I fully agree with Jim on these points. Resonant antennas for each
band/frequency are the way to go. An the coaxial choke balun properly
constructed and installed very close to the feedpoint is and excellent
performer. It handles lots of power without problems and is inexpensive to
built. Ugly however.
Other than the above, everything else is considered a compromise. Of course
if you only have room and supports for one wire antenna, a balanced fed
system with a 238 works and will allow you to migrate to all bands. As to
antenna length, all you can get up works. Feedline length, that length
necessary to go between the feed point and the tuner works. Don't worry
about the feet and inches. Just let the tuner deal with the Xc and Xl and
j components of the antenna system.
My favorite antenna is the old fashion folded dipole about 1/4 wave above
ground, made of open wire line and fed with open wire line and terminated
into a good 4:1 balun.
73
Bob, K4TAX
----- Original Message -----
From: "Jim Brown K9YC" <k9yc@audiosystemsgroup.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>; "Stuart
Rohre" <rohre@arlut.utexas.edu>
Sent: Friday, April 11, 2008 5:37 PM
Subject: [TenTec] Twinlead and Balanced Tuners
>I strongly urge anyone using or considering balanced feedline and
> and/or a balanced tuner to study the part of my RFI tutorial that
> talks about antennas and balance. Almost any ham antenna is likely
> to be at least a bit unbalanced by its surroundings -- trees,
> buildings, ground slope, antenna length -- even with a balanced
> feedline. While the "skew" of the pattern may be minimal, the
> NOISE pickup on the feedline will usually NOT be minimal. This
> noise pickup is caused by common mode current, which is the result
> of antenna imbalance. Killing this common mode feedline current is
> a major benefit of a good choke balun (W2DU or better).
>
> The coaxial choke baluns described in my tutorial are MUCH BETTER
> choke baluns, because they present a MUCH higher common mode
> impedance. Because all forms of twinlead have significant leakage
> flux that heats a ferrite core, it is NOT practical to wind a
> choke with twinlead. This is a MAJOR disadvantage of feeding
> antennas with twinlead.
>
> http://audiosystemsgroup.com/RFI-Ham.pdf
>
> http://audiosystemsgroup.com/NCDXACoaxChokesPPT.pdf
>
> A FAR better way to do it is to use big coax to keep losses low
> (RG8 if it's a low dipole, RG11 if it's a high dipole) and a good
> tuner. The 229 and 238 tuners are EXCELLENT tuners (very good
> efficiency, very good tuning range). My 80/40 fan dipoles are all
> within 3:1 on 30, 17, and 12 meters, and work VERY well with a
> 238. I can't easily install a tri-bander on a tower, so I use
> 20/15/10 fan dipoles. All of these antennas have serious coax
> chokes at their feedpoint, and they are very quiet on receive.
>
> Quite a few members of NCCC and PVRC (two premier contesting
> clubs) are using my chokes on a variety of antennas, and report
> similar results.
>
> IMO, the benefits of twinlead feeding an all-band one-size-fits-
> all wire dipole are a fignewton of the imagination of guys who are
> casual operators and don't do much weak signal work. They ARE an
> EASY way to work all bands, but they are NOT a GOOD way to work
> all bands.
>
> 73,
>
> Jim Brown K9YC
>
> On Fri, 11 Apr 2008 14:36:31 -0700, Stuart Rohre wrote:
>
>>On the subject of the tuner being balanced itself
>
>
>
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