I heard some one say that a balanced tuner is very
expensive to build. I used some quarter inch copper tubing
from the hardware store to build a four inch coil. This is
the tubing for a ice maker. I then built a four inch coil
and tapped it to a multi position switch from radio shack.
I had a used large old condenser and hooked the whole
thing up in a L network design. I left the ground
connection floating on both input and output and connected
it directly to ladder line. A current balun was made out of
25 ft of coax and put at the input of the tuner. Works
great on my 330 ft loop. No second coil is necessary.
Everything is balanced and nothing heats up. Kind of ugly
looking, but the electrons don't seem to mind.>>
Bruce,
I looked at this issue extensively when the whole myth about
moving baluns came out.
Moving the balun to the input of an unbalanced network does
NOT make life on the balun or the system easier for the
truly difficult problem, common mode isolation. It does not
change a thing to the better for common mode currents or
isolation, and it actually makes the system worse on higher
bands where network physical size and unwanted stray
capacitance affects balance.
It takes exactly the same common mode impedance and common
mode current and voltage capacity in the balun if it is
located at the tuner output or at the tuner input when the
network is a floating unbalanced network. The core (if
used) will get just as hot, and current unbalance (except
for stray capacitance or network transmission line effects)
will be exactly the same.
If you draw it on paper and trace the path from one lead of
the transmission line you will see exactly what mean. There
is a direct connection from one side of the balanced antenna
terminals to the balun, and this means the balun has
EXACTLY the same common mode problems. The only thing you
modify is the differential impedance, and it is extremely
easy to solve that issue with any current balun so you
really just fix something that is largely a non-issue to
start with.
Now if you used a real balanced network with series
impedances in each leg and in particular some perfectly
ground reference point for the shunt elements, you would
make common mode life easier for the balun BUT the drawback
is you now have a balanced voltage source which may or may
not supply balanced currents. The symmetry of the network is
also critical. You have, in essence, exactly the same
expense and difficulty as simply building a balanced tuner
of any standard configuration.
A manufacturer would be misleading customers if it claimed
they had a balanced tuner when using an non-symmetrical
floating network with a balun on the input. It would be no
better than the same balun on the output, and likely much
worse on upper bands.
I'm afraid there is no free lunch. It has to be a balanced
network which means at least double the cost of the
expensive components, or you can simply build a good balun
and use an unbalanced network on the balun input. Building a
very good balun for the output is less than half the overall
cost of using an expensive true-balanced network, I know
because I priced this stuff out dozens of times.
There might be a marketing or sales advantage to customers
who feel good about a balun on the input, but that would be
dishonest or incompetent engineering by the manufacturer to
claim it did anything for system performance.
73 Tom
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