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
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
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,
so I would trust it.
3. Are there any disadvantages when closely stacking beams that I have
failed to notice?
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