Wind Loading and Center of Lateral Aerodynamic Center of Pressure for a 2M-20 Element VHF Cross Yagi Antenna - PowerPoint PPT Presentation

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Wind Loading and Center of Lateral Aerodynamic Center of Pressure for a 2M-20 Element VHF Cross Yagi Antenna

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Title: Wind Loading and Center of Lateral Aerodynamic Center of Pressure for a 2M-20 Element VHF Cross Yagi Antenna


1
Wind Loading and Center of Lateral Aerodynamic
Center of Pressure for a 2M-20 Element VHF Cross
Yagi Antenna
  • Jeffrey Annis K9VS
  • Waukesha,Wi

2
Introduction
  • About 2 years ago we decided to upgrade our 2M
    1/2 wave vertical to a Yagi antenna.
  • With 20 element cross Yagi assembled and ready
    for installation something didnt seem right with
    the mounting location.
  • Being a practicing engineer and involved in
    model aeronautics, proceeded to evaluate the
    antenna center of aerodynamic pressure.

3
Introduction
  • This investigation provided the impetus to write
    the following paper and share knowledge gained
    with fellow hams.

4
2M-20 Element Cross Yagi
5
Introduction
  • A misnomer among many hams an antenna center of
    gravity is not at the same location as the center
    of aerodynamic pressure.
  • If light duty tripods and rotators are used,
    mounting the antenna at the correct position to
    prevent damaging cart-wheeling torques is of
    major importance.

6
Internet Good Information Source
  • Several good articles in QST and QEX magazines.
    Couple listed as references.
  • Several national and international organizations
    related to Wind Engineering.
  • American Society of Civil Engineers has standard
    A7-98 section B, dedicated to wind loading of
    structures.

7
Internet Good Source of Info
  • This standard has a wind zone map of the United
    States for a once in 50 year occurrence of
    maximum wind speed.
  • Per the map, maximum wind for the mid-west vary
    between 80 and 90 MPH.
  • Based on this, recommend designing an antenna
    system to withstand 100 MPH.

8
Yagi Antenna Wind Loading
  • Wind loading can be the most significant load on
    an antenna because wind force varies with the
    square of wind speed.
  • Aerodynamic drag on objects depend on shape,
    surface roughness, drag coef., air density,
    frontal area, and mostly wind velocity.

9
Drag Equation Reynolds No.
  • Drag Force FCd(?/2g)AV2
  • Drag Coefficients are determined empirically
    through wind tunnel testing and vary only with
    Reynolds Number.
  • Reynolds Number ReVD/?

10
Antenna Structural Shapes
  • Structural shapes typically used to construct
    Yagi antennas are solid rods or round tubes.
  • These antenna structural elements typically have
    very high aspect ratios, length to diameter
    ratios, and therefore can be considered infinite
    cylinders.

11
Drag Coef.- Infinite Cylinder
12
Center of Aerodynamic Pressure
  • Worst case wind loading and cart-wheeling moments
    are assumed to occur when the antenna mast is
    perpendicular to the wind.
  • Primary drag elements are the boom and vertical
    elements.
  • Contribution from horizontal elements is small
    and therefore neglected.

13
Center of Aerodynamic Pressure
  • Center of pressure is calculated by summing each
    contributing drag force times the distance to the
    reflected end and dividing by the total force.
  • Cp(Sum FxL)/Sum F

14
Calculation Spread sheet program
15
1/10 Scale Antenna Model
16
1/10 Scale Antenna Model
  • A 1/10 scale model was built to empirically
    validate spread sheet calculated results.
  • An alligator clip attached to an aluminum tube
    simulated a mast and allowed easy movement of the
    attachment point for wind testing.
  • Aerodynamic balance point found to be 6 from
    reflector end.

17
Generic Wind Loading Curves
18
Results
  • Center of pressure is located 61 inches from the
    reflector end.
  • Correlated very well with 1/10 model testing
    where empirically determined location on model
    was at 6 inches from reflector end.
  • Manufacturer recommended location more than 14 .5
    inches off from C.P.

19
Results
  • In an 80 MPH wind, this can cause a significant
    turning moment of 35 ft-lb on a light duty
    rotator or tripod.
  • Resulting off-center cg is minimal if an upper
    thrust bearing is used with rotator.

20
Results
  • Optimum design is to have both aerodynamic center
    of pressure and center of gravity at the rotator
    rotational axis.
  • Can be accomplished by adding flat drag plates or
    counter weights to the antenna boom.

21
Conclusions
  • Analytical methods presented can be used to
    calculate prior to installation, the center of
    pressure for a VHF/UHF antenna.
  • Proper location of the antenna attachment point
    can prevent costly wind damage to the rotator or
    antenna supporting structure.
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