[TowerTalk] Broken OWA monobander - Help!
Don Moman
ve6jy@freenet.edmonton.ab.ca
Sat, 25 Nov 2000 14:07:30 -0700 (MST)
Having built 4 of the 10m OWA's and 4 15m OWAs, all with 48' booms (plus
yagis for 40 and 80m patterned after them) I can say that your SWR
expectations are very reasonable. These are flat! When they start to go
up, they really go up but that is after over a 1 mhz wide flat spot (on
10m) of typically under 1.1:1. Only one of all the beams was out to lunch
and it was a balun problem (brand new, a replacement worked fine).
You don't need to raise them to 35' either - they are eaily tested just
on step ladders and the SWR curve won't change much more when you go
higher.
I would suspect the coax and/or balun first, not the antenna.
GL
Don VE6JY
On Fri, 24 Nov 2000, Dan Levin wrote:
> 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
>
>
>
>
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VE6JY Don Moman email: ve6jy@freenet.edmonton.ab.ca
Box 127 Lamont, Alberta email forwarding: ve6jy@rac.ca
CANADA T0B 2R0
(780) 895-2925
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