[TowerTalk] Broken OWA monobander - Help!

Sat, 25 Nov 2000 20:31:47 +0000

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<tt>Hi Dan, I didn't model your anntenna(I use eznec) but I would tend
to believe the numbers you got from the MFJ analyzer. The model SWR numbers
sound a little too good to be true.</tt><tt></tt>
<tt>gl es 73 Steve VE3TU</tt>
Dan Levin wrote:
<blockquote TYPE=CITE>I am building a three stack of 10 meter OWA design
yagi's for the upcoming
 10 meter contest.&nbsp; I just raised the first one to 35', and it
doesn't
 exhibit anything like the impedence curve that I expected.&nbsp; 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.&nbsp;
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".&nbsp;
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.&nbsp;&nbsp; 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"&nbsp;&nbsp;&nbsp;&nbsp; .625"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; .5"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
.375"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Total length&nbsp;&nbsp;&nbsp;
Spacing
 6"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 67.75"&nbsp;&nbsp;&nbsp;
2.5"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 32.375"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
108.625"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 0
 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 69.75"&nbsp;&nbsp;&nbsp;
2.5"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 32.125"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
104.375"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 44"
 6"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 67.75"&nbsp;&nbsp;&nbsp;
2.5"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 22.375"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
98.625"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 69"
 6"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 67.75&nbsp;&nbsp;&nbsp;&nbsp;
2.5"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 20.5"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
96.75"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 131"
 6"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 67.75"&nbsp;&nbsp;&nbsp;
2.5"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 20.625"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
96.875"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 193"
 6"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 67.75"&nbsp;&nbsp;&nbsp;
2.5"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 16.125"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
92.375"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 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&nbsp;&nbsp;&nbsp; 30 + 10 = 1.7
 28.1&nbsp;&nbsp;&nbsp; 29 + 8&nbsp; = 1.7
 28.2&nbsp;&nbsp;&nbsp; 29 + 6&nbsp; = 1.7
 28.3&nbsp;&nbsp;&nbsp; 29 + 6&nbsp; = 1.7
 28.4&nbsp;&nbsp;&nbsp; 30 + 8&nbsp; = 1.6
 28.5&nbsp;&nbsp;&nbsp; 31 + 8&nbsp; = 1.5
 28.6&nbsp;&nbsp;&nbsp; 33 + 9&nbsp; = 1.5
 28.7&nbsp;&nbsp;&nbsp; 35 + 8&nbsp; = 1.5
 28.8&nbsp;&nbsp;&nbsp; 35 + 7&nbsp; = 1.4
 28.9&nbsp;&nbsp;&nbsp; 33 + 8&nbsp; = 1.5
 29.0&nbsp;&nbsp;&nbsp; 28 + 8&nbsp; = 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.&nbsp; Just changing the length of the first director
doesn't fix
 it.
Any ideas of what might be wrong?&nbsp; 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.&nbsp; But that, by itself, doesn't
account
 for the very low feed impdenence.
Could my instrument be bad?&nbsp; Can anyone think of what I might have
done
 wrong?&nbsp; I am pretty sure that the element lengths are as modeled,
to within
 .125" or so.
Thanks a million!
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ***dan,
K6IF
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