[TowerTalk] re:Force 12 Stack

[Pete Smith]

At 09:30 AM 2/7/2000 -0800, Jay Terleski wrote:
>
>
>Ron,
>
>The C4 C3 stack will work fine.  I am guessing you will use the single
>feed line models.  You must compensate the Stack by not using identical
>feed line lengths but feed one with a small delay line length of coax to
>compensate the driven element (sleeves) difference in horizontal spacing
>when pointing in the same direction.

An excellent point, though the offset for the C-4 is a small one.  The
C-4XL requires significantly more.

>
>
>The other thing I would suggest is that you shorten your proposed
>spacing to 27 ft.

Also agreed -- my modeling showed 28 feet, but that's within the margin of
error.  you can find some details, and pattern plots, in my 8/9-95 NCJ
article titled A Preliminary Look at Improved Stack Spacing for Short-Boom
Tribanders.

>
>C3s are short booms and modeling has shown us that 27 feet is about the
>best and widest compromise for gain and pattern.  
>
>Also model the stack to make sure your setting these antennas to obtain
>the pattern you want.  At 100 feet and 75 feet you will have a very good
>DX signal but probably not so good a signal to US.  You might consider a
>third C3 at 50 feet.  Then you could also put the upper one in phase
>with the one at 50 feet and have a very wide spaced stack.  This
>provides high angle lobes for E-skip and F skip close in work.


I have modeled something very close to this, and the results are
disappointing.  Even though the middle antenna is undriven, it seems to
"participate" anyway, to the detriment of gain and F/B.  A better solution
might be a 1X/2X/3X stack (antenna heights of 1/3, 2/3, and the full
tower), with provision for switching among them.  Running all 3 antennas
together does a good job of concentrating power in the first lobe and
cancelling the second, and you could still drive the bottom one only, for
domestic purposes. 

73,  Pete N4ZR

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