John,
Probably the best approach is a 4:1 "current" balun, which tends to
float the connection to the twinlead and should reduce current on the
outside of the shield of the 75 ohm coax. However, in this
configuration, regardless of the type of balun used, the twinlead will
radiate vertically polarized signals since the currents in the two wires
of the twinlead are not equal, nor are they 180 degrees out of phase, as
they would be with a perfectly balanced antenna.
In modeling this antenna, using the built-in feedline model in EZNEC,
for example, will not show the radiation from the twinlead and as a
result the gain, pattern, and impedances won't be accurate.
I can model this if you tell me know the height(s) of the antenna, and
wire description (gauge, insulation). Also, is there a connection to
ground at the existing balun? How long is the RG-11 run and how is it
routed (right angles and/or parallel to antenna wire; on ground, above
ground, buried)?
73, Terry N6RY
On 2013-06-03 6:51 AM, John Geiger (AF5CC) wrote:
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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