Section 2.0 - Loads

ID #1161

Is it acceptable to use Ka = 0.6 for feedlines and also consider the feedline ladder (consisting of angle rails and rungs) to be shielded?

The intent of the standard for determining the EPA of waveguides was to provide 2 methods. In one method, each component of the waveguide ladder would be considered independently. Shielding of the waveguide ladder components would be acceptable for this method in accordance with Section 2.6.9.4. When determining the total wind load to be considered for design of the supporting structure, either the Ka factor, in accordance with Section 2.6.9.2, may be used, or a Ka factor equal to 1.0 could be used with the consideration of shielding of the waveguide ladder and lines from the tower members. But in no case can shielding of the waveguide ladder and lines from the tower members be considered in conjunction with a Ka factor less than 1.0. In the alternate method, the waveguide ladder and lines can be treated as a cluster, in accordance with Section 2.6.9.5, with the width of the cluster equal to the maximum out to out dimension considering all lines and waveguide ladder members (i.e. rails, rung, etc.). The same Ka and shielding criteria limitations described above would remain applicable. In summary, shielding of a waveguide ladder and lines from the tower members shall only be considered when Ka is equal to 1.0. Shielding of various appurtenance components by other appurtenance components (e.g., coax shielding a waveguide ladder component) has no impact on the use of Ka, per Section 2.6.9.2. In other words, the intent of Ka is to reduce the EPA appurtenance due to the presence of the tower members and is independent of the method used to determine the EPA of the appurtenance itself.

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Last update: 2015-01-30 14:53
Author: John Erichsen
Revision: 1.0

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