[TowerTalk] wind load vs Rohn specs

[Kurt Andress]

Bill Coleman AA4LR wrote:

> On 9/12/00 4:21 PM, Kurt Andress at K7NV@contesting.com wrote:
>
> >> >Antenna area measurements should be made with a calculator. Simply summing
> >up
> >> >the Length x diameter of the members is more accurate.
> >
> >For clarification:
> >In my mind, the statement means that we take element #1 and sum up the
> >exposed
> >areas of all its sections, then we do the same for every other element.
> >Then we
> >add all the individual element areas together to get a total area for the
> >elements. The boom is done just like one of the elements.
>
> Ah ha! This is EXACTLY the Vertical projection.
>

Hi Bill,
Not exactly, one view of the yagi does not cover the exposures that are of prime
interest. More below.

>
> The Vertical projection makes more sense to me. It accounts for the drag
> of all portions of the antenna, but it leaves out vertical members such
> as the boom-to-mast plate, or maybe truss support members or wires above
> the boom or elements.
>
> >I've never ever meant to imply that the leading elements shield the others
> >from
> >the wind. I don't believe that is possible for the normal yagi element
> >spacings.
>
> Well, actually they do, to some degree. But it's probably too hard to
> calculate accurately, so why not just assume that all members are in
> clear air, even those that obviously cannot be (such as the boom versus
> the elements). This gives a kind of "worst case" area, except for the
> vertical members it leaves out.
>
> Or is the intent to add the area of the vertical members as well? Such a
> sum would be neither the Vertical or the Horizontal optical projection.
> Rather, it is a sum of the projections of the unassembled components.
>

Or, the assembled components as they are presented to the wind. As with most
things like this, there are a multitude of possible load cases (sets of
conditions) that can occur. Narrowing the effort down to just having to examine
the most important ones keeps it managable.

For figuring out the projected area of the thing, we first need to know the
orientation to the wind and then look at what is exposed at that attack angle.

I agree with what I think I've read previously that it is best not to try and pick
the fly dung out of the pepper.

>
> >> Could you dig up some references?
> >
> >Communications Quarterly, Spring, 1993, Determination of Yagi Wind Loads
> >Using the "Cross-Flow Principle", by Dick Weber, K5IU
>
> I'm sure this will answer all my questions. I am currently searching for
> a copy of it. Thank you.
>
> >The essence of the article is that the forces acting on the antenna
> >members act
> >normal to their major axis. This means all element loads act parallel to
> >the boom,
> >the boom loads act parallel to the elements.
>
> Intersting. In aerodynamics, all drag is considered parallel to the
> airstream. Forces normal to the airstream are lift.
>

You will find that the information presented there does not reinvent aerodynamics,
it just reinforces it as applied to the yagi antenna. The "apparent" revelation is
simply that the resultant force from wind acting on a piece of the antenna is a
vector acting 90 degrees to the major axis of that piece. One of the two
components of that vector (in a two dimensional analysis) is the drag and it acts
downwind as you expect.
It is all quite sensible as you will see.


>
> >Since, the loads on an antenna are developed this way, there just is no
> >way for
> >them to generate a peak load somewhere in between 0 & 90 degress azimuth.
>
> OK, I can accept this. But I don't see how this has an effect on the
> computation of this projected area number, unless we have the elements or
> the boom "shadowing" other pieces.
>

Well, here's how it does. The approach identifies the two worst load cases, so we
don't have to go trying to analyze the antenna at 90 different orientations (0
thru 90 deg @ 1 Deg intervals) to figure out what is going on, although we still
can because it is possible, we don't have to.
Because this method of looking at the yagi shows us that the two worst load cases
occur at 0 and 90 degrees, we know we only need to compute the projected area at
those exposures to end up with the two highest loads the antenna will apply to the
tower. These are also the two load cases used to design the antenna itself. Design
the elements at 0 deg (elements broadside) and the boom at 90 deg (boom
broadside), that is where they are most highly loaded. Again, this is for a two
dimensional analysis, we can add the third dimension (non-horizontal wind like on
a hilltop) and still get there following the same fundamental approach, just more
math.


>
> >> So far we've talked about yagi's. What about other configurations,  like
> >> quads? How do we compute their "projected area"?
> >
> >I'd be doing it the same way. Sum of all wires and spreaders in the 0
> >direction,
> >boom the same as above.
>
> So, in the zero direction, do we take the area of the entire boom, or
> merely it's cross-section. (assuming the elements are much longer than
> the boom)
>

Only the cross section (end of the boom, that's what is exposed to the wind at 0
deg) if you think it is important, I think it is "in the pepper", along with the
edge of the boom/mast plate.

But, it's easy enough to account for them. The end of a 2" OD boom is .022 SqFt
FPA, a 1/4" thick x 12" tall edge of a boom/mast plate is .021 SqFt FPA. Is that
significant when the total elements in clean air are 9.5 SqFt FPA? Your call,
looks like pepper to me!

>
> >Well, I guess you have missed the 20 or 30 posts I've made in the last 2-3
> >years
> >on this matter.
>
> No, I haven't missed them at all, Kurt. I've gone back and read them and
> tried to understand them.

Sorry they didn't get the job done. Maybe I'll get it right sometime. You can
reread my previous response to be a reply to your suggestion that I condoned the
current state of confusion about antenna values. I have been real consistent about
that for over two years, and never have I suggested that is was ok for everything
to be stuffed up.

>
>
> >Please explain your new method of dealing with the problem, exactly how
> >would I
> >take my 1" diameter x 72" long piece of tubing and arrive at its "Flat Plate
> >Equivalent Area."
> >Then I can think about what I'd do with that number.
>
> As I understand it, flat plate equivalent is an old method. You don't
> need a wind tunnel, just a wind balance, perhaps on a moving vehicle.
>
> Bascially, you characterise the drag of an object by equivocating it to a
> flat plate of a given size. You mount the object on the balance, then
> construct a flat plate that properly balances the object in the wind. The
> result is the area of the flat plate (which is assumed to have an aspect
> ratio close to 1)
>
> The Wright Brothers used one at the turn of the century.
>
> Such a number is a very accurate representation of the drag presented by
> an object.
>

Ok, thought it was something like that, had to make sure.
Here's where I would expect to encounter problems.

We currently need flat projected areas to directly use the tower designs, like
those in the Rohn Catalogue. We haven't been able to get all antenna manufacturers
to do that.
I think we would have the same problem with the new value, specially if they had
to go do full scale testing. That is a stretch, so it could be done
mathematically. But, still think we'd see the same reluctance to do it.

Now, if we changed the antenna number, we'd need to also change the EIA and other
specs to make everything compatible. We've been told by tower2sell that the
standards are undergoing revision, so we should probably just hang tight and see
what comes out of that, and go from there... Might make things easier?

Here's a bit of trivia. EIA used to have an antenna spec, RS-409, Minimum
Standards for Amateur Radio Antenna. That was in sync with EIA-222-C. At least one
amateur antenna mfgr adhered to it (now SK). Does anyone know what happened to
409?


73, Kurt, K7NV

YagiStress - The Ultimate Software for Yagi Mechanical Design
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