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[TenTec] "Tuning" antennas; baluns

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Subject: [TenTec] "Tuning" antennas; baluns
From: banderso@eni.net (Barry Anderson)
Date: Tue, 23 Sep 1997 16:19:41 -0400
At 07:56 AM 9/23/97 -0800, you wrote:
>I think what we have here is a misunderstanding of each others terminology.
>There are several absolutes involved.
>        1: An antenna is resonant when standing waves exist along it's
>           length. No amount of tranmission line massaging will change that.

        Not Completely true ... hanging a feedline on the antenna WILL
change its resonance. The more mismatched the feedline is to the antenna,
the more effect on resonance.

        An antenna "system" consists of BOTH the antenna and the feed line.
Regardless of the resonate nature of the flat top, or the length of the
feedline, the antenna must be looked at as a system. A tuner (or matcher)
actually causes the "system" to resonate. Although standing waves will form
on the feeder portion of the system, radiation can be all but eliminated by
the use of balanced lines, where equal and opposite opposing currents cancel
virtually all radiation. In this case, the feeders are actually part of the
antenna ... and the tuner will resonate the whole mess. Because of the
almost complete absence of dielectric losses, one can roam to any frequency
at will and still enjoy a highly efficient antenna system.  In this case,
the length of the feedline WILL have an effect on the initial resonance of
the antenna system, which will be brought into parameters with the tuner.

         Using coax to feed a balanced antennae is an idea whose only saving
grace is the simplicity of running the feedline. In this case, the only way
to make the system function to any degree of efficency, is to attempt to
choose a coaxial line type with an impedance that is as close as possible to
that of an already resonate flattop and use a balun to choke off the antenna
currents which would flow on the outside shell of the coax.  In this case,
the tuner will still attempt to resonate the whole mess, but because of
different levels of opposing currents flowing between the different parts of
the unbalanced line(abet small in this circumstance), radiation will not be
"all but eliminated."  If this rare set of circumstances were to occur, they
would occur at ONLY ONE FREQUENCY ... at which time losses would begin to
increase. The +/- j components of the match will certainly effect the
resonance of the antenna system.

         Matching the antenna to the feedline with a matching device, such
as a gamma match, only puts a tuner at both ends of the feedline ... and the
one at the antenna end cannot be easly tuned. The standing waves which will
be present on the line because of its impedance differances with the flat
top, will NOT be opposite and equal and will not balance out. Because of the
voltage differances between the inner and outer parts of the coax, losses
will occur in the dielectric. In effect, some radiation take place WITHIN
the coax as well as radiation from the outer shell of the coax, itself. 

        IMHO ...
>        2: An antenna consists of the radiating elements. Hopefully
>           a transmission line will not radiate rf.

        See above ...
>        3: The transmission line delivers rf from the generator to the
>           load. (Antenna)
>        The antenna system consists of all of the above.
>By the way, the Amidon baluns were operated well within their ratings and still
>failed after short periods. The load on the baluns was approximately 300 ohms
>and I never run more that 1500 watts of rf, the baluns were spec'ed at 3kw as
>I recall.
>73 de KL7HF

Sorry about all the bandwidth ... 73

Barry Anderson K3SUI
Frederick, MD

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