original dimensions (scale factor 1, tip length 68", base
length 68"):
https://s26.postimg.org/gsllvt4y1/LY5_T-2.png
Let's say Vytenis wants the vertical to resonate at 3.550 MHz. That's a scale
factor of:
3.401 / 3.550 = 0.958
One (impractical) way to adjust the antenna would be to apply that scale factor
to each of the 16 taper sections.
https://s26.postimg.org/s3o9k6bt5/LY5_T-3.png
Sure enough, that moves the resonant frequency exactly as desired. But a more
realistic approach would be to shorten either the tip or the base section by
this amount:
876" * 3.401/3.550 = 839.208" (new total length)
then
876" - 839.208" = 36.792" (length reduction)
Both the tip and the base sections are 68" long so the new section length would
be 68" - 36.792" = 31.208". Let's first shorten the tip (with the scale factor
reset to 1) and see what the new resonant frequency is:
https://s26.postimg.org/e8pz1phe1/LY5_T-4.png
Not bad but certainly not exact. Now what if we shorten the base section (with
the tip reset to 68"):
https://s26.postimg.org/csegjkeh5/LY5_T-5.png
That's much closer. Whether or not it is feasible (with plenty of helpers) to
collapse the second section down into the first with the antenna raised, thus
shortening the base section to achieve the desired resonant frequency, is
another matter. Total weight of the VA-1 is about 21 lb (9.5 kg). When laid
flat (horizontal) the moment at the base is about 500 ft-lbs (678 Nm). That's
a hand full. The one that Vytenis has is heavier.
And of course it goes without saying that nobody is going to adjust an 80m
antenna to the nearest 0.001 inch!
Links:
DXE VA1 80m Vertical: https://www.dxengineering.com/parts/dxe-7580fs-va-1
DXE VA2 (Heavy Duty) 80m Vertical:
https://www.dxengineering.com/parts/dxe-7580fs-va-2
Assembly and instruction manuals:
VA1:
https://static.dxengineering.com/global/images/instructions/dxe-7580fs-va-1.pdf
VA2:
https://static.dxengineering.com/global/images/instructions/dxe-7580fs-va-2.pdf
If anyone would like the AutoEZ format (*.weq) VA1 model contact me off-list.
Dan, AC6LA
http://ac6la.com/
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