Incidental to L. B. Cebik's modeling of antennas of the end fed so
called Zepp type, (a dipole), he spoke on current balance and its effects.
Summary:
Although the traditional Zepp feeder was a balanced line, and in the
Zepp end fed, one wire does not connect to anything, with the other wire
to the end of the antenna, Cebik found this only caused a 10 per cent
difference in the line current in the two conductors when modeled.
As a practical matter, you can use on a regular dipole center feed, the
cable choke to block current from the outside of the coax shield, as
long as the shield is being led off at right angles, and its effect is
to confine the currents to the inside shield and the center conductor
and in balance, (equal and opposite). This would force equal currents
into the dipole legs, if each is independent of other coupling such as
nearby conductors, trees, buildings, etc.
Folks usually use one type of coupling or another. A wound balun to
transform from balanced dipole center feed to coax, or a bead "balun" of
isolating ferrite beads to block outside the shield currents, or a cable
choke of coiled solenoidal coax to again block the outside the shield
current. Only one of these is needed or desireable, as the mechanically
loading on the antenna must be considered so as to not cause unfavorable
wire stress in winds.
The wound wire balun coils may have a different number of bands covered
than the bead choke. And depending on how you wind the cable choke, it
may cover fewer bands than exist from 2 to 30 MHz. You often found
those broken into cable chokes effective at 80 to 40m, (maybe 30m); and
others for 20m and up.
A lot of individual home built isolation devices in the above classes
have been built over the years. The problem is that there is no
comprehensive table of bands covered, losses, efficiency, power handling
for all types homebuilt and commercial except for what Jerry Sevick has
covered in his books as a result of experiments a number of years ago.
His work was done with some self built instrumentation, as Antenna
Analyzers were not widely available at that time. To his credit, in the
back of his first book, he included details on the measuring
instruments, and they were good basic instrumentation. He described how
he measured or estimated efficiency, measured balance, etc.
Some additional information may be gleaned from the intensive work Frank
Witt did on tuners, which were mostly commercial tuners as I recall, but
most of those have a built in balun of the core type.
-Stuart
K5KVH
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