[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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