[Antennaware] Dipole-balun question

Stephen Kangas stephen at kangas.com
Wed Jun 5 11:48:14 EDT 2013

John, I agree with Terry that you'll need a true "current" balun, a Ruthroff
design may also be possible, depending on answers to Terry's questions PLUS
(1) what band(s) are you interested in and (2) what 2:1 VSWR bandwidth are
you aiming for?   If you want to learn & design your own, start with (the
late) Sevick W2FMI book "Understanding, Building, and Using Baluns and
Ununs" along with the freebie balun & smith chart calculator programs
available.  If you want it built for you, I recommend Balun Designs
(www.balundesigns.com) but you'll likely need to have Terry or someone else
here help you with some specs from modeling if you want him to build the
perfect balun for this.  I'm curious tho: what's preventing you from
adjusting wire lengths on your antenna to more ideal lengths?

Terry, it is possible to "see" in a model the twinlead radiation component
and Its effect on characteristics, eg the freebie MMANA-GAL program does a
good job of that, among others.

Stephen W9SK

-----Original Message-----
From: Antennaware [mailto:antennaware-bounces at contesting.com] On Behalf Of
Terry Conboy
Sent: Wednesday, June 05, 2013 7:47 AM
To: John Geiger (AF5CC)
Cc: antennaware at contesting.com
Subject: Re: [Antennaware] Dipole-balun question


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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