Not so sure about the balanced pre-amp, it doesn't makes double balance,
only one stage should be centered, when the
aerial is balanced - the connection to one side is just as good as
balanced connection, the only improvement you can add
is better linearity. The only result of a balanced amplifier is that it is
more difficult to make, but probably isn't so balanced
after all.
> As long as the background noise without pre-amplifier is 10-20dB above
the background band noise
> it is totally waste to use a pre-amplifier, it may only worsening the
result.
I don't buy this. The reason for the balanced connection (or center tap) to
the loop is to reduce the so called 'antenna effect'. Without the balance
the antenna will be sensitive to the E-field of the intended signal as well
as all the other unwanted signals and noise in the area, coming from all
directions. Also, the desired null off the side, typically 40db for a well
balanced loop, will be greatly reduced if the loop is connected simply to
one side.
One common way to achieve a 'balance' is to isolate the tuned winding from
the input to the amplifier with a one or two turn isolated pickup winding. A
ferrite core can also be used that couples one turn from the main winding to
the amplifier or receiver. John Andrews uses this approach. I call both of
these a 'floating connection' rather than a balanced connection', but they
achieve the same thing because neither is effected by E-field signals.
Bill A
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