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Dish Surface Size & Wind Pressure

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Post Dish Surface Size & Wind Pressure

Here is a guide to the increase in the surface size of a larger dish - how much more signal can hit the dish. Also the wind load figures in a 100mph wind. This is the amount of wind pressure hitting the dish. If the dish was flush at the end of tube, the full pressure would push on the dish. As, we have open air around the dish, the wind disperses - so the true constant pressure (CP) figures to your dish are in yellow below. It's a bit like try to fill your hand with water from a running tap. Because of the flow, only so much can retained - the retained amount is the Constant Pressure.

Dish Size------------------ -surface area sq cm---------surface area sq inches------------------100mph windload in lbs
40cm-----------------------------1256--------------------------194-------------------------------4268
60cm-----------------------------2827--------------------------438-------------------------------9636
70cm-----------------------------3848--------------------------596-------------------------------1232
80cm-----------------------------5026--------------------------780-------------------------------1716
90cm-----------------------------6361--------------------------985------------------------------21670
100cm----------------------------7853-------------------------1217------------------------------26774
110cm----------------------------9503-------------------------1472------------------------------32384
120cm---------------------------11309-------------------------1752------------------------------38554
125cm---------------------------12271-------------------------1902------------------------------41844
150cm---------------------------17617-------------------------2730------------------------------60060



You will notice that a 70cm dish has one third of the surface area of a 1.25 dish. The wind pressure on a dish is 22lbs per square inch @ 100 mph. The total surface area wind pressure - end column - on 1.5 dish is a massive 60060 lbs hitting your dish - a lot of the impact is lost, so the constant pressure is around 1000 lbs.

These are the figures for the constant pressure actually being applied to your dish - as if you were pressing upon it with the shown amount of weight.

Wind Speed in MPH---------30----------50---------70---------90--------100

40cm-----------------------6----------17---------32---------54---------66 lbs
60cm----------------------12----------34---------72--------146--------160 lbs
70cm----------------------18----------51--------100--------164--------203 lbs
80cm----------------------24----------66--------131--------216--------266 lbs
90cm----------------------30----------84--------164--------272--------336 lbs
100cm---------------------37---------104--------208--------337--------446 lbs
110cm---------------------45---------125--------246--------407--------539 lbs
120cm---------------------54---------150--------293--------484--------642 ilbs
125cm---------------------58---------162--------318--------526--------697 lbs
150cm---------------------83---------233--------457--------755-------1001 lbs

Wind pressure figures for a mesh dish - in a 40 mile per hour wind - would be 40% less than a solid dish - BUT - only up to a 50mph wind. Over 50mph, the wind will make the mesh dish act the same as a solid dish. The reason for this is: the high wind flow creates turbulance around the holes on the mesh - preventing the wind from passing through. Basically too much wind to get through a small hole at the needed rate.

Now, if you are using a ground pole as a mount for a 1 metre dish.

Wind at 100 mph will put 446 lbs applied pressure on the dish. The base of the pole will then have a massive leverage upon it. If cemented in, it could bend or break at that point. The amount of leverage on the pole will depend upon it's length. A 3 metre high pole with a one metre dish up top - I estimate as a 3 to 1 leaverage ratio. So, the wind applied pressure at 100 mph will be transferred to the base of the pole multiplied by 3 = 1358 lbs. Bad news if the pole breaks where the cement joint is.

Another tip I picked up - and this is real - do not use plastic tubing to as a mount for your dish. Duh!

Figures are relativly accurate and are based on a tempature of 40f. The wind pressure will be higher when colder and less when warmer. It also depends on other conditions. Altitude will also increase the AP, so if your dish is roof high - you can add 15-20 percent to the above figures.

All calculations are based on a formula for calculating wind on solid objects and have been done separately over a period of 5 months
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