[TowerTalk] Force 12 C31XR Modeling

[w5wz@w5wz.com]

David,

I can't help you with the modeling; however, I can give you some insight 
on converting the F12 C-31XR/C-51XR from single to per-band feedlines.
I have a 3-high stack of C-31XR, each band with its own feed line.  When 
I got the antennas, they were all single feed line.  I assembled and 
checked out that way first, then did the conversion to multiple feed 
lines.
The conversion from single feed line to per-band included:
1. A driven element must be repositioned to the other factory-installed 
element-to-boom bracket.  I think it is the 15m DE.
2. The tips of the same DE must be changed - I can't remember now if 
they are shortened or lengthened - it is whatever the manual indicated.
3. Lose the ROMEX to the same DE, replace with a CAL-AV Labs bead balun.
4. Either make and install or modify the coil hairpin across the same DE 
feedpoint.  Again, no recall, but the manual led me down the correct 
path.

--Scott, W5WZ


> Message: 1
> Date: Tue, 21 Feb 2023 12:47:43 -0500
> From: David Needham <aa4vt.dave@gmail.com>
> To: towertalk@contesting.com
> Subject: [TowerTalk] Force 12 C31XR Modeling
> Message-ID:
>         <CALNJPz2r40uawT-yntXvZ=Ecx5H-GU9aT9Q3C_hORMfsZ-_0YQ@mail.gmail.com>
> Content-Type: text/plain; charset="UTF-8"
>
> Hello Towertalkers,
>
> I have built an EZNEC model of the Force 12 C31XR utilizing a single 
> coax
> feedline for a friend that is rebuilding a used one. I have a pretty 
> good
> model working with EZNEC Pro2+ v 7.0 and AutoEZ, but I had to make an
> assumption for the 14/2 Romex with the ground wire removed that is used 
> for
> a transmission line from the 10m to 20m Driven Element.
>
> I found that someone had posted a MINNINEC model back in 2010 or so, 
> but
> they used separate feedlines and didn't use the transmission line. I 
> came
> up with that the impedance of the transmission line must be about 120 
> ohms
> to get the approximate curves with the stock dimensions. Has anyone 
> else
> done this and has a better number?
>
> He is experiencing high SWR on 15m, minimum of 1.86 at 21.126 MHz. It 
> would
> require drilling out rivets, but I am finding that the spacing between 
> the
> 15m and 20m element may need to be adjusted, at least according to the
> model. Has anyone had this issue and corrected it?
>
> In the JK Antennas version of the C31XR, I see they have changed over 
> to
> aluminum angle iron. Has anyone retrofitted a Force 12 C31XR with angle
> iron or square tubing replacing the Romex transmission line?
>
> Any help would be greatly appreciated. He is trying to get us ready for 
> CQ
> WPX Phone contest at the end of March.
>
> --
> Thank you!
> David Needham, AA4VT (formerly K4AJA and K8AJA)
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