[TowerTalk] Broken OWA monobander - Help!
Jiří Šanda
jirka@jimaz.cz
Sun, 26 Nov 2000 00:14:07 +0100
Hello Dan,
as an builder - and modifier of 13 pcs of OWA Yagis i can say the following
:
I tried to model your design and got a little bit different data than you
did - the explanation why is clear since you used NEC4WIN95 which does not
model the OWA correctly - first director is too close. I used the
EM-professional (and of course NEC4 I have not access to ) are the only
programs known to me which models this problem more correctly. Here is the
expected results from me. The model is not correct since the first .75"
section is too thick and the .5" is too short to obtain correct data. Anyway
my agreement of modeled and measured data on my 13 antennas was + - 3 Ohm
both real and imaginary. However I have found that the sensitivity of OWA to
correct dimensions is very big. - difference of +0.5" in length of the
driven element adds about 10j Ohms !!!
It will be probably your problem. All this is provided that your measured
data are correct, you ca nverify it by any SWR meter - it is easy. Just
expect that the data measured with small power are more optimistic compared
to reality measured by MFJ or by SWR meter with high power.
Here are results of my model:
IMPEDANCE
normalization = 50.
frequency resistance reactance impedance phase VSWR
(MHz) (ohms) (ohms) (ohms) (deg)
source = 1; node 38, sector 1
27.9 32.1879 -5.45229 32.6464 350.39 1.5845
28. 34.0725 -3.38623 34.2404 354.32 1.4799
28.1 36.0681 -1.45904 36.0976 357.68 1.3887
28.2 38.2431 .248614 38.2439 .37 1.3075
28.3 40.6573 1.59203 40.6884 2.24 1.2335
28.4 43.3229 2.34192 43.3862 3.09 1.164
28.5 46.1319 2.16985 46.1829 2.69 1.0967
28.6 48.7451 .703136 48.7502 .83 1.0296
28.7 50.4447 -2.34931 50.4993 357.33 1.0488
28.8 50.1515 -6.75971 50.605 352.32 1.1444
28.9 46.8713 -11.4134 48.2408 346.31 1.2762
29. 40.6044 -14.4867 43.1112 340.36 1.4636
29.1 32.7139 -14.567 35.8105 336. 1.7366
All the best
Jiri OK1RI
-----Original Message-----
From: owner-towertalk@contesting.com
[mailto:owner-towertalk@contesting.com]On Behalf Of Dan Levin
Sent: Friday, November 24, 2000 11:22 PM
To: tower talk
Cc: L. B. Cebik
Subject: [TowerTalk] Broken OWA monobander - Help!
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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