[TowerTalk] Broken OWA monobander - Help!
Maurizio Panicara
i4jmy@iol.it
Sun, 26 Nov 2000 13:08:39 +0100
The SWR curve partly depends by how one decides to design (optimize) the
antenna.
The OWAs I built following my preferred criterias had a quite sharp SWR
increase over a certain frequency where gain and F/B are best. Below a much
lower frequency the SWR continues to be low and flat, althought the other
antenna parameters, gain and F/B, aren't any more satisfactory.
The flatness is not the only parameter to care about, because an OWA cell
can continue to show a very broad and low SWR curves but in a frequency
range where antenna gain, and expecially F/B, are not any more optimized .
By the way, the software I use and the real model built agreed very much in
results and didn't require any further adjustment.
73,
Mauri I4JMY
----- Original Message -----
From: "Jiří Šanda" <jirka@jimaz.cz>
To: "Dan Levin" <djl@andlev.com>
Cc: "towertalk" <towertalk@contesting.com>
Sent: Sunday, November 26, 2000 12:14 AM
Subject: RE: [TowerTalk] Broken OWA monobander - Help!
> 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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