Hi Patrick,
First, this entire balun and tuner thing has gotten way out
in fantasy land because people started assuming things
without doing a paper analysis or measurements. QST even
keeps repeating this myth.
Moving the balun from the output to the input of a floating
L or T changes things, but it does NOT make things better
for the balun. Say I have a string-of-beads balun on the
output of a T network and with a given load on low
frequencies measure core heating and balance. If I go
through the hassle of floating that network and moving the
balun to the input....nothing significantly changes. On the
high frequency end balance almost always gets worse.
If I float a double or balanced network and put a balun on
the input, the common mode isolating impedance of the tuner
that allows balance is at very best no different overall
than a tuner that forces equal voltages on the line
terminals and probably worse. The balun does have an easier
life in this case, but for all practical purposes it could
be eliminated entirely. The system does NOT have improved
isolation of parallel current induced by the antenna on the
feedline.
What this means is I can double the cost of a single ended
network tuner from say $150 in raw materials to over $300,
and wind up with about the same electrical problems. On the
other hand $30 more invested in the balun of a conventional
T or L could make a significant improvement in overall
performance.
I liken this problem to the popular misconception that
measuring currents without regard to phase can indicate
"balanced" operation of a feeder. Someone just didn't think
about the problem.
> I've read your comments & analysis of "T" tuners before
with interest but
> what's your opinion on the "L" - double balanced" tuner
concept with
> variable inductors in each leg (used with open wire
feeders)?
Look at what you do to the system. If the center floats, you
now add two series reactances in each feeder terminal. The
value of reactance is relatively small, so the feeder does
not have high isolation impedance from ground. The
reactances have a large voltage drop and so this establishes
a VOLTAGE drop that is nearly equal (if reactances are
equal) becuase the large circulating currents in the tank
circuit dwarf the line currents, but now we just have a
balanced voltage source that is roughly feeding balanced
out-of-phase voltages to the feeder.
The ideal balun would be a floating current source, so why
did we do all that work to get a roughly balanced voltage
source? Why didn't we just link couple into a floating above
ground network and be done with it?
Is this a
> viable application of the "L" style tuner or is there a
better "T"
> configuration equivalent for balanced feeders?
I'd just put my money into better current balun and use a T
network.
73 Tom
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