[Towertalk] Yagi Rotational Center

[n4kg@juno.com]

Recently, there was a recommendation made 
to the effect of running a string from the reflector
tip diagonally across the boom to the end director
tip, claiming (if I remember correctly) that the
point at which this string crossed the boom was
the Rotational Center.

I believe that recommendation to NOT be correct.

My thinking is that as the director is shortened,
the string will move closer to the boom and the
crossover will move TOWARD the Director which
is clearly not the desired result.

There are two approaches that will lead to determining
the center of rotation.  

An iterative measurement approach (either actual or on 
a scaled drawing).  Select a point on the boom, measure
from that point to the tip of the reflector and to the tip of 
the end director.  If they are not equal, select another 
point which will reduce the longer dimension and lengthen
the shorter dimension.  Repeat this process until a point 
is found on the Boom which is equidistant from the 
reflector tip and the end director tip.  THAT is the
Rotational Center.

Mathematically, calculate the rotational radius to 
the reflector tip and the director tip from points along
the boom until they are equal.  

(R/2)^2 + Br^2 = Rr  
where R = Reflector Length, 
Br = distance on boom from reflector to test point

(D/2)^2 + Bd^2 = Rd
where D = Director Length,
Bd = distance on boom from end director to test point

When you find Br and Bd such that Rr = Rd,
you have found the rotational center.

I like the scaled drawing approach best :-)

Tom  N4KG




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