On Thu, 2008-05-08 at 15:17 -0700, Stuart Rohre wrote:
> 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.
The fact is that the unconnected wire still has stray capacitance to
space and the other wire. That causes a small current at the high
impedance end of the feed line, at the antenna. And the end impedance of
the half wave radiator is high enough its current is small so even at
the open end of the feed line the currents aren't wildly different and
so when transformed by a quarter wave the currents aren't badly out of
balance.
>
> 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 desirable, 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.
Its not hard to check the isolation properties of the current balun or
choke with a Boonton RX-meter that measures parallel R and C with an R
range from a few ohms to an open circuit and a C range from -10 or 20 pf
to 120 pf or something like with a frequency range from 500 KHz to 250
MHz. A vector impedance meter may work but I don't think it has as wide
a resistance range. I know the basic tool used by Collins transmitter
designers when developing PA plate feed chokes was that RX-meter and the
one I have is handy for such measurements though its not instant
reading.
>
> 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.
Since I've run into problems tuning the balun instead of the antenna and
know excess voltage on the balun will lead to core saturation, I much
prefer a balun on the input of the floating or balanced tuner or a
totally air cored link coupled tuner for balanced lines. I've built them
for 160 m through 70 cm, though some cover several bands.
>
> -Stuart
> K5KVH
>
73, Jerry, K0CQ
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