Stan Griffiths wrote:
> >K7LXC@aol.com wrote:
> >
> >> In Statement #1 you say " The flat projected areas will."
> >>
> >> In Statement #2 you say "we get to use the larger areas contained
> in the
> >> oval outlines of the Rohn diagrams".
> >>
> >> Aren't these contradictory? My question still remains - "WHICH
> figure do
> >> we use to determine Rohn antenna load capacities vis-a-vis amateur yagis?"
> >>
> >> Cheers, Steve K7LXC
> >>
> >
> >Hi,Sigh, this can be difficult to explain!Ok , Both of the oval and
> >rectangular
> >values in the Rohn drawings are projected
> >area.
> >The Oval one is larger, because the round members have less drag, hence, less
> >load, hence, lower "effective area". The larger oval value generates the same
> >applied load on the tower as the rectangular one.
> >If the antenna is made with round tubing, use the oval area. If it is made
> >from
> >box tubing, use the rectangular area.
> >Is that any better?
> >
> >73, Kurt
>
> Hi Kurt,
>
> I think you are totally correct in what you say, above, assuming you know
> that the antenna manufacturer's square foot rating was calculated as HyGain
> has just done, that is, Projected Area without regard to element shape.
> Some antenna companies REDUCE their area specifications with regard to
> element shape so I think there is risk of using this "area reducing" factor
> twice in the same installation . . . once for the antenna loads and once
> again for the tower ratings . . . this may result in a dangerously
> overloaded tower. What do you think?
>
Hi Stan,
We are in agreement! But I think that all antenna companies have applied a shape
reduction factor to their areas. That's a guess on my part. There is an EIA
spec,
RS-409, Minimum Standards for Amateur Radios Antennas. The one I have is dated
1973.
This spec says to use a .66 shape factor for round pieces and uses .004V^2 to
calculate the wind pressure (like 222-C). It also says the max torque due to
wind
shall be calculated. There is not a specific instruction in the spec on what or
how
to report the antenna area.
I think the absence of coordinated specifications for antennas and towers has
led to
the problem.
73, Kurt
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