[TenTec] Balun

[Stuart Rohre]

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