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Tonno
ES5TV
tonno.vahk@mail.ee
----- Original Message -----
From: "Tõnno Vähk" <tonno.vahk@mail.ee>
To: <towertalk@contesting.com>
Sent: Friday, July 27, 2001 7:05 PM
Subject: [TowerTalk] The best stacking distance for long-boom yagis - 0.5
wavelength!!
>
> I have done a lot of modelling and also field testing and found out that
it
> is possible or even favourable to stack long boom 10m and 15m yagis only
0.5
> wl apart!
>
> I hope you can confirm or argue with me!
>
> Especially it concerns OWA designs (5 to 7 elements) where my modelling
> results with AO show that stacking them by 5-6 m on 10m and 7.5-8m on 15m
> gives the best pattern and in most of the cases also the highest gain!!!
The
> second best stacking distance is roughly twice as much - 1 wl.
>
> 1 wl distance yields higher gain for the stack of 2 yagis only after the
> higher antenna is raised up to 2 wl or more and even then by maximum 0.5
db
> as far as we remain in practical heights. In free space though the gain is
> approx. 1db more for 1wl than for 0.5 wl.
>
> Based on my results I would advice anybody considering a 2-stack of OWA
long
> boom yagis at a hight of less than 2 wl to stack them 0.5 wl apart.
> Especially that holds true for the upper antenna at 1.5 wl - then place
the
> second at 1.0 wl and you have an awesome pattern and best gain.
>
> That is contrary to general perception that you don't stack yagis less
than
> 0.7 wl (for long booms at least) and I wonder if AO is wrong or rather
most
> of the suggestions are based on free space modelling and that is not the
> same as above ground! As far as most of the Yagi optimizer programs are
not
> capable of modelling stack above ground and only in free space I guess
many
> guys have come to wrong conclusions. It is quite some effort to create AO
> files for yagis with correct tapering and to model the stacks!
>
> I modelled following 15m designs: 5el owa 38' boom, 6 el owa 40' boom, 6el
> owa 48' boom (the famous NW3Z beam) and w6qhs 6el 35' boom.
>
> The pattern is smilar for all. When you go wider than 0.5-0.6wl you start
to
> have a growing fat lobe right up at 90 degrees. This is very irritating
and
> eats up a lot of radiation energy. The worst pattern (highest 90 degree
> lobe) is at about 0.75wl and it is also the point of the lowest gain for
the
> stack. So the higher antenna at about 2wl we have about the same gain for
> 0.5wl and 1wl stacks. But pattern-wise it is different. Both have the main
> lobe at about 7-8 degrees and it is 18-19 dbi above average ground. 0.5wl
> has the second lobe at 22 degrees and it is about 4-5 db down and all the
> other lobes are 20-30 down. 1wl has the 2nd lobe at 50 degrees and about
10
> db down, all the other lobes are 15-30 db down.
>
> Only the 48' boom owa and w6qhs had a small gain advantage for 1wl but
> pattern-wise I would choose 0.5wl any time, especially for contesting
where
> I don't care so much about F/B but am interested in the 2nd lobe at 22
> degrees for sure!! And most of the stacks are for contesting.
>
> In my mind with 38' and 40' boom OWAs there is no question and I would
stack
> them 0.5wl any time. NW3Z 48' boom is an extreme example. The higher beam
at
> 30m (100') the gain of the stack at 7-8 degrees is: lower at 22m (72') -
> 18.63 dbi and lower at 15m (50') - 18.86 dbi. So only 0.2 db difference. I
> would select the pattern of 0.5wl!! Putting the lower one at 72' lets you
> add another beam at 50' and rise your gain to 19.36 dbi with remarkable
> pattern and F/B!!
>
> Still I see many guys stacking them at 0.75 and even 1.25 wl, the latter
> gives 18.02 dbi and is a serious crime against the pattern.
>
> The close spacing does not mess up the swr curve, impedances remain the
> same!!! The stack can be optimized for best swr and F/B or course if it is
a
> rotating tower and antennas are fixed.
>
> We put up stacks of 2 40' boom 15m OWAs and 2 30' boom 6 el 10m OWAs at
> ES5Q/ES9C QTH. Both stacks are with 0.5 wl spacing. 10m tower is 56' high
> and 15m tower 92' high. Both upper antennas rotate and lower ones are
fixed
> to US. Both stacks are "killers" as people like to say:). US stations
report
> on average 2 S-units advantage to stack vs. upper beam and 3 S-units vs.
> lower beam!!!! Hard to believe but I have run enough A-B-C tests to be
sure
> of that!! The swr curves of the stacks are perfect.
>
> At first we put up 15m stack at 80' and 45'. It simply did not work. Stack
w
> as as strong as the higher beam in most cases. After rising it to 92' and
> 67' it really started to play!
>
> Another advantage of having the lower beam higher is that it is clear from
> obstructions because should you face a little hill in front you might be
> better off with single upper antenna than a stack where the performance of
> the lower antenna is nulled by the obstruction.
>
> I guess I am not the only one with similar ideas as I noticed OK guys also
> stacking their OWA yagis very close.
>
> I hope OK1RI and other guys who have experimented with OWAs and done
> computer modelling will either challenge my thoughts or prove them.
>
> I have also following questions:
>
> 1.How bad is the exsistence of the high fat lobe at 90 degrees? Does it
> effect also near field - other antennas on the tower??? Does it cause
> interference?
>
> 2. Can I trust AO on that? I have found that there is almost perfect match
> between AO results and field measurement as far as I can go (impedances,
> SWR, etc.)
> so I would trust it.
>
> 3. Are there any disadvantages when closely stacking beams that I have
> failed to notice?
>
>
>
>
>
>
>
>
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-----
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