On 11/13/2010 11:21 AM, Jim Brown wrote:
On 11/12/2010 4:22 PM, Stuart Rohre wrote:
Most external baluns are of heavier construction and therefore lower
loss potentially, than built in baluns common to 1970's tuner designs.
Loss in ferrite chokes and in baluns wound with ferrites is NOT a
function of HEAVY construction, but depends primarily on the
characteristics of the ferrite material (the MIX), how the chokes are
wound (mostly the number of turns), and the size of the core.
The LF end of the useful range depends on the raw inductance of the
winding which comes from the magnetic material and the turns. The first
stop band going up is when the length of the wire (loaded by dielectric
of the insulation and the core) gets close to 1/2 wavelength. Going to a
bigger core adds more wire per turn and tends to lower that HF end. Its
easiest with a tiny core to get the broadest bandwidth, like the cores
in a TV splitter that work from 10 to 1000 MHz (and cost $2.98 retail
assembled).
Likewise, HEATING in the core depends on the loss, the TX power level,
the balance of the antenna, AND the length of the transmission line into
which it is inserted. Using a balun on Field Day (usually 100W) and
saying it worked fine doesn't mean much. A FAR better test would be
running 1.5kW through it into difficult lengths of line and with an
unbalanced antenna (like a Windom) and noting whether or not it got hot.
True. I remember a balun the antenna boys at Collins built and had us
test with 50 KW from our 250KW VOA transmitter we were developing. The
first try smashed the core. They had to redesign for better cooling a
couple times.
Although I own and love a half dozen Ten Tec tuners, I don't use
twinlead, so I've never given the balun a torture test. :)
I prefer a link coupled tuner with balanced output, the I don't have the
limitations of the balun on the tuned side.
73, Jerry, K0CQ
73, Jim K9YC
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