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Re: How best to park your satellite antennas



----- Original Message -----
From: "Edward Cole" <kl7uw@acsalaska.net>
To: "i8cvs" <domenico.i8cvs@tin.it>; "Graham Shirville"
<g.shirville@btinternet.com>; "AMSAT-BB" <amsat-bb@amsat.org>
Sent: Wednesday, September 03, 2008 5:42 AM
Subject: Re: [amsat-bb] Re: How best to park your satellite antennas

> At 09:16 AM 9/2/2008, i8cvs wrote:
> >----- Original Message -----
> >From: "Graham Shirville" <g.shirville@btinternet.com>
> >To: <amsat-bb@amsat.org>
> >Sent: Tuesday, September 02, 2008 6:35 PM
> >Subject: [amsat-bb] How best to park your satellite antennas
> >
> > > Hi all,
> > >
> > > I cannot remember this question being discussed before!
> > >
> > > If you have a nice array of VHF and UHF antennas - 2 stacked X  8 foot
> > > long yagis one ach band plus a 2 foot S band dish, is it best from
> > > wind loading and other reasons to park it:
> > > vertically pointing straight up (the dish has a hole at the bottom)
> > > pointing to the horizon towards the prevailing wind
> > > somewhere else?
> > >
> > > I know that at SSTL in Guildford they are usually parked pointing
> > > straight up but I have never been able to find anyone with a really
> > > good explanation for doing so.
> > >
> > > If you look at 193.51.07.114 you can see the sort of array that I am
> > > talking about!
> > >
> > > Many thanks
> > >
> > > Graham
> > > G3VZV
> >
> >Hi Graham, G3VZV
> >
> >Pointing straight up with the main supporting boom in direction of the
> >dominant wind is the best because the dish will offer the minimum surface
> >to the wind while both yagi's are aligned in a way to be seen by the wind
> >as about a single antenna.
> >
> >The above is my experience with less damages into the windy gulf of
> > Naples
> >
> >73" de
> >
> >i8CVS Domenico
>
> Generally, large radio astronomy or space tracking dishes are parked
> at zenith because this presents the smallest area exposure to
> winds.  Also, the dish load is balanced on its pedestal.  For a
> satellite array the long yagis present as much wind area as a small
> 2-foot dish.  One could point the array directly into the wind but
> this would present the full area of the dish to wind loading.  If the
> array is pointed 90-degrees from the wind direction and pointed to
> the zenith, wind area would be minimized.
>
> For my new 16-foot eme dish there is another consideration (living in
> Alaska).  That is snow loading.  A dish pointed straight up will fill
> with snow and rain definitely overloading it.  The best stow position
> is on the horizon and pointed 90-degrees to the prevailing
> wind.  Large dishes should have tie-down wires for further
> stablization in high winds.
>
> So it varies with the situation and type of antennas.  The idea is to
> minimize wind area.
>
> 73, Ed - KL7UW

Hi Ed, KL7UW

I am happy that you agree with me so please read the following.

A flat reflector with diameter D = 1.128 meters has a surface
S = 1 square meter

At a wind speed of 120 Km/h with the wind perpendicular to it
the force applied to the mounting frame is  70 Kg (kilograms)

If the reflector is a deep dish the above force must be multiplied
by K=1.6 and it becames 112 Kg / square meters

If a wind at 120 Km/h is applied perpendicular to standard
antennas with a net total metallic surface of 1 square meter
K= 1.4 and the force applied to the mounting frame becames
98 Kg / square meters

With Passive Reflectors and Antennas with flat radome K=1
and for antennas with aerodynamic Radome K= 0.8

It follow that the force applied to the mounting frame of a deep
dish with D = 60 cm when pointed directly into a wind direction
at 120/Km/h is about 31 Kg

Consequently pointing straight up with the main supporting boom
in direction of the dominant wind as I suggested that is the same that
you suggested using different words:

> If the  array is pointed 90-degrees from the wind direction and
> pointed to the zenith, wind area would be minimized.

it seems to be the best parking position because the dish will offer
the minimum surface to the wind while both yagi's are aligned in a way
to be seen by the wind as about a single antenna.

In addition the wind will see the main supporting boom as a single point
wich surface is only the small diameter of the pipe.

In the above conditions the torque applied by the wind to the Elevation
rotator is the minimum possible and the same for the Azimutal rotator.

Infact if you point the antennas stright up at 90 degrees elevation and
90 degrees from the wind direction if you disconnect the mechanical
joint between the azimuthal rotator and the mast you will see that under
wind conditions the antenna will search the minimum load against
the wind positioning itself as naturally did a flag or as suggested by you
Ed and by my self in the above messages.

73" de

i8CVS Domenico




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