[Elecraft] RE: Attic Ant.-Closed Loop vs. Open Ends?

[Stuart Rohre]

Now this method is a bit convoluted, but you could put extra long leads on 
an ohm meter that will measure low ohms.  Start at the feedpoint with one 
lead, and experimentally tap onto the antenna where you think the 55 foot 
point is.   If you have found the center, you will have the resistance of 55 
feet of wire indicated on your meter.  You likely know the gauge of your 
wire, thus, you can use a wire table to predict what 55 feet of that wires 
should measure on the resistance scale.  You will have to zero out the extra 
long test leads you make to reach to the far side of the loop.  The easiest 
way is to make a test lead of 55 feet and then have short lead and the meter 
with you as you seek the center on the far side.

Other means of accomplishing the same thing, are to find equal capacitance 
point on far side to each side of the feed point insulator, or equal 
inductance.  However both of those methods may suffer from incidental 
coupling within the attic.  The low ohms method is done at DC to overcome 
those disadvantages.

-Stuart
K5KVH