[Antennaware] Dipole-balun question

John Geiger (AF5CC) af5cc at fidmail.com
Mon Jun 3 09:51:32 EDT 2013

I had previously said that I had a G5RV type dipole that I made myself.  After repairing it today following a run in with a tree branch last Wednesday, I realize that it isn't quite that.  It is more of an "off center fed dipole". It isn't the classic "Windom off center fed" where one leg is twice as long as the onther.  On my antenna one leg is approximately 50 feet long, and the other leg is around 60 to 65 feet.  It is fed with 300 ohm radio shack twin lead-about a 25 foot run, which goes to a 4:1 voltage balun, and then to 75ohm RG-11 coax to the shack.

Given this new revelation, that it is a off center fed dipole, could someone model this and suggest what is now the best balun for the twin lead to coax junction?  I know for a Windom the feedpoint impedance is supposed to be 200ohms, but this doesn't follow the 2/3 to 1/3 leg length ratios, and also having the feedpoint to coax at the end of a length of 300ohm twin lead probably complicates things as well.  The center of the antenna is probably 25-30 feet high, and the longer leg slopes to probably 20 feet high, and the shorter leg is probably 10 feet high.  

I am assuming that it isn't quite a balanced antenna, since the legs aren't equal, so how crucial does a balanced to unbalanced (coax feedline) transformer become?  My main interest is trying to keep the RF off of the coax shield, although I haven't had any real problems with RFI in the shack.

73 John AF5CC

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