I was going to let this one go, but after sleeping on it for a week (?),
the subject and commentary seemed to be one of those recurring persistent
semi-myths that stick around and unfortunately influence decisions on
expensive hardware. And so I decided to take a break from chasing some
maddening amplifier glitches.
As you describe your usage, someone who makes use of over-under yagi's to
optimally select incoming vertical angles, or rotates them independently to
cover two directions at the same time, the simple answer is that you should
use the single antenna optimization. That's why you don't hear this
subject brought up over and over, because the antennas are at their
"weakest" being used separately, and need their best performance when used
separately for incoming elevation angle or split direction. Then you get
whatever extra you can when you run them stacked (power on both in the same
direction) . That they interact is a simple fact. That is why YO has the
stacking business in there, for someone who wants to use them ALWAYS with
both under power in the same direction.
As to the stacked 2 element 40's, sorry guys, been there, done that. As
one goes to 20m and above, the interactions with ground (related to
distance in wavelengths) are diminishing significantly and it's cleaner.
But on 40 meters at ordinary heights one is wading in interaction to the
point of making a stack of 2 element yagis not worth the trouble. Note
that I'm NOT referring to using them separately.
Short version: I'm telling you this because the tools which give correct
answers for other significant closely related variations confirm it.
Long Version:
The pair of 40 two el in a stack will not give you the intuitive 3 db extra
gain, even if you allow an optimizer to work SIMULTANEOUSLY on the lower,
AND on the upper, AND on the height above ground, AND on the phasing, to
optimize to stack gain. (You can do that in AO Pro.) A two element in
free space is a cardioid in the vertical pattern that contains the boom,
and the radiation straight up and down is barely diminished from straight
ahead. So two element yagis in stacks have significant interaction, and
the yagi solution for both even in free space depends heavily on the
spacing. With horizontal polarization over ground, the ground reflection
also plays heavily into the solution, and if you let AO Pro play with
everything all at one time, the solution for the upper 40m 2 element is not
the same as for the lower, because the lower is closer to the ground and
has interactions BOTH with the ground AND the upper yagi.
Please note that the solution you get from TA (terrain analysis) on
stacking PRESUMES NO INTERACTION AT ALL between antenna and antenna, or
antenna with the ground, e.g. that the single antenna pattern is NOT
modified by proximity with the other antenna or proximity to ground. That
disclaimer IS in there somewhere in the fine print. It also makes TA
useless for dealing with two element yagis.
For forty meters, the interaction with ground is so significant at ordinary
heights, that when we were designing and considering the 5 element 40m quad for
N4AF/NY4A (on a 180' stretch of catenary strung between two towers oriented
to Europe), and comparing alternatives, it was quite clear, from both AO
Pro and EZNEC Pro NEC4 output, that a height had to be picked, designed to,
and then maintained. We picked 84 feet for the center height of the
diamond quad elements. We went for that solution precisely because
stacking 40m 2 element yagis for Europe was so significantly inferior and
we needed to cover all the incoming angles. The quad at 84 feet had no
notches in the vertical pattern. AND we also picked it because Howie did
not want to put up stacked 40 meter 3 element yagis (longer boom) because
of his hurricane exposure (and nobody was arguing with him on that :>)).
But in the modeling, the quad beat or equaled even stacked **3's** in any
combination at any useful takeoff angle to Europe, and the 5 element never
needed to be switched for incoming angles, and this was true even if AO
monkeyed with the yagi to maximize for the stacked configuration. And if
the three's couldn't beat the quad, for sure the two's would not come
anywhere close. And since this probably seems an outrageous claim to many,
I can show you the proof.
You can see this verified with impartial readings off the Reverse Beacon
Network. Go to http://www.reversebeacon.net/analysis/ Enter 02/19/2011
as the date. Click on the Europe bar. When the station selection expands,
click on S50ARX. Enter W3LPL, K3LR and NY4A under stations. Look at the 40
meter spots. On the right side of the graph for the zulu day, Saturday
evening was for propagation one of those wide open very "smooth"
propagation evenings analogous to reflections off a lake on a windless day.
In particular, starting around 19Z or so, see how NY4A's strength goes
through three jumps up to a plateau. You can see that LPL and LR climb
them as well. These plateaus follow the propagation changing modes
(numbers of hops and angle) as the band opens into the evening. Note that
the quad engages all three modes cleanly and fully as they open, without
any fading. From our area at that distance, if the longest path is cleanly
open, the path is open from the shorter LPL LR distances on 40. If
anything LPL and LR have a propagation advantage on us. LPL and LR both
have excellent stacked 3 or 4 (depending on how you count them) element 40m
yagi's. But note how as the fourth mode is engaged, both LR and LPL fall
off because they are not cleanly engaging the mode, most likely because the
increasing elevation angle is starting into a notch in the yagi vertical
pattern. Also note that NY4A carries the best signal for most of the 24
hours.
I deliberately picked LPL and LR because no one will attempt to blame the
difference on a poor implementation of a concept. Those guys are their own
proof of excellence, reinforced year after year with their top end scores.
The outcome you see demonstrated in the RBN plot was clearly predicted in
NEC4 modeling runs over ten years ago, based on nothing but the different
designs of the antennas, before the quad was ever erected. The performance
of a stack of 40m two el yagis is only another run of the same tools on a
configuration with less aluminum using the same methods and principles that
correctly predict this outcome.
So again, forget optimizing 2 element 40s for the stack, you will do
almost, but not quite as well in any direction by putting up a single 2
element. You will do better than a 2 el 40 stack with a single 3 element
on your tower. Only go for a pair of two elements when you plan to use
them at separate azimuth and elevation, and then use the single yagi
dimensions. BUT if you are going to optimize for 2 elements, optimize each
yagi separately at the height it will be used on the tower.
73, Guy.
PVRC since 1963
On Sun, Feb 26, 2012 at 4:02 PM, George Dubovsky <n4ua.va@gmail.com> wrote:
> OK, I know that there is interaction between the antennas, and between the
> ground and the antennas; all that shows up in the models. But knowing that
> the antennas will be used as upper, lower, and both, what is the default
> condition to optimize and tune (each Yagi) to? Does everyone do it
> differently? I have seen enough difference in the tuning in YO (for
> individual Yagi in free space, single Yagi at one height, and Yagis at a
> certain stacking distance) to make me ask the question.
>
> Guy, I have 2 over 2 on 40, at 145' and 85', and I do believe that it
> works as a proper array when the Yagis are pointed in the same direction.
> And it definitely works when I split them apart and beam in different
> directions.
>
> Is there a YO guru in PVRC?
>
> 73,
>
> geo - n4ua
>
>
> On Sun, Feb 26, 2012 at 3:21 PM, Guy Olinger K2AV
> <olinger@bellsouth.net>wrote:
>
>> This is true, always has been. Typically a commercial yagi is designed
>> for single use, far and away the most common use. Stacking three plus
>> elements does produce the kinds of improvements proffered for stacks. The
>> difference in a stack of single use design and a stack designed as a stack,
>> probably cannot be detected by the other end, with other issues, like
>> feedline loss, or corrosion issues a far greater villain. Further, even in
>> stacks, a lot of the use is still lower only or upper only, STILL
>> vindicating design for single use.
>>
>> Stack interaction is VERY evident if you stack two element beams, as
>> their interaction is huge, rendering two element stacks not worth the
>> effort. The free space pattern of a yagi needs have an over/under null to
>> stack "well". This is very easy to model.
>>
>> 73, Guy.
>>
>> On Sat, Feb 25, 2012 at 5:21 PM, George Dubovsky <n4ua.va@gmail.com>wrote:
>>
>>> I am trying to optimize a pair of HyGain 204BA for use in a stack. I am
>>> using YO v7.61 and, mostly, I think I know what I'm doing at a basic
>>> level
>>> ;-)
>>>
>>> I intend to mount one at 70' and one at 110' (that seems to work in HFTA
>>> for this location), but that's not my problem. I can optimize an antenna
>>> at
>>> either height, and I can design a new Beta match. But when you select the
>>> stacking feature of YO (using 40'), the element spacing seems to want to
>>> be
>>> different and the matching of course changes. What that seems to be
>>> telling
>>> me is that when I select upper, lower or both using a stack-match only
>>> one
>>> condition is going to be correct, and the others will be a compromise.
>>>
>>> It can't be this tough - I feel that I am mis-interpreting the YO
>>> instructions or something. For instance, It may be that the stacking
>>> feature of YO is only intended to tell me what the combined pattern will
>>> be, but I don't get that from the manual. (My manual is marked Version
>>> 7.5).
>>>
>>> What is the proper sequence of steps for optimizing a stack?
>>>
>>> 73,
>>>
>>> geo - n4ua
>>> _______________________________________________
>>> Antennaware mailing list
>>> Antennaware@contesting.com
>>> http://lists.contesting.com/mailman/listinfo/antennaware
>>>
>>
>>
>
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