EPA DEFINED!
The last answer was only partially correct (75% passing) on projected area,
et al. So here is my "sort of correct way" of finding "EPA". I mean "sort of"
because there are a number of variations and interpretations that can be
used to arrive at slightly different answers. Calculating EPA is an art form
and is subject to one's own style and taste. The end result should be within
10% to be called "close enough."
Let's define some terms:
PA - Projected Area - length x width.
FPA - Flat Plate Area: converts round areas to flat areas by multiplying PA
times .67
EPA - Effective projected area: The projected area (PA) times the shape
factor. (PA x Ca). The term Ca varies with the shape (round, flat, other) and
the length to width ratio.
Ca - shape factor of appurtenance. for rounds, varies between .8 and 1.2 for
L/W = 7 to 25 respectively. For flats, varies from 1.4 to 2.0 for L/W = 7 to
25 respectively.
Where did the confusion on EPA, FPA and PA come from -- The EIA-222 and its
various revisions. For Rev C the world used FPA and when Rev D came out the
procedures were changed to EPA. Many antenna manufacturers have still not
updated their catalogs. I suspect that they do not like the larger EPA
numbers or they have limited manpower.
There are a lot of antenna manufacturers out there and many of them use
different values and don't say which method they are using. The EIA-222-F
(current version) say that if the antenna is made up of rounds you can
conservatively multiply FPA by 1.8 to get EPA. One of the best methods that
some antenna manufacturers publish is the thrust value (T) and the wind speed
(V). To get EPA = T/(.00256 x V x V) (I can't do the squared thing in plain
text)
Now the problem gets worse, because almost none of the antenna manufacturers
publish information on their antennas with "ICE" and ice controls the design
of guyed towers (but it is "optional" whether or not to design for ice and
how much ice). The majority of commercial towers are designed for 1/2" radial
ice with 75% of the basic wind speed wind pressure.
So the trick is to figure out which is the value given is PA, FPA, or EPA.
This is mainly done by looking at the antenna and determining the length and
width of each antenna element and find its Ca x PA. The total of all elements
is the EPA. For ice you can repeat the process by adding 1" (for 1/2" radial
ice) to all lengths & widths. Or for a quick approximation if the antenna is
made up of many different diameters, take the (EPA/1.2) and divided it by the
total length of all the round elements. This will give the average diameter.
Then EPA(1/2") = (D+1") x (L +1") x 1.2
Life can never be simple can it.
Tower2sell@aol.com
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|