[TowerTalk] Broken OWA monobander - Help!

[Dan Levin]

I am building a three stack of 10 meter OWA design yagi's for the upcoming
10 meter contest.  I just raised the first one to 35', and it doesn't
exhibit anything like the impedence curve that I expected.  Any hints,
sugggestions, etc. on what might be wrong and how to fix it would be most
welcome.

The antenna was built out of an old KLM Big Sticker 6 element monobander.  I
took the KLM elements, and removed them from their black plastic insulators,
leaving me with 6' pieces of 5/8" tubing tapered at one end to 1/2".  I
joined two of these with 12" of 3/4" .058 wall tubing at the center, then
added back on the KLM 3/8" tips.   The driven element is split at the center
with a fiberglass rod, and fed with a Force 12 B1 balun.

Here is the design (half-elements):

.75"     .625"      .5"       .375"         Total length    Spacing
6"        67.75"    2.5"      32.375"         108.625"         0
          69.75"    2.5"      32.125"         104.375"        44"
6"        67.75"    2.5"      22.375"          98.625"        69"
6"        67.75     2.5"      20.5"            96.75"        131"
6"        67.75"    2.5"      20.625"          96.875"       193"
6"        67.75"    2.5"      16.125"          92.375"       282"

The antenna models in NEC4WIN95 to show between 38 and 53 ohms real and less
than 10 ohms imaginary. The SWR for 50 ohm coax is less than 1.3:1 across
the band, with a dip to 1.08:1 at 28.8 Mhz.

The real antenna looks like this, according to my MFJ-259B (vs. 50 ohm coax)

28.0    30 + 10 = 1.7
28.1    29 + 8  = 1.7
28.2    29 + 6  = 1.7
28.3    29 + 6  = 1.7
28.4    30 + 8  = 1.6
28.5    31 + 8  = 1.5
28.6    33 + 9  = 1.5
28.7    35 + 8  = 1.5
28.8    35 + 7  = 1.4
28.9    33 + 8  = 1.5
29.0    28 + 8  = 1.8

I wouldn't care, except that I plan to stack this antenna with a StackMatch,
and the power distribution will be uneven if the other antenna behaves as
expected.

The problem is that I can't figure out how to 'break' my model to give the
above results.  Just changing the length of the first director doesn't fix
it.

Any ideas of what might be wrong?  My best current guess is that the driven
element is too short, probably because my model doesn't accurately deal with
the rod and feed wires from the balun.  But that, by itself, doesn't account
for the very low feed impdenence.

Could my instrument be bad?  Can anyone think of what I might have done
wrong?  I am pretty sure that the element lengths are as modeled, to within
.125" or so.

Thanks a million!

            ***dan, K6IF




--
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com