[Elecraft] open wire feeders

[Guy Olinger K2AV]

Would it were that all "baluns" were the same.  There is a limit to how
much blocking can be put between the balanced line and the coax.  What most
people fail to account for in the balanced line business is how much COMMON
MODE current and voltage there can be.  Unbalanced current on the feedline
is always common mode current.  Most will never see it in a model because
they DO NOT add the single conductor that gives the model the common mode
path that exists in reality.

I had a blocking need at the end of a long 450 window line run.  I was
doing very poorly in the matching, balancing business for this antenna.
 When I modeled everything literally, EZNEC said that with a PERFECT block
there was 955 volts RF RMS across the common connection in the balun !!!
 Gads.  There is NO balun ANYBODY makes that will stand up to that and work
right.  It will (and did) smoke ferrites.  Conversion to an isolation
transformer (which is not a balun...not inherently broadband) stopped all
the issues.  Of course an isolation transformer is a single band solution
without a tuner, as the series inductance has to be tuned out somehow.
 Running the balanced line to an isolation transformer (not a balun)
IMMEDIATELY followed by a tuner will handle common mode current.

But then, who talks about isolation transformers any more.

73, Guy.


On Sat, Dec 31, 2011 at 1:40 PM, Robert G. Strickland <rcrgs at verizon.net>wrote:

> Don...
>
> Many hams - as I have done/do - use coax to get out of the house,
> connect it to a balun, and then connect twinlead from the balun to an
> antenna. There is a lot of commentary on this setup, but perhaps another
> time through would be helpful. The questions arise:
> - if the coax is short, say under ten feet, is this setup more or less
> equivalent to running the twinlead all the way from the antenna to the
> transmitter?
> - again, if the coax is short, will RG8 or 213 be sufficient to the task?
> - is there any advantage of one balun ratio to another [1:1, 4:1, 9:1]?
>
> Happy New Year, and thanks for your contributions here on the Elecraft
> reflector.
>
> ...robert
>
> On 12/31/2011 17:42, Don Wilhelm wrote:
> > George,
> >
> > Short question, long answer follows --
> >
> > Do to constraints at home, I no longer use open wire or ladder line
> > feeders, but when I did use them, I found several things were true if
> > you did not want them to radiate (and create RF in the Shack).
> > My first rule is to use balanced antennas - off center fed antennas are
> > famous for feedline radiation and RF in the shack.
> > The second rule is to run the feedline away from the antenna at right
> > angles for as great a length as you can manage, but certainly for a
> > quarterwavelength - The feedline can pick up radiation from the antenna
> > if this rule is not followed.
> > Third is to run the feedline correctly - use nice gentle bends if you
> > must change direction, support it using as few hangers as possible (if
> > you can put the feedline under tension, you can get away with very few
> > supports) but support it so it is stable even in the wind.  Do not run
> > it parallel to other conductors, but you may cross a conductor at right
> > angles if necessary.  The line should be spaced away from other objects
> > by at least 3 times the spacing of the conductors.
> >
> > Lastly, If I could, use a true balanced tuner, link coupled is best, so
> > if you see a Johnson Matchbox at a hamfest, get it.  If you must use an
> > unbalanced tuner, use a good balun at the output (see K9AY's info on
> > baluns).  BTW, do not assume that a 4:1 balun is the thing to use, the
> > feedpoint impedance in the shack can vary wildly from very low to very
> high.
> >
> > If you do encounter a high impedance feedpoint on any band, that will
> > place a high RF voltage point at the shack end - add or subtract some
> > feedline to bring the feedpoint impedance down.
> > If you do not understand how the  feedpoint impedance changes with the
> > length, take a look at the Antenna article on my website www.w3fpr.com
> >
> > 73,
> > Don W3FPR
> >
> > On 12/31/2011 11:25 AM, W2bpi1 at aol.com wrote:
> >> Those of you using open wire feed lines. How do you keep RF out of the
> >> shack? 73 George/W2BPI K2/100
> >>
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> --
> Robert G. Strickland, PhD, ABPH - KE2WY
> rcrgs at verizon.net
> Syracuse, New York, USA
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