Wind load of the B5


#1

I’m sizing up a tower to be built as the hub of a rural town’s network. I need to come up with specs for the tower. Since different vendors post their antenna wind loads at different speeds, I want to normalize it all down to effective square footage of surface area.

I notice, for instance, that the A5-360-18 has a wind load rating of 35 LBF at 100 MPH. Doing some simple math (100 MPH is 26 lb/sf), that’s a perfectly understandable 1.34 SF. Sounds right.

Now a certain other company rates its 25 dB 5 GHz dish (40 cm) at 77 LBF at 125 mph, which comes to just under 2 square feet. That’s a bit more than the area of the circle, but still understandable.

But the B5 integrated radio, which is also 25 dB and about the same physical diameter (but a bit heavier/deeper), is spec’d at 86 LBF at 100 mph, which comes to about 3.3 square feet. Since I want to put seven of these on the tower (ain’t GPS wonderful?), that’s a lot of load! But it also sounds like the radio was over-spec’d. I don’t see why it’s more than 2 sf.

Anyone have any answers? Thanks.


#2

Hi Fred,

Wind load is the product of area (A), pressure § and drag coefficient (Cd). The difference you are seeing is likely the Cd since the objects under comparison have similar area.

We used a very conservative Cd value (plus safety margin) from a product design perspective, which results in stronger mounting hardware, but I see the challenge it creates for tower loading calculations.

For reference in your own tower loading/structural calculations, the B5 area we used is 1.674 square feet.


#3

Thanks. That’s a good explanation, and it helps keep the tower load numbers reasonable.