[NLRS] 432 beam questions (k1fo/424b)

[Dr. Gerald N. Johnson]

One thing you have to consider and watch closely is that the boom
diameter has a significant effect on the element length. There is no
tolerance there. Either the data from Pfixbecke's NBS report or a
computer program based on DL6WU work some decades age is reliable.
www.ifwtech.co.uk/g3sek/diy-yagi/ele.exe Its a dos program that will run
in a modern windoze dos window.

GM3SEK's discussion at: www.ifwtech.co.uk/g3sek/diy-yagi/index.htm#BoomCorr
is dead on accurate. Then you can compare to the K1FO table.

Steve K1FO said you could use the same element dimensions with fewer
directors down to about 12. Sometimes in his tables there's a note to
shift the whole batch some. He says below 12 elements they aren't
optimized. My computer analysis
(www.geraldj.networkiowa.com/papers/2012/HowCloseBa.pdf) showed the 9
element using his dimensions works quite well and gives the same gain at
432 on a 1 meter boom as M2 does on a boom half again longer. Tests at
two CSVHF conferences of the first one I built gave gain greater than
the computer gain. I used ele.exe to adjust the elements for boom 
length. There was no boom in the computer models.

The length of the T match wires beyond the shorting bars is not very
critical, more of a convenience for adjustment. Its a real pain when
adjusting and you get to the end of them and have to replace them. Those
extensions do have some effect so its not normal practice to cut them
off. Point the antenna at the sky when adjusting the shorting bars (keep
them spaced the same on both sides) to keep from adjusting for
reflections. A yagi that size can show an increased SWR from a person
walking in front of it at 30 or 40 yards. K1FO designs tend to show a
dry match best at a higher frequency than 432.1 because they go down in
frequency when wet. More when covered in ice.

That 132 mm spacing spec is just the starting point. Adjust for better SWR.

It will be hard to butt splice 12 gauge wire and lapping won't give the 
same results as a complete wire. Thin wall hobby shop tubing that will 
just take 12 gauge wire might be a better splice or extension, so long 
as you don't have to slide the shorting bars past it. The closest that 
will fit and it will be a little loose is 3/32" inside diameter. The 
next smaller won't go over 12 gauge wire unless you opened it up with a 
drill and it will be very hard to hold it while drilling it. A 3/32" 
collet in a jeweler's lathe would work best.

73, Jerry, K0CQ

On 10/28/2013 1:12 PM, Brad Johannes wrote:
>
>
> Hello all, I'm in the middle of a tower install along with some
> new(used) beams.I have a 24 element 432 beam from Vince (who thinks
> it might have been Rons 'RYT) in which case maybe Ron can contact me
> directly for the history, but I thought I'd throw it out to the group
> for advice also. The beam appears to be a Cushcraft 424b, that was
> partially? converted to the original K1FO specs.When I put the beam
> on 10' mast in the box of the pickup truck yesterday, I had a SWR of
> 1.7:1 at 432.1It showed dips in SWR at 430 and 435 of 1.4 and 1.3
> respectively. The element dimensions are closer to the k1fo specs
> than the 424b, but it is 24 element, and not the 25 as show in the
> k1fo literature.The T match is built like the
> k1fo:http://www.directivesystems.com/DSFO432-25.htm My driven element
> is 340mm as shown as well as the reflector at 348mm, my directors
> vary by a couple mm from show (but I'll admit I wasn't measuring
> perfectly)My T match bars are spaced at 128mm, vrs the spec 132mm.The
> distance between driven, and #12 t match wire is close to the specd
> 17 and 15mm.The main difference I have is the #12 tmatch wires are
> 109mm instead of the 128mm shown. I plan to solder another piece of
> #12 copper to the ends of these to get them at the 128mm, and
> test/trim from there. Or should I get the #12 wires at the 128mm and
> then de solder the t match bars and move for lower swr? Am I barking
> up the right tree??? And other advice? 73, Brad KB0HNN
> ______________________________________________________________ NLRS
>