Lostgallifreyan wrote:
. . .
Note that the idea that has caught my attention is to use transformers..
there would be no direct current contact between either the line and the
antenna, or the line and the receiver. This isn't my own idea, it's just a
wheeze that looks most interesting because it looks like it ought to work,
and if it does it certainly pretects the receiver input rather well. It's
beginning to look as if choosing coax or twin line isn't so important as
knowing if that transformer scheme is valid, so long as I do something to
block common mode currents with a choke balun on the line.

An ideal transformer will effect perfect current balance. What I don't
know, and have never seen any theoretical or experimental work on, is
how effective a real physical transformer can be made to be in that
regard. There will be capacitive coupling between windings which could
be a source of common mode current (current imbalance), and it might be
necessary to take some or a lot of care in the winding to maintain good
balance. And of course the system will have the windings' impedance
between the conductors, necessitating care in transformer design for
that reason. A fundamental advantage of the balun type connection is
that the balance improves as the winding coupling becomes more intimate,
while that tends to work against a conventional transformer. It would be
an interesting study -- I regret I don't have the time to dig into it more.

Roy Lewallen, W7EL

Roy Lewallen wrote in
:

Lostgallifreyan wrote:
. . .
Note that the idea that has caught my attention is to use
transformers.. there would be no direct current contact between either
the line and the antenna, or the line and the receiver. This isn't my
own idea, it's just a wheeze that looks most interesting because it
looks like it ought to work, and if it does it certainly pretects the
receiver input rather well. It's beginning to look as if choosing coax
or twin line isn't so important as knowing if that transformer scheme
is valid, so long as I do something to block common mode currents with
a choke balun on the line.

An ideal transformer will effect perfect current balance. What I don't
know, and have never seen any theoretical or experimental work on, is
how effective a real physical transformer can be made to be in that
regard. There will be capacitive coupling between windings which could
be a source of common mode current (current imbalance), and it might be
necessary to take some or a lot of care in the winding to maintain good
balance. And of course the system will have the windings' impedance
between the conductors, necessitating care in transformer design for
that reason. A fundamental advantage of the balun type connection is
that the balance improves as the winding coupling becomes more intimate,
while that tends to work against a conventional transformer. It would be
an interesting study -- I regret I don't have the time to dig into it
more.

Roy Lewallen, W7EL


I'll give it a go, though I don't know if what I try will have enough rigour
to give useful results back. I just sent a cheque for some FT114-43 toroids
which are big enough to consider a screen between windings, though as Richard
Clarke said, it might be awkward finding the best point to tie that screen
to. Re impedance of windings, I have no idea or plan except to aim for close
spacing (within each coil) and neat symmetry, and glue to keep them in place
so that whatever results it remains steady.